Calculate actual quench height
 

Reassembly on 8BA after Refresh & installing Ross domed Pistons w aluminum connecting rods after issues with Pistons from previous rebuilder. I’ve left one head off for calculating Quench Height / piston-head clearance. I was concerned as piston dome is projecting into combustion chamber area .170 more than the flat top piston. But actually a lot more space than I thought it would be? These are the numbers:
A-Piston dome height above block head mate surface=.170
B-Head surface to combust space piston C/L =.255
C-Compressed gasket =.060
Quench height B+C-A=.145?
Is that about where it S/B? Being my only ever Flathead it seems like too much space. I didn’t check previous setup but it had to be even more with flat top Pistons by the elimination of the dome. I think the calculation is right but sounds like a lot? Comments please!

I only question it as on typical GM OHV engines I’ve rebuilt I think I recall it’s been between .055-.065 with aluminum connecting rods & even less with steel? Does the Flathead compression which for stock I think is about 7:1 increase that quench height that much more than a OHV engine with 9:1 or 10:1 compression?

Re: Calculate actual quench height
 

Just recently done a 69a block with Ross dome top pistons and Edelbrock heads got 0.060 to 0.065 thou clearance piston to head which I left at that . The Edelbrock combustion chamber is almost a mirror image of Ross dome top piston profie.
Phil NZ

Re: Calculate actual quench height
 

Anthony be sure to check both side of engine. Do not assume they are alike. You have aluminum rods in your flathead? For what reason may I ask.

Re: Calculate actual quench height
 

Connecting Rods supplied by original builder along with rings & new Pistons. Not sure I agreed but he said the original piston used in rebuild either had had Mfgr issues that came out as a recall notice or detonation issues. He supplied the new Ross Domed Pistons, rings & new connecting rods at his cost. Even though engine had only 2000 miles when prob came up several years had gone by & it was out of warranty but as said he covered the new parts cost.

Re: Calculate actual quench height
 

I think you can take .020" off the edelbrock heads. However, If in doube torque the heads down on the gasket youare using and then take a larhe dianeter piece of rosin core solder and ruit through the spark plug ia several placed to check head clearance.. The Alum rods will have a large effect on balance.
Gramps

Re: Calculate actual quench height
 

Why not check the piston to head clearance on the engine. You can use clay or aluminum foil balls. Place the balls in several areas around the edge and the center of the piston. You can check it with or without the head gasket in place. I would check it without. The piston you are checking needs to be at TDC. Place the balls on the piston and install the head and tighten it down. Remove the head and then carefully check the height of the balls. If you are checking the clearance without the head gasket, add the gasket thickness to the ball measurements (normally around .050). As ideally you want around .050 clearance total, the ball test without the gasket should be near zero.

Re: Calculate actual quench height
 

Its important to check the outer edges of the piston to the dome of the head. The Ross piston top radius is usually not the same as the dome of the heads. Not sure how much of the dome you have checked so I will explain how I do it just in case it helps you.
I always use clay material placed on top of each piston. Here's how I do it I place the clay in four locations one dead center of the dome then along this center line I put clay on each side of the dome as far out towards the outer edge of the dome as possible. The last area I clay is at the bottom center of the piston once again as close to the outer edge of the piston dome as possible. Its important to do all eight pistons to the domes of the heads. Now you can rotating the engine enough so all eight pistons are at TDC during rotating the clay has now been compressed to the clearance dimensions. Once the heads are removed simply use a razor blade to slice the now compressed clay along its center line remove one side of the clay exposing the edge of the clearance. Now use a Vernier caliper placed on the edge to measure the true clearance at all for locations of the piston tops. Also I use the clay for valve clearances measurement as well
If your block was not decked off the crank center line you will find from side to side different readings. If the head gaskets your using are copper composition type you can reuse the gaskets after your checking is complete I always check clearances using the head that will end up on the engine. Just be sure to use the same gasket on the same side you used for the clearance checking to keep things equal when assembling the engine. Using the aluminum rods on the street can be done without any trouble {I have thousands of miles on mine} however you will find the engine will rev up nicely but the down side is this may effect your fuel economy.
Ronnieroadster

Re: Calculate actual quench height
 

I think I came up with the head clearance with hard measurements, brought #1 cylinder turning crank to the mark on pulley verified TDC, put a magnetic indicator base with indicator on the head deck of block. I checked the top of piston to block deck=.170 did same on head to get depth @ center of where piston dome will enter=.255, I measured a used head gasket as compressed dimension @ .055-.065 used .060. I’m not concerned with interference clearance issue, actually the opposite. I came up with .145 as piston to head clearance which seems to much to me. As Phil Gillespie said he recently did same w domed Pistons & ended up about .060 which is what I was expecting? Thinking compression will be pretty weak @ .145?

