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Discussion Starter #1 (Edited)
Is there a way of telling how much lift a motor has room for with the heads already on. The heads are Edlebrock RPM that they say are set up and good to go up to 575 max. lift. I just need to know if there a way of testing with the heads already on how much clearance I have and that will tell me what the motor will allow as max lift and still be safe. I am running a 488/510 matching cam now.
Thanks in advance for the info.
 

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Yes. Use test valve springs and a dial indicator with the rocker arm off. You can manually push the valve down until it contacts the piston. Usually check at TDC, but with high lift the valve may be closer at positions before or after TDC.

Steve
 

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The only true way would be to clay the domes. But the heads are all ready on. You could cycle the engine to overlap, and approx 10 to 7 degrees before TDC check the exhaust clearance, and 7 to 10 degrees after TDC check the intake. You would do that by getting a spring compressor handle and a dial indicator and put it on the retainer. At the overlap of the valves are the closest they will be to the pistons. I always go for .100 claerance, but seen it as low as .025 in a comp engine. Good luck. Dave
 

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Man, this is a great question and some great anwers...

I'm not sure on heads when they say it's okay up to .575" lift, does that mean that anything over that will cause the valve springs to coil bind or the spring retainers will bottom out on the guides or seals?

So assuming with a weak test spring there is a little bit of fudge room in overall .575" lift quoted, I would think the valve could probably move overall for this test of maybe .600" or a bit more.

If at TDC he can read with the dial indicator plus .600" available movement of the valve, does it matter that he check it anywhere else other than TDC?

I would think if the dial indicator reading cam out at anything much over the head's .575" rating that his 488"/510" rated lift of his cam would be safe.

Just a question of my own on this same topic, if I were running the stated heads with .575" max lift rating AND ASSUMING there was enough valve to piston clearance, could I run a cam with a rated lift of .575"?

I'm just courious where the fudge is in the heads themselves.
 

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If the heads were sold stating that they were good to .575 lift. I would think that that is what the valve springs are rated at that installed height before coil bind. you can't just take the rocker off and check at TDC for the simple fact that it doesn't simulate the cam opening the valve. That's why if you can put a test spring on one intake and one exhaust, You put the rocker on, adjust it, put a degree wheel on and get it set to TDC with a pointer. The critical part is 7 degrees before TDC on overlap (that's when the exhaust in closing and the intake is opening) for the exhaust valve, and 7 degrees after TDC on the intake. With the test springs on, and dial indicator on the retainer going the same angle of the valve stem, When you get to about 10 degrees before, start checking the clearance by pushing down on the retainer and write down the reading. Then go a couple more degrees and recheck. you'll get a real good idea how close and how fast the clearance changes. Check it all the way past TDC and the clearance will get bigger. Then do the same to the intake side. To me, exhaust clearance is more important because the piston is chasing the valve up towards the head, as compared to the valve chasing the piston. Both are inportant, don't get me wrong. I always try to have at least .100" (tenth of an inch) on both. On the one engine I built I ordered my JE pistons that were cut for max lift. I ended up with .200 valve clearance and cut my compression. DUH! It still worked out. This is kinda long, but you'll get the picture. Dave
 

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If the heads were sold stating that they were good to .575 lift. I would think that that is what the valve springs are rated at that installed height before coil bind. you can't just take the rocker off and check at TDC for the simple fact that it doesn't simulate the cam opening the valve. That's why if you can put a test spring on one intake and one exhaust, You put the rocker on, adjust it, put a degree wheel on and get it set to TDC with a pointer. The critical part is 7 degrees before TDC on overlap (that's when the exhaust in closing and the intake is opening) for the exhaust valve, and 7 degrees after TDC on the intake. With the test springs on, and dial indicator on the retainer going the same angle of the valve stem, When you get to about 10 degrees before, start checking the clearance by pushing down on the retainer and write down the reading. Then go a couple more degrees and recheck. you'll get a real good idea how close and how fast the clearance changes. Check it all the way past TDC and the clearance will get bigger. Then do the same to the intake side. To me, exhaust clearance is more important because the piston is chasing the valve up towards the head, as compared to the valve chasing the piston. Both are inportant, don't get me wrong. I always try to have at least .100" (tenth of an inch) on both. On the one engine I built I ordered my JE pistons that were cut for max lift. I ended up with .200 valve clearance and cut my compression. DUH! It still worked out. This is kinda long, but you'll get the picture. Dave
Dave, I understand what you're saying, but I'm still confused.

