Author Topic: Quench and different piston heights.  (Read 261 times)

Offline Greasy Harley

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Quench and different piston heights.
« on: December 18, 2020, 05:47:19 PM »
I am working on a 350 that I rebuilt about 10k ago. It never ran correctly, it had detonation problems.
Upon examination I have confirmed that my quench was not nearly tight enough. I used a .041 head gasket and my pistons were all below the deck.
Thing is, they're all slightly different heights below the deck.
The piston heights vary by .006" with the deepest one @.020" and the highest one @.014"
Is that a lot?
So I am leaning toward .026" MLS head gaskets which will yield a quench of .040" to .046"
I understand this is on the outside edge of ideal quench, but the only other feasible option would be a 0.15" steel gasket that sets two pistons @.029" from the head. this is on the dangerous end of quench.

Would it be better with the .015" gasket? I have read multiple places to NEVER go less than .030"

So first: how bad is a .006" variance in piston/deck height?
Second: a little to much clearance is surely better than not enough, right?

FWIW: This is not a high performance engine, it's just an old Chevy 4x4 truck.
I am also installing a slightly bigger cam with a longer intake duration to better match the compression ratio, Pretty sure my mismatched cam was also a contributor to my pinging problem.

I am grateful for any advice,
Russ

Offline Mudsport96

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Re: Quench and different piston heights.
« Reply #1 on: December 20, 2020, 10:23:09 AM »
Do you have the measurements for each cylinder available?

Are the variations side to side, or on the same side of the engine?

I have run as tight as .038 on an engine, but the rotating assembly was well built.

Tolerances can stack up too, a little more bearing wear on one cylinder, that rod is machined a hair off. Hell the crank trhrow could be machined wrong.  A .006 difference in my opinion isn't that bad if you arent building a race engine. Hell if its opposing banks you may even be able to get custom thickness gaskets to get them closer.

Offline old cars

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Re: Quench and different piston heights.
« Reply #2 on: December 20, 2020, 10:30:36 AM »
I have a 1975 350 in my shop now that has an original 48,000 miles on it. This engine had never been apart before. Steel shim original head gaskets. Pistons measure .035 front of block to .028 rear of block on one side and .035 to .030 on other side, from top of deck. These are the original pistons. This is not uncommon. So yes .006" difference front to back of block is common. Don't go to tight.
What compression ratio do you have? Also do you have some cam specs.

Offline tayto

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Re: Quench and different piston heights.
« Reply #3 on: December 20, 2020, 11:04:59 AM »
Harley, I recently built a similar engine this summer. 191 TBI heads (pocket ported and casting flash cleaned up 65cc), used summit flat tops and eagle rods. Had engine decked and ended up running a cometic 0.023" gasket, quench was in the 0.035" to 0.040". Made 325 fwhp @ 5300 rpm and 390 fwtq @ 3000. I suspect the torque was higher but dyno pull was only recorded @ 3000 rpm & up. We ran timing from 34* and ended up down to 26* making the most power. This was all on shell 91 octane (ethonol free here in canada). I got it installed in my truck and put around 750 miles on it before I discovered i had no oil @ top end. We ended up scraping the block and currently getting another one machined. I will make sure to keep quench at 0.040" or under, I think it made a difference. Truck ran strong at the start I definetly need to regear the rear end as the truck lugs a lot when in OD and causes pinging. I think with steel rods it would be ok to run quench @ 0.030",i don't think I would want to run any tighter but have heard of others running as tight as 0.025".

Offline tayto

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Re: Quench and different piston heights.
« Reply #4 on: December 20, 2020, 11:36:33 AM »
Just to add a bit more. Engine loved running with no vac advance can hooked up. I unfortunately didn't have time to dial in but for the most part it seemed to like 10* vac advance. Only time I really heard rattle was when i accelerated with the converter locked. I would lightly put my foot on the break to unlock the converter and then accelerate to get around this. That or simply drop the shift lever to 3rd. Got 16 mpg, with basically no tuning in the rocky mountains. Once engine is back in I plan to go to 4.11s (currently have 3.42s), this should nake the engine not lug @ highway speeds, as this seemed to be where i had the most problems with detonation (low rpm high load). Currently running the crappy stock manifolds and single restrictive exhaust system, planning on going to duals and some "worked on" log manifolds. trying to avoid headers but I have a feeling i will eventually go to them.

Offline old cars

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Re: Quench and different piston heights.
« Reply #5 on: December 20, 2020, 11:52:39 AM »
Standard head gaskets typically have a compressed thickness around .040-inches. That works well for stock and most street performance builds if the piston is even with or slightly below the deck at TDC. If the rotating assembly height is taller than the deck height, the piston will protrude past the deck surface and may require a thicker gasket to give adequate clearance.
You should never be less than .035-inches. The quench distance should be increased if the piston has enough rock at TDC to protrude above the deck. High RPM and aluminum connecting rod applications will require additional clearance as well due to their expansion at operating temperatures and high speeds.

Offline Cliff Ruggles

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Re: Quench and different piston heights.
« Reply #6 on: December 21, 2020, 03:06:46 AM »
Ideal is .035", some builders go tighter with steel rods.

Never over .040" for any reason here, and I'm nicely rewarded with engines that use higher compression, manage pump fuel just fine, make great power, take LESS timing and fuel to be happy, and they run much cooler than building the same engine with a lot of quench in it.

If you are using cast pistons with tight bore tolerances I know builders who consider .025" minimum for steel rods......
« Last Edit: December 22, 2020, 05:00:36 AM by Cliff Ruggles »