head machining leaves wide cylinder face

OhioRick · 03-30-2014, 10:01 AM

I had my 3.0 heads remachined and with the hemispherical shape of the combustion chamber, the surface that would sometimes contact a head gasket (not on mine) is obviously wider. The cylinder bore will also be 3mm larger (98mm with JE pistons) which leaves an even wider flat area exposed.

I will try to post a photo, but I wonder if I will have clearance issues without beveling or blending the chamber more to narrow the width. Any thoughts from the pelicans?

cgarr · cgarr's Garage

If your going from 95 to 98mm cylinders usually the head is chamfered but I have seen some that were not which will leave a step or sharp edge. If the sealing surface is just cut, unless its at the max of .25mm should not change a lot.

OhioRick · 03-30-2014, 11:42 AM

thanks Craig. The heads were done before I decided to go 3.4 so they have not been relieved. Here is a photo showing the cylinder sitting on the head and the flat which is about 1.6mm on each side. It is still smaller than the JE piston dome so I don't think it will be an issue, but wanted some second opinions.

HawgRyder · HawgRyder's Garage

I would hit the exposed sharp edge with a de-burring tool myself...just to get rid of the possibility of pre-ignition.
Sharp edges tend to glow in combustion chambers.
Bob

cgarr · cgarr's Garage

I would chamfer that but then I had some race heads in, the guy didn't want it done afraid of loosing combustion volume. But as Hawg said at least take the sharp edge off it..

cgarr · cgarr's Garage

The tops of the J&E pistons will be about 97mm here is a head © that was chamfered for 98mm cylinders but the rod bearing let go and it took out the head in this area.

OhioRick · 03-30-2014, 12:40 PM

Good advice and so quick. I will take off the sharp edge and try not to damage the critical face. I'm going to like being here during my rebuild. Thanks so much.

JFairman · 04-01-2014, 05:21 AM

Years ago I bored out a BMW R90S to fit R100/7 pistons and I beveled that edge by hand with a file and then smoothed it down nice with fine to ultra fine sandpaper until the whole combustion chamber was polished .
That was only 2 cylinders though.

I just installed heads I bought around 6 years ago with new valve guides from a '76 turbo on to my '87 turbo. The machining was done before I bought them. The '76 heads were on 95mm bore cylinders and the '87 cylinders are 97mm bore so I took the '76 heads up the road to Xtreme heads in Palm City, Florida to have the bevel cut in to the edge of the sealing surface.

The low compression 97mm turbo pistons probably wouldn't have hit it but I didn't want to chance it or have the sharp edge running hotter than the rest and possibly causing detonation. The original '87 turbo heads have that edge beveled too.

Xtreme Cylinder Heads And Castings did a real nice job and the owner Bill is a good guy.
Here's a couple pics of the bevel they cut.

I did the exhaust port smoothing and intake porting with a die grinder using carbide burs and port sanding cones on threaded arbors. The valve seats had already been cut so I protected them with tape and the new valve guides were already in so I put short pieces of fuel line over them to protect them while grinding in there.