Piston To Valve Clearance Problem - Help - Chevy Nova Forum
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Old 16th-February-2008, 09:32 AM   #1
leejoy
 
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Piston To Valve Clearance Problem - Help

Well I just threw the new BBC together - new cam (roller) and stuff. I put a head gasket on, installed the head, set lash on the valves, installed "temporary" valve checking springs, hooked up a dial indicator, and checked piston to valve clearance. It looks like I have a problem. I've only got about .030" of clearance on the intake. The exhaust is fine.

I was told to have at least .080" clearance. I'm not even close.

I have SpeedPro L2399F60 pistons. Aluminum, domed.

Standard 4" non-stroker crank.

The cam is a custom solid steel roller from Comp Cams. It's the 11-771-8 on a solid billet. (-9 series core).

Now what do I do? I already messed up by installing the oil pan and all gaskets - took me forever to do that. I thought for sure I would have enough clearance. Now I may have to do the whole darn thing over again...

Do I have to get the pistons cut or what? Is that too much material to remove from the pistons? That would mean removing and reinstalling every piston - that's a ton of work for me. Do I have to change the cam?

what should I do? - Help..........
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Old 16th-February-2008, 10:05 AM   #2
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First step is to check the cam alignment. Make sure the timing chain is installed correctly (dots lined up right, keyways on the 0* ones if it's a multi-keyway set)

Then you can degree the cam, and make sure that it's opening and closing at the manufacturer's specs.

http://www.compcams.com/Technical/In.../Files/145.pdf
http://www.carcraft.com/howto/116_07...aft/index.html

After that, you may need to adjust the timing chain to make the cam line up to specs.

If that *still* doesn't solve the problem, you have a few options:

- advance/retard the cam to increase clearance
- modify the pistons for additional clearance (and rebalance)
- use lower ratio rocker arms to reduce the valve lift
- use a different cam
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Old 16th-February-2008, 10:26 AM   #3
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Quote:
Originally Posted by leejoy View Post
Well I just threw the new BBC together - new cam (roller) and stuff. I put a head gasket on, installed the head, set lash on the valves, installed "temporary" valve checking springs, hooked up a dial indicator, and checked piston to valve clearance. It looks like I have a problem. I've only got about .030" of clearance on the intake. The exhaust is fine.

I was told to have at least .080" clearance. I'm not even close.

I have SpeedPro L2399F60 pistons. Aluminum, domed.

Standard 4" non-stroker crank.

The cam is a custom solid steel roller from Comp Cams. It's the 11-771-8 on a solid billet. (-9 series core).

Now what do I do? I already messed up by installing the oil pan and all gaskets - took me forever to do that. I thought for sure I would have enough clearance. Now I may have to do the whole darn thing over again...

Do I have to get the pistons cut or what? Is that too much material to remove from the pistons? That would mean removing and reinstalling every piston - that's a ton of work for me. Do I have to change the cam?

what should I do? - Help..........

I just ran into this last weekend, also. Excpect with a 350 small block. I tried moving the cam timing all over the place with no luck. Its seems the TRW now SpeedPro pistons (L2256F), I have are not releaved for a 2.02 sized valve. I have plenty of clearance up/down. My issues is the o.d. of the valve and the radius of the relief is less than .010". This is the very reason I use clay to check p/v clearance. So I ordered/rented a piston notching tool from Isky Cams. You buy the tool full price, use it and return it and they refund the price minus 40.00. The stem of cutter goes through the valve guide and its powered with a drill. Only draw back is the intake valves must be removed from the head. I have several heads laying around unassembled so its not going to be that big of an issue. Maybe someone local will have a head unassembled that you could use. I'm going to try and use the tools this weekend and if I ever figure out how to post pics I'll do that also.

Last edited by Granny 67; 16th-February-2008 at 10:48 AM.
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Old 16th-February-2008, 12:05 PM   #4
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What heads are you using?


The valve depth in the head is important, as is how much the deck of the head and block have been milled.
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Old 16th-February-2008, 01:40 PM   #5
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I checked mine with the chain 1 tooth off on acident and was realy close also... Reset the timing marks on the cogs and it was good.