At TDC the piston is at it's highest point of the stroke and obviously closest to the valves. As an example, if you were to have your test spring in place on both valves and at TDC you physically measure your .100" over the max rated lift of your cam, why check anything else? Regardless of any other rotation of the crank, the valve has more than enough clearance.

Does this not make sense or am I missing something?

Thanks for the explanation of the valves in relationship to the piston and I get all of that. Valve timing is always running in front of or after TDC. Two cycle motors are even worse on the port timing and blow back through the carb.
 

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You're understanding what I'm saying but not what I'm telling you. Try to picture the piston at TDC at the comperssion stroke. At TDC you will have, probably, a bunch of piston to valve clearance. That's on compression with the valves closed. Now picture the piston coming up to the valve on the exhaust stroke. (overlap, when the ex is closing and the intake starts to open) The valve is wide open and the piston is chasing it up. Just before the piston gets to the top, the other TDC, The piston is gaining ground on the valve to get all the gases out. Approximately between 10 and 7 degrees of crank timing before TDC is the closest the piston will be to the valve before the valve shuts and the intake chases the piston down. The intake usually never catches up to the piston on the down stroke. Unless the clearance isn't enough. under lower rpm you can get away with it. But almost always, when a piston hits the valve, it's the exhaust. All you need is to float the valves, over rev the engine past it spring capabilities, and you have a crash. So, when you are going to check P-V clearance, you need to do it with a check spring on both valves. Have the pushrod, rocker adjusted and locked down, and a dial indicator on the retainer. Now as you cycle the enging to the overlap stroke, about 10 degrees before, stop and push down on the open ex valve. Read the dial indicator to see how much clearance you have. Move it a couple more degrees and check it again. You will see that the clearance is less. Do it all the way to TDC and past. Now set up on the intake the same way. As the piston hits TDC, it's all ready starting to open. Check the clearance. Go past TDC a couple degrees when the valve is chasing the piston. By the time you get to 10 degrees after TDC, you'll gain a bunch of clearance. Or, you can pull the head, put a thin layer of clay(about .150) put the head on with one or two bolts, Install pushrod, rocker, adjust, and cycle the engine. You'll feel it squish the clay. Go slow. Pull the head, cut the clay with a razor blade on top of each valve and measure the thickness. There is your clearance. Hope this helps. Let me know what happens. Dave
 

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You're understanding what I'm saying but not what I'm telling you. Try to picture the piston at TDC at the comperssion stroke. At TDC you will have, probably, a bunch of piston to valve clearance. That's on compression with the valves closed. Now picture the piston coming up to the valve on the exhaust stroke. (overlap, when the ex is closing and the intake starts to open) The valve is wide open and the piston is chasing it up. Just before the piston gets to the top, the other TDC, The piston is gaining ground on the valve to get all the gases out. Approximately between 10 and 7 degrees of crank timing before TDC is the closest the piston will be to the valve before the valve shuts and the intake chases the piston down. The intake usually never catches up to the piston on the down stroke. Unless the clearance isn't enough. under lower rpm you can get away with it. But almost always, when a piston hits the valve, it's the exhaust. All you need is to float the valves, over rev the engine past it spring capabilities, and you have a crash. So, when you are going to check P-V clearance, you need to do it with a check spring on both valves. Have the pushrod, rocker adjusted and locked down, and a dial indicator on the retainer. Now as you cycle the enging to the overlap stroke, about 10 degrees before, stop and push down on the open ex valve. Read the dial indicator to see how much clearance you have. Move it a couple more degrees and check it again. You will see that the clearance is less. Do it all the way to TDC and past. Now set up on the intake the same way. As the piston hits TDC, it's all ready starting to open. Check the clearance. Go past TDC a couple degrees when the valve is chasing the piston. By the time you get to 10 degrees after TDC, you'll gain a bunch of clearance. Or, you can pull the head, put a thin layer of clay(about .150) put the head on with one or two bolts, Install pushrod, rocker, adjust, and cycle the engine. You'll feel it squish the clay. Go slow. Pull the head, cut the clay with a razor blade on top of each valve and measure the thickness. There is your clearance. Hope this helps. Let me know what happens. Dave
Dave, thanks.

