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PostPosted: Wed Jan 20, 2021 6:13 pm 
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You want to aim for .040" total piston to head clearance or "quench distance". I like to see the pistons at zero deck or even at .005-.010" out of the hole, then I use the correct head gasket to get the piston to head clearance I want.

If you're getting new pistons, then all you have to worry about is that the deck surfaces are straight and parallel to the crankshaft centerline.

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PostPosted: Thu Jan 21, 2021 1:10 pm 
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Thank you. The quench distance depends on the lift of the cam used as well. Is getting the starting point you mentioned best after the cam selection? Or it doesn't matter as it can be adjusted with the head gasket later? I also read that for high pressure turbo applications head gasket is a liability so it's best with no head gasket but o-ring the seal so the pressure will not blow the gasket out. I know this kind of machining is out of my league but I'm just curious...


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PostPosted: Fri Jan 22, 2021 6:20 am 
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The quench distance is separate from everything else and does not depend on the camshaft. Piston to valve clearance depends on the camshaft, but quench does not. A tight quench is a blessing in several ways. It helps combustion, can add horsepower, and can even decrease chances of detonation.

The camshaft should be the last part chosen as correctly choosing a cam depends on the compression ratio. Machining the block, getting pistons, and machining the heads affect the compression ratio.

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PostPosted: Fri Jan 22, 2021 12:49 pm 
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OK thank you for explaining that. I thought for some reason it was valve to piston clearance. Just looked up quench distance. What can I say, a newbie ;-)


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PostPosted: Mon Jan 25, 2021 6:38 pm 
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Finally The block, heads, pistons are at the machine shop. He says either .020" or .030 over will clean up. He also wants to get on it asap (grumpy old guy I better oblige ;-) I need the bearings (1010 both main and conrod) and decision on the pistons. Domed pistons will be best for 10.5+ compression ratio. Can I please get some recommendations? Original connecting rods.


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PostPosted: Mon Feb 01, 2021 7:58 pm 
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Happy Monday,
today I finally got the word from the machine shop. .020" cleaned up the ridge nicely. Now time for parts ordering
Are .020" over pistons special order? I was recommended Auto tec 1000513 but these are .030" over. The machinist also said that I can use the current valves as they are not bad and he can grind them. I'm not sure if they are the bad original kind (he said they have 3 grooves) so I have to decide if I need new valves as well. He's already put in new guides. I'm still going to deck the heads for more compression ratio but first I have to measure the cc on the combustion chambers so I know how much to deck them. but I can have him install new cam bearings and freeze plugs after he finishes cleaning the water passages and hot tanks the block.


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PostPosted: Tue Feb 02, 2021 5:41 am 
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jacek wrote:
Happy Monday,
today I finally got the word from the machine shop. .020" cleaned up the ridge nicely. Now time for parts ordering
Are .020" over pistons special order? I was recommended Auto tec 1000513 but these are .030" over. The machinist also said that I can use the current valves as they are not bad and he can grind them. I'm not sure if they are the bad original kind (he said they have 3 grooves) so I have to decide if I need new valves as well. He's already put in new guides. I'm still going to deck the heads for more compression ratio but first I have to measure the cc on the combustion chambers so I know how much to deck them. but I can have him install new cam bearings and freeze plugs after he finishes cleaning the water passages and hot tanks the block.


Change the valves. They will break.

I can get pistons in any bore size, in .001" increments. I'm a Racetec dealer. You will need to have all your machine work done before ordering pistons though, heads cc'd, deck height measured, etc.

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PostPosted: Wed Feb 03, 2021 12:00 pm 
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OK Thank you. What about using 4bbl engine pistons in 2bbl engine? I inquired about that and this is the response I got:
" These pistons are the 4-barrel pistons that can be used in 2-barrel engines as a performance upgrade to give a little more compression."
should I worry about detonation with these?
Has this been tried?


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PostPosted: Wed Feb 03, 2021 12:40 pm 
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As long as you are not trying to put an open chambered domed piston in a closed chamber head, it will work. But with the 2v open chamber, as soon as you start raising the compression you are increasing the chance of detonation. It can be controlled up to a point as long as the compression doesnt get too high. But honestly, with what you are trying to do, I would put a flat top in it and mill about .050 off the heads. That will drop you down to about 68 cc's. A flat top at zero deck, and that's probably about all the compression those chambers are gonna tolerate. I ran domed pistons in an open chamber head in a street motor 35 years ago, but pump gas wasnt the garbage it is today.


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PostPosted: Thu Feb 04, 2021 2:17 pm 
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The way to get some turbulence in open chamber heads is to cut them down some - 70~74cc, less if possible - then run flat top pistons at zero deck. You'll not have a ton of compression but it'll work way better and be less prone to detonation.

Pick your valves before they put in the guides - guides need to be compatible with the valves. IIRC, not recommended to run stainless against an iron valve guide. Brent can better address that. Stainless valves typically run with phospor-bronze guides.

KV is right about pump fuel. Several years ago I could run my 10.5:1 iron head 4V on 91 + 1 gallon of VP110. Now to keep pepper off the plugs, it has to be no less than a 50/50 mix. That's $6/gallon for fuel, at current prices in 5 gallon quantities.

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