As patman stated just something to check out.
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Old 16th-February-2008, 01:54 PM   #6
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The 2399 piston has very shallow reliefs.

I ran into problems with a 544 lift cam using the 2399 piston....


a small fly cut would be great and easy....and not off set the balance that much..... I think its Isky has a tool for flycutting the piston in the block using the head as a guide.
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Old 16th-February-2008, 01:55 PM   #7
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This is why you always do a mock up assembly first. I'd do the degree wheel and I'd also use the clay to see what's hitting and where.
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Old 16th-February-2008, 02:52 PM   #8
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Quote:
Originally Posted by stock z/28 View Post
What heads are you using?


The valve depth in the head is important, as is how much the deck of the head and block have been milled.

don't know how much has been milled. using stock GM iron oval port heads from 1968-1970. (fully reconditioned & ported)
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Old 16th-February-2008, 02:54 PM   #9
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Quote:
Originally Posted by Paul Wright View Post
This is why you always do a mock up assembly first. I'd do the degree wheel and I'd also use the clay to see what's hitting and where.
Hey Paul...........are you overseas now? I remember you saying something about Iraq? (not yet, I'll know more on the 19th)~Paul W.

anyway, I will do as you say with the clay. I was going to do that anyway, and I will likely have to degree the cam. maybe it's advanced by 3-5 deg. that would do it.

Lee

Last edited by Paul Wright; 16th-February-2008 at 05:51 PM.
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Old 16th-February-2008, 02:59 PM   #10
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You can taker some Play-Do (yea steel it from the kids) past some on the piston. Rotate the assembly.

Then see where the piston and valve is close.

Could be a very small area that you can correct yourself with a Dremel.

Any true performance machine shop can fly cut the pistons with ease.

Good job on the check, lot of people never ever check!

Al
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Old 16th-February-2008, 05:54 PM   #11
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Quote:
Originally Posted by veno View Post
The 2399 piston has very shallow reliefs.

I ran into problems with a 544 lift cam using the 2399 piston....


a small fly cut would be great and easy....and not off set the balance that much..... I think its Isky has a tool for flycutting the piston in the block using the head as a guide.
I guess my paragraph was to long!!!
GEESSHHH! This is a tough crowd!
I say use clay, so does Mr. Wright.
I say try Isky cutter if cam timining doesnt change issue, so does Mr.Veno.
Big AL always has the answer.
I say, I give up!

Last edited by Granny 67; 16th-February-2008 at 05:56 PM.
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Old 16th-February-2008, 09:57 PM   #12
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guys - check out these pics

I also learned that my timing gear on the crank is set for 4 deg advanced - that makes my problem worse right? How much improvement would I get (approximately) if I put that at 0 deg advanced (straight up)?

you can see in that last pic with the clay that there is little or no clearance. I poked a nail into the clay in all areas and found only about .030" of thickness everywhere - very tight.

Lee




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Old 16th-February-2008, 10:38 PM   #13
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check your cam card. It may be ground 4 degrees advanced. With the sprocket advanced 4, you are really 8 degrees advanced.
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Quote:
Originally Posted by Paul Wright
Building a small, high rpm engine
with the perfect bore, stroke and rod ratio is very impressive...
like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar.

Cubic inches is like Indiana Jones with a cheap pistol.
.
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Old 17th-February-2008, 12:53 AM   #14
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By any chance did they install a larger diameter intake valve?


Just a guess but I have seen many instances where the new seat was not machined deep enough when a larger diameter valve was installed.

Big block valves are kinda deep in the chamber, and its kinda tempting to have the seats be a bit high in the head, and this will cause valve clearance issues,

I look for an installed height of around 1.880 with a stock retainer and standard valve length.
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Old 17th-February-2008, 07:35 AM   #15
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If someone installed a bigger valve they could also have the valve to high in the chamber. Like Jeff said is the spring height right? Check your valve train geometry and pushrod length too. Best of luck. RM
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