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Discussion Starter · #21 ·
Actually I assumed the compression increase from the 78 L-48 (8.5:1) to the L-82 (9.1:1) was due to the different piston design. I didn't realize there was a thinner head gasket. I did some research and I've read the L-82 originally came with .018 gaskets... Eric may be right about .015 also.
But the .026 gaskets interest me.
Maybe this is a dumb question but should I have a detonation problem with the Summit cast iron cylinder heads and .026 gaskets or should I stick with the .039 gaskets bringing it to about 9.1:1??
 

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Discussion Starter · #22 ·
I also read that back in the 70's Chevy did not sell .018s for replacements for L-82s but rather listed thicker .026s ... perhaps because they figured that any head work would also include shaving the heads for straightness. So they sold a thicker gasket to compensate for shaved heads to keep compression ratio around 9.1:1...
 

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You're actually correct on the original shim gasket being .018" from Chevy originally.....took me a minute to remember back to using them and the uncoated replacement you could buy was .018".
The modern Fel-Pro coated version today is .015".

Personally, I'd rather have the tigher quench, it'll probably be less detonation sensitive even with the higher compression ratio that results....at least, that's been my experience. The qunch effect is more important.
 

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Discussion Starter · #24 ·
You're actually correct on the original shim gasket being .018" from Chevy originally.....took me a minute to remember back to using them and the uncoated replacement you could buy was .018".
The modern Fel-Pro coated version today is .015".

Personally, I'd rather have the tigher quench, it'll probably be less detonation sensitive even with the higher compression ratio that results....at least, that's been my experience. The qunch effect is more important.
Eric-- Thanks, you have been very helpful. Just curious, why do you think it would be less detonation sensitive even with the higher CR? Because of the chamber design? And... I guess detonation can always be addressed at least to some degree with less advance--right?
 

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Tighter quench helps to eliminate the dead spots in the chamber....those dead spots in the burn are where detonation gets it's start.
Tons of info out there on why you shouldn't use thick head gaskets to lower compression because defeating the quench effect actually makes the engine more detonation prone, even though you lowered the compression ratio.

I know, it seems counter-intuitive.
 

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Discussion Starter · #26 ·
Tighter quench helps to eliminate the dead spots in the chamber....those dead spots in the burn are where detonation gets it's start.
Tons of info out there on why you shouldn't use thick head gaskets to lower compression because defeating the quench effect actually makes the engine more detonation prone, even though you lowered the compression ratio.

I know, it seems counter-intuitive.
Okay, thanks. I think I will go with the .026 gaskets... Thanks for your help!
 

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Discussion Starter · #27 ·
There is a .026" thick composite gasket available if you want to tighten up your quench a bit more than the .039" will have.
Mahle/Clevite Original/Victor-Rienz #5746, it's a stainless core graphite faced gasket, works really well even with aluminum heads. .026" x 4.100" bore
Mahle Original, Head Gasket, Chev SB, 4.100" Bore, .026" Compressed, Each - Competition Products

Eric--My engine is stock 4" bore 350. I notice 4.1" bore on these gaskets. Will they work with 4" bore? Also, I have seen others at 4.125" Do all these work with 4" stock bore?
 

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Eric--My engine is stock 4" bore 350. I notice 4.1" bore on these gaskets. Will they work with 4" bore? Also, I have seen others at 4.125" Do all these work with 4" stock bore?
Yes, gasket bore is always actually bigger than cylinder bore....both to allow for the lead-in chamfer at the top of the bore and for overbore sizes.
Smallest gasket bore I've ever seen for a 4.00" bore SBC were 4.040" offered....but I don't remember who, maybe SCE or Cometic. Pretty much a racer thing to maximize compression ratio.

The standard Fel-Pro dimension for their common composite SBC gaskets for 4.0" bore SBC is 4.166", if that tells you anything....basically made to fit nearly everything possible with fewer part numbers needed to stock.
 