Re: Calculate actual quench height
 

Do as Ronnie says. However I use tin foil balls. With your method you are assuming the radius of the head and the radius of the piston are the same. The outer edges might show something completely different.

Re: Calculate actual quench height
 

I actually am satisfied I have the true clearance +or- .005 and am not concerned about interference. My question is, is .145” which it is too much & will it give me weak compression as it’s more than twice what it should be? I was expecting around .060”

Re: Calculate actual quench height
 

Your vale reliefs in head look much more shallow than a factory set of heads . Your fear of low compression may be made up by those shallow valve reliefs . Im just looking at a picture but had the factory heads on the bench today . The only way to tell is to cc them and compare to factory .if it ran with the flat tops it should run better with these pistons. Did you ever take a compression reading with the flat top pistons and these heads ?

Re: Calculate actual quench height
 

Your doing things the hard way and making this more difficult (an less accurate) than it needs to be. Folks with years of experience are attempting to help you out here!!

Re: Calculate actual quench height
 

Before having the issued that caused the Pistons to have to b replaced it seemed to run OK. Hard for me to say ‘yeah great’ cause I never drove another Flathead. Thx for all the ideas & help. I will verify with clay or use The idea Gramps offered w solder wire.
If that shows approx .145” too I’m not gonna worry about it! As Ggmac says if it ran with the flat tops it’ll run faster w the domed.

Re: Calculate actual quench height
 

>>>Before having the issue[ ] that caused the Pistons to have to b replaced it seemed to run OK.>>>


Then it should run even more OK with domes with no apparent head-slap danger. But might shake a bit more with the Al rods. If it was mine, I'd probably run it as-is to see how it goes. Then go from there. Jack E/NJ

Re: Calculate actual quench height
 

Anthony,
I'm not sure my method is technically optimal but I mill the heads I intend to use ,usually EAB's, so that when I rotate the motor the pistons lift the heads ( held
in place with 3 studs and hand pressure ) ever so slightly, no gaskets in place . I apply prussian blue to the piston crowns and the valves. The heads have been bead blasted so any contact will show up on the head in the form of a , you guessed it, a blue mark. I relieve the area of contact and go at it again and again ( ave 5-6 hours/
head ). In all cases one bank makes initial contact at 3 o'clock, the other bank at 9 o'clock. I use BEST head gaskets that when torqued under the head yield .045 to
.048 thickness. That .045 to .048 is my clearance. I limit rev's to 5000 and with a
bit of distributor dialing in 87 oct corn gas works well. I don't see how you can get
closer than that.
OMHO,
Charlie ny

Re: Calculate actual quench height
 

As said this is more what you have to watch. "Its important to check the outer edges of the piston to the dome of the head" and yes the pistons dont match the dome shape theres far more room in the center. Check the deck height and see if the rim of the piston is above or below. Ive just taken off .055 of my cast heads for a volume of 74.5cc and there was still way more to come off if I need too [but my machinist hates me when I walk in].. Do as Brian taught me remove the gasket and check it from there you can go to zero and so long as you have installed gasket thickness you're fine. HOWEVER, is yours supercharged?

Re: Calculate actual quench height
 

Talkwrench>No Super Charger but with the potential of .145 head clearance could be a good candidate;).
Charlie ny>Makes sense & sounds like a very good foolproof method!
Jack E/NJ>Agreed!

Thanks again to you all for the great advise / Ideas!!
I’ll be firing it up by the end of the week, 1st time in over a year!

Re: Calculate actual quench height
 

The reason for closing up the piston to head clearance, has nothing to do with raising the compression, It's to create a high degree of turbulence in the combustion chamber just before the spark fires. This turbulence causes a more complete burn of all the AF in the cylinder. This we get more power and better economy.
Gramps

Re: Calculate actual quench height
 

Just wanted to report on results of ‘aluminum foil balls’ clearance check. Turned out .140 at center & .095 at edges. As the aluminum foil test shows more than enough clearance ( Probably too much), I’m still gonna go with it as stated earlier & agree with several poster’s, “If it ran OK before the Domes then it should run even more OK with them. Run it as-is & go from their”
Will report after I get it running.

Re: Calculate actual quench height
 

Youre correct you do have plenty of clearance at .140 & .095 at edges.
My clearances with recent check were 0.058 to 0.065 so I opted to go with that as ideal being 0.050. With your engine I would perhaps reduce the clearance but its your engine.
Also depands upon what head gaskets in use, copper or composite as crush on these can differ a lttle also.
Phil NZ