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Old 02-21-2016, 10:18 AM   #1
Shoebox
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Default Combustion chamber reshaping

Ok, I realize there are variables so a definite Yes or No answer may not be possible, looking for opinions..........

39 Merc bored to 3 5/16, 4 inch crank, probably a stock Merc 8CM cam,or something close. With the original 81T heads milled to that desirable .050" clearance, will the combustion chamber need some reshaping????

Thanks-
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Old 02-21-2016, 11:13 AM   #2
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Default Re: Combustion chamber reshaping

First that comes to mind is with a 8cm cam you have to machine the nose of it to make it fit with the dizzy.
And if so use the camgears from the old setup and not from the merc crank.
Saying you can run or not run after milling a head is a real gamble...better clay it and be safe.
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Old 02-21-2016, 01:02 PM   #3
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Default Re: Combustion chamber reshaping

If you're milling it to have .050 clearance, than the only reason it would need to be reworked is if the head dome profile/radius differs from the piston crown radius. I've noticed that some of the cast pistons tend to have a different shape than ones like Ross (forged). Also, it depends on the true crankshaft stroke deck height, etc..

As 'flatheadmurre' noted, the only true way to know is to mock-up the block (with the actual crank, rods and pistons you're using - which means the block has been bored) - then see where you are at with the heads. You'll probably find that the clay will have an inconsistent thickness - due to the dome/piston shapes not being the same. I'd say you can get as close as about .040 on a street engine and be okay (it is what I aim for).

I recently created a 3D CAD model so I could have my new Navarro heads CNC milled to make the chambers exactly match my pistons (Ross, custom ones). I found that the Navarro domes really didn't match the Ross domes -- more 'peaked/pointed' than a true consistent radius.

If you have access to a CNC mill and somebody who can redo the chambers, might not be a bad idea. My guy just used a 1/4" ball mill and I smoothed the final machine work out with a die grinder.
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Old 02-21-2016, 01:37 PM   #4
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Default Re: Combustion chamber reshaping

Flatheadmurre, I failed to mention regarding the cam, I expect to have the original cam reground to whatever profile deemed best, by someone such as "Pete".

I would of course mock up and clay or tinfoil ball, just thought I'd see the likelyhood of reshaping, cause I do not have access to a CNC mill and have never done much more than a little matching of ports to gaskets.

Thanks for any and all input.
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Old 02-21-2016, 02:29 PM   #5
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Default Re: Combustion chamber reshaping

I take .040" off the head, this is usually too much, and like B&S. look for 45-5- thou clearance. another trick is to flaten out the exit of the transfer area to 45 degs or so. Yes, most of us don't have CNC machines. I often wondered how much power gains are made this way. However closing up the piston clearance does improve throttle responsr amd econmy.
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Old 02-21-2016, 02:33 PM   #6
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Default Re: Combustion chamber reshaping

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I will tell you my experience and you can judge for yourself. On my C59A engine I have iron C7RA-A heads. When I milled for 50thou piston to head clearance, I found that the edge of the chamber moved in toward the centre of the piston by about 1/4", and the edge actually kissed the piston. This necessitated reshaping the chamber to match the dome, which I did by hand by marking the gasket outline on the head with a sharpie.
I imagine your experience will be the same.
A very worthwhile exercise, as it made the engine very responsive. thanks JWL.
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Old 02-21-2016, 03:27 PM   #7
Shoebox
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Default Re: Combustion chamber reshaping

Hey Bassman,
Thanks for the response,.... what all did you use to do the reshaping, die grinder, flap wheel, stones ??

As a side note, I was going to ask you regarding your screen name, are you a bass fisherman or do you play bass guitar (as I do) checking out your profile answered that question.

Thanks!
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Old 02-21-2016, 03:51 PM   #8
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Default Re: Combustion chamber reshaping

I'll post my recent experience with surfacing and reshaping. I got good advice here to do the reshaping before surfacing because of "slips" of the grinder. I used a shape duplicator to get the shape of my new piston domes and it revealed that the chamber in the heads were much more pointed than my new Egg pistons. After claying the heads with a used gasket I marked the gasket outline on the heads. The area that needed clearance was toward the outside edge because of the peaked chamber. I used a carbide cutter in an air die grinder taking vey small bites per pass. When I got to what I thought was very close I used a worn 60 grit flap wheel on a 4" grinder to smooth the chambers. Be careful with the grinder and keep it at a steep angle to avoid cutting into the gasket area. Then I had the heads cut .050". I put the heads on without gaskets to check for interference. I put black shoe polish on the pistons to mark the head where there was contact. Once I had clearance again polished with the flapwheel being VERY careful to stay off the gasket surface. Don't expect to go out there in an afternoon and whip this job out. I have many hours of putting the heads on and off checking clearances. It makes the cost of new heads seem attractive.
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Old 02-21-2016, 05:47 PM   #9
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Default Re: Combustion chamber reshaping

Quote:
Originally Posted by Shoebox View Post
Hey Bassman,
Thanks for the response,.... what all did you use to do the reshaping, die grinder, flap wheel, stones ??

As a side note, I was going to ask you regarding your screen name, are you a bass fisherman or do you play bass guitar (as I do) checking out your profile answered that question.

Thanks!
I'm primarily an upright bass player these days. We don't have bass fish here in New Zealand.
I used a grinder and a flap wheel. Obviously, it requires a delicate touch, and slow and easy is the mantra.
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