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Discussion Starter · #29 ·
Thanks Eric for clearing that up! BTW I did get the 5746 Mahle .026 gaskets. Heads supposed to be delivered 10/13. QUESTION: I understand the thread holes for the temp sensor are 3/8" NPT. Current sensor is 1/2". Will any 3/8" GM sensor work with my gauge? My temp gauge is out of a 73 Grand Prix (pretty old stuff) ... I put a couple examples below. I would rather not drill and re-tap. Any thoughts?


 

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There is a difference in sensors between those used to trigger an idiot light(on/off), and those used to give a variable readout like a gauge or for fuel injection computer. The two types don't interchange functionally.
Type you need will have to match the instrument cluster in your car....I'm assuming idiot light, and not gauge package dash??

You could also just drill the hole in the head bigger and tap to 1/2" NPT.
 

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Discussion Starter · #31 ·
There is a difference in sensors between those used to trigger an idiot light(on/off), and those used to give a variable readout like a gauge or for fuel injection computer. The two types don't interchange functionally.
Type you need will have to match the instrument cluster in your car....I'm assuming idiot light, and not gauge package dash??

You could also just drill the hole in the head bigger and tap to 1/2" NPT.
Yes I would need a sensor for a variable readout. It's an old gauge from 73... Actually the 4 center gauges in picture below are from a 73 Grand Prix. When I installed them to replace idiot lights, the replacement for 73 Pontiac GP and 78 Chevy sb were same part# and 1/2" NPT. I'm wondering how to find the right one that would work with my gauge and be 3/8" NPT. It seems all the sensors listed for the old engines are 1/2".


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Discussion Starter · #33 ·
You might be able to use a 3/8" male npt to 1/2" female npt adapter (link shown below) to fit your current temp sending unit into the new head.
Good idea! I will need to check to see about space...Currently using passenger head hole--back near #6... Header might be in the way... Also have a possible option at front of intake...
 

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Good idea! I will need to check to see about space...Currently using passenger head hole--back near #6... Header might be in the way... Also have a possible option at front of intake...
i was thinking the same thing for you gauge fix. i bet you could find that way cheaper at the local hardware or plumbing supply.
 

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Discussion Starter · #35 ·
i was thinking the same thing for you gauge fix. i bet you could find that way cheaper at the local hardware or plumbing supply.
Yes that's a good idea too! Checked it out today and there is room to use the adapter and still put the sender on the passenger head. The intake hole currently plugged is right up against radiator hose so that won't work... Thanks guys!
 

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Discussion Starter · #36 ·
Got my heads last night...Started today... Prepped heads, installed rocker studs, blew out water passages for debris, tested intake valves for leaks (read that it is a good idea). No leaks! Cleaned and painted new heads and began to disassemble engine... Should get back on it Saturday...

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Discussion Starter · #37 ·
Removed intake ,headers and plugs tonight. I would like opinions on my plugs... #8 definitely fouled. Recently I rebuilt my carb and changed to larger jets...resulting in way too rich with black smoke out of both pipes. So I went back to the smaller jets. I wonder if #8 is fouled with gas?? There was no blue smoke. Also #1 & #2 seem a little whiter than the rest... ANY THOUGHTS??

RIGHT TO LEFT #1, #3, #5, #7

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RIGHT TO LEFT #8, #6, # 4, #2

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Discussion Starter · #38 ·
I did a compression check on #8 and is at 180 psi where it has been historically... So I can only assume #8 plug was fouled when I had the wrong jet in the carb and it was running way too rich a few weeks ago... Heads coming off today.
 

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Discussion Starter · #39 ·
BIG SURPRISE... Heads are all unbolted and ready to pull. Then I examined the pushrods and below is what I found!!! several of the pushrods were chewed up on the rocker end resulting in different lengths... Inside of several of the rocker cups also chewed up a little. Lifter ends are fine. Not sure how to explain this... except it obviously had to do with valves not staying in adjustment. I had installed Summit stamped steel rockers and hardened pushrods a few years ago... as I remembered the cheapest available. But I figured Summit stuff is pretty good... ANY IDEAS OR SUGGESTIONS???

Rod on right rear is normal
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