Max Moritz - Machining Valve Pockets
 

My 3.2SS rebuild from a '79SC is progressing. I have mocked up the engine to do the piston to valve clearance.

The pistons are 98mm Mahle Max Moritz and the cams are DC20. My deck height is about 1.1mm, the heads have been chamfered. The heads are off my '79SC. Here's a link to the pistons

-> http://forums.pelicanparts.com/showthread.php?s=&threadid=221887&highlight=max+mo ritz

My piston to valve clearance has shown a paltry 1mm. The timing of the cams is 2.2-2.4mm. With this non-radical cam I was surprised but looking at the pistons maybe it makes sense. I think these pistons were designed for the SC cams, they have very shallow valve pockets. I proved the valves are hitting at the pockets by applying some white grease to the valve