Need to chamfer heads to fit RS pistons??

[scottc]

Question for the RS replica engine builders:

I have heard that it is necessary to chamfer the edges of the combustion chamber on the 2.4 heads when fitting Mahle 2.7 RS pistons in order to provide room for the piston dome to fit into the head.

My casual inspection of all my parts seems to indicate that this is not necessary. My original 2.4E pistons have the dome extending all the the way to the outer diameter edge of the piston.
My new RS pistons have a 1/8" wide annular flat surface surrounding the dome on top of the piston.

Since this 1/8" (which is hard to measure since the dome is slight and the blend smooth) corresponds closely to the increase in bore (84mm-90mm is close to 1/4") I have assumed that Mahle put this flat there to allow the larger bore piston fit into the head without interference.

Can anyone shed some light on this?

Thanks in advance,
Scott

[Eagledriver]

Scott,

I know that when my 2.4 was upgraded with 2.7 Mahle's the heads were cut for the new pistons. You could assemble the engine and see how much clearance you have with some clay or solder. Then cut them if they are too close. You may fine you have to cut the piston pockets deeper if you use an "S" cam. You can then get all the cutting done at once. I had to have the pistons cut even though the engine was supposed to be all stock RS parts.

-Andy

[Bill Verburg]

When I looked into that for my 2.4 Andial made no mention of it.

[Peterfrans]

I used 2.2S heads on my engine and had then machined to accept the RS P/Cs. Of course I use S cams.

With this I have a piston to head clearance of 1.05mm with a .25 mm gasket under the barrel.
The piston to valve clearance is 1.76mm

I ended up with a compression ratio of 1:8.95


As my case was also align bored and shaved, and the heads were resurfaced I might have lost some room. It has also to do with tolerances. If you are mixing and matching parts (as I did) you might build up tolerances and end up with to little room.

[scottc]

Thanks for the quick replies fellas.

I am not sure my explanation was clear enough. So I will try again.

Eagledriver and Peterfrans;

I'm aware of the need to have the heads surfaced to mate with the new and larger cylinders. I am aware of the need to check for deck heigh clearence and valve pocket clearence. The chamfer cut I am refering to is a small anglular cut done on the outer diameter of the combustion chamber to allow clearence for the dome. I am not talking about the mating surface where the head seals to the cylinder which is where one would normally adjust the deck height by machining the head. Also aware of altering deck height with varoius thickness base gaskets.

Bill Verberg;
I suspect you got my drift.

I have a attached a crude sketch to try to illustrate my point.
Thanks, Scott

[Bill Verburg]

I have found Andial to be very good on that subject, they did have a 2.9 p/c set which did require a head chamfer for the 93mm pistons.

[TRE Cup]

you will have to chamfer the heads for the larger bore pistons- period!

[scottc]

TRE Cup;

Thanks for the definitive answer.
I guess that the 1/8" annular flat is not enough to allow the dome to fit into the head.

Scott