I do understand your explanation, spot on…

I feel that I'm learning more on the subject from every post. I truly enjoy this information. But…

The reason for my persistence is this. Assuming you’re running the .488”/.510” cam and you set the piston at TDC and then check the available travel of the valve with the dial indicator AND the available travel of the valve (with a safety factor) is greater than the max lift rating of the cam, why go through the added trouble of checking the clearance at any other rotational position of the crank? The valve will never travel further than that at any rotational position of the crank.

Thanks
 

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You're understanding what I'm saying but not what I'm telling you. Try to picture the piston at TDC at the comperssion stroke. At TDC you will have, probably, a bunch of piston to valve clearance. That's on compression with the valves closed. Now picture the piston coming up to the valve on the exhaust stroke. (overlap, when the ex is closing and the intake starts to open) The valve is wide open and the piston is chasing it up. Just before the piston gets to the top, the other TDC, The piston is gaining ground on the valve to get all the gases out. Approximately between 10 and 7 degrees of crank timing before TDC is the closest the piston will be to the valve before the valve shuts and the intake chases the piston down. The intake usually never catches up to the piston on the down stroke. Unless the clearance isn't enough. under lower rpm you can get away with it. But almost always, when a piston hits the valve, it's the exhaust. All you need is to float the valves, over rev the engine past it spring capabilities, and you have a crash. So, when you are going to check P-V clearance, you need to do it with a check spring on both valves. Have the pushrod, rocker adjusted and locked down, and a dial indicator on the retainer. Now as you cycle the enging to the overlap stroke, about 10 degrees before, stop and push down on the open ex valve. Read the dial indicator to see how much clearance you have. Move it a couple more degrees and check it again. You will see that the clearance is less. Do it all the way to TDC and past. Now set up on the intake the same way. As the piston hits TDC, it's all ready starting to open. Check the clearance. Go past TDC a couple degrees when the valve is chasing the piston. By the time you get to 10 degrees after TDC, you'll gain a bunch of clearance. Or, you can pull the head, put a thin layer of clay(about .150) put the head on with one or two bolts, Install pushrod, rocker, adjust, and cycle the engine. You'll feel it squish the clay. Go slow. Pull the head, cut the clay with a razor blade on top of each valve and measure the thickness. There is your clearance. Hope this helps. Let me know what happens. Dave
Dave, I have to just get this out. This is the most accurate and concise description for the process I have ever read in the site. Awesome explanation. And I fully agree that the exhaust valve clearance is underrated in terms of importance. Much more risk with the EV, and the main reason the exhaust clearance is always greater than the IV. Kudos.

rob
 

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My dad used to work on school buses back in the late '60s and through the '70s. When he first started there were a lot of the buses powered with 348's. Of course there were no automatic transmissions in those days for buses. When drivers would miss a gear when shifting it typically ended with an exhaust valve through a piston. He rebuilt many motors from that cause.
 

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My dad used to work on school buses back in the late '60s and through the '70s. When he first started there were a lot of the buses powered with 348's. Of course there were no automatic transmissions in those days for buses. When drivers would miss a gear when shifting it typically ended with an exhaust valve through a piston. He rebuilt many motors from that cause.
That would happen if they over reved and had valve float due to not enough valve spring pressure.
 

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Dave, thanks.

I do understand your explanation, spot on…

I feel that I'm learning more on the subject from every post. I truly enjoy this information. But…

The reason for my persistence is this. Assuming you’re running the .488”/.510” cam and you set the piston at TDC and then check the available travel of the valve with the dial indicator AND the available travel of the valve (with a safety factor) is greater than the max lift rating of the cam, why go through the added trouble of checking the clearance at any other rotational position of the crank? The valve will never travel further than that at any rotational position of the crank.

Thanks
You are correct in your theory, however in a wedge head combustion chamber you will never have enough room to get full lift at tdc
 

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Thanks Rob. I appreciate it. OK Dan. When the piston is at the TDC compression stroke, you will have a bazillion (kinda like the deficit) clearance to the piston. The reason you check it on overlap of the valves is that at 7-10 degrees before and after TDC with the valves opening and closing, at that point is where the valves will be the closest to the piston. Now, with a cam that is as small, but bigger than stock as you have, you'll probably have no problem with clearance. Now, you put a dome piston and a .560-.600 lift cam in, now you need to check. Also, if you advance or retard the cam on the crank with the various slots in the lower cam gear, you change the mechanics of the cam timing. Not ignition timing, cam timing. So if you advance the cam on the sprocket, it may change the clearance a bunch either way on the P-V area. Once you start checking this stuff, it gets in your blood. If you pay someone to build an engine for you, they should do all this. But most of us do it ourselves because we can't afford to just get it done. Plus, when you do it yourself, you actually know what is in, and what your clearances are when you're talking about your stuff. I've been doing this for quite a few years and as I became more acclimated to do my own stuff, started buying one tool at a time so I didn't have to keep borrowing from anyone. Everything I do is in my garage. Only the maching work gets sent out and done to what I tell the machine shop to do. If it isn't right, it's on me. now I do take advice and I never stop learning. But I enjoy passing on what I've learned so someone else can benefit from it. I know this is long, but I hope you'll get the point. Dave

And Ray, I agree with you about the clearance. But you still need to check on overlap. And most on this site don't know what a wedge block is. They're doing some neat things with the 348/409's.


This is my rag. Getting ready for the Blue Suede Cruise at Norwalk next year.

http://s37.beta.photobucket.com/user/IT427/media/IMG_0798.jpg.html?sort=3&o=29
 

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Thanks Rob. I appreciate it. OK Dan. When the piston is at the TDC compression stroke, you will have a bazillion (kinda like the deficit) clearance to the piston. The reason you check it on overlap of the valves is that at 7-10 degrees before and after TDC with the valves opening and closing, at that point is where the valves will be the closest to the piston. Now, with a cam that is as small, but bigger than stock as you have, you'll probably have no problem with clearance. Now, you put a dome piston and a .560-.600 lift cam in, now you need to check. Also, if you advance or retard the cam on the crank with the various slots in the lower cam gear, you change the mechanics of the cam timing. Not ignition timing, cam timing. So if you advance the cam on the sprocket, it may change the clearance a bunch either way on the P-V area. Once you start checking this stuff, it gets in your blood. If you pay someone to build an engine for you, they should do all this. But most of us do it ourselves because we can't afford to just get it done. Plus, when you do it yourself, you actually know what is in, and what your clearances are when you're talking about your stuff. I've been doing this for quite a few years and as I became more acclimated to do my own stuff, started buying one tool at a time so I didn't have to keep borrowing from anyone. Everything I do is in my garage. Only the maching work gets sent out and done to what I tell the machine shop to do. If it isn't right, it's on me. now I do take advice and I never stop learning. But I enjoy passing on what I've learned so someone else can benefit from it. I know this is long, but I hope you'll get the point. Dave

And Ray, I agree with you about the clearance. But you still need to check on overlap. And most on this site don't know what a wedge block is. They're doing some neat things with the 348/409's.


This is my rag. Getting ready for the Blue Suede Cruise at Norwalk next year.

http://s37.beta.photobucket.com/user/IT427/media/IMG_0798.jpg.html?sort=3&o=29
First Ray, that's what I didn't get, but do get now.

Dave, this is getting clearer every post.

I think I have it now. I went back and re-read your first few posts and I was missing a key item, the rocker and pushrod.

So if I have it correctly, it goes like this.

With the cam in.. Okay I have to stop now and as I just thought of another question that just popped into my little grey cells.

All of your measuring is with a cam installed, but what if you don't have a cam yet and you want to know how large you can go without crashing? For the sake of this post, the motor has heads that will take any amount of lift, so we can eliminate the heads in our conversation.

This is of course all hypothetical for a fictitious new motor build...

Based on what I have read from your posts and if I want to determine how large of a cam I can put in, I would have the heads on and no cam in the motor. (Remember, new motor and it's in build stage so no cam yet installed) So I start with test springs on both valved of say #1 cylinder. No cam so no pushrod or rocker arms. I set up the 1" stroke dial indicator and install the degree wheel on the crank with a pointer setup. Also remember no valve clearance issues with the heads.

So I start by finding TDC with a spark plug probe in #1 spark plug hole. I roll up till it stops and roll back around opposite until it stops, compare the two readings and split the difference, that's TDC. I roll the crank around to the TDC and then I may have to adjust my degree wheel to the new TDC. This will also give me a point of reference to check the harmonic balancer mark to TDC.

Now that I have true TDC, I back wards rotate the crank to around 15° BTDC. Now I forward roll the motor to 10° BTDC and I can push the exhaust valve down and take a reading from the dial indicator. Then I roll a couple more degrees and repeat the process marking down that reading. I'll repeat that every couple degrees until TDC. Now starting at TDC I'll push down the intake valve and mark that reading. I'll roll it another couple degrees, again taking another intake reading. I'll repeat this until about 10° ATDC.

So I've taken all these readings and know what I have available at 10° BTDC for the exhaust and I know what I have from TDC through 10° ATDC for the intake.

I think I just painted myself into a corner with the question quickly going south. As I'm thing through all this, I'm thinking that at 10° before or after I do NOT have max lift achieved by the cam and rocker. So without having a cam to try, how do you find out what cam limits your pistons have?

This would be so much better face to face, but that doesn't help others.

So am I in a catch 22 with my question?

As I'm typing my mind is churning up what if's and I think I'm just getting deeper and deeper and might be going down for the third time, so to speak...

I'm sorry about the rambling on with this subject, but it is fascinating to me and when through, I hope to have a better understanding on the subject. In the past I've never been subjected to this problem. My engine builder did it with my 392 hemi and other times it's just been mostly stock components when this is never a problem.

So if you can humor me with some more of your teachings, I would be very grateful.

Hope you all are enjoying your Thanksgiving with family and friends. We're not leaving for our outing for another hour. I'm sure I will over eat, as that is the goal of this great day.

Dave, thanks again for indulging me with your wisdom.
 

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Hi Dan. Happy Thanksgiving to you and everyone else on this site. OK. You have the crank, block heads, and imaginary cam. And if you knew exactly what the lift, ramp speed, and duration was you probably could theoretically figure out the clearance. But without it, not gonna happen. If you were building an engine from the bare block, you could put the crank in the block with the #1 and #5 main bearings in. All this will come apart to reassemble. After the crank is in, you'd put #1 piston and rod in with the dial indicator set to measure TDC. Install the timing cover that you're going to use, without the seal if you want. Install the balancer you're going to use and the timing indicator. Get TDC on the dial indicator and see if the pointer is at zero. If it isn't, correct it. File it, paint it, whatever. Now you have TDC on you balancer for timing purposes. Remove the balancer, and cover. With the cam and timing set installes, you now put your degree wheel on and get the piston to TDC. With your makeshift pointer, I used a coat hanger gringing the point on, set the wheel to TDC. You can put the head on with no gasket, a couple of bolts and the check springs on the one cylinder. Drop in a set of lifters, solid even if you have a hydraulic cam, for checking purposes. Pushrods, guideplates, and rockers, and adjust them. Get your dial indicator on the retainer. Now you would be ready to check the clearance. Now when you get to overlap you would start at 10 degrees before TDC and push the retainer down. Check the reading. May be .200. Turn crank to 7 degrees. Recheck retainer. now it may be .150. Check every couple degrees, write numbers down. Whatever the smallest number you get will be the clearance. Then you can add the thickness of the head gasket to your number. That's the total. now switch the dial indicator to the intake and do the same procedure on the intake AFTER TDC. That's what you do before you assemble the engine in case you don't have enough clearance. You could have the pistons flycut if there isn't enough. If that happens, then you would go through the procedure again to check your numbers. Then you're ready to assemble. Measure twice, assemble once. It takes time, but then YOU know it's right. You don't want anything crashing around to junk your stuff up. So to answer your question, you need the cam in there. And I'm getting a typing lesson. lol Dave
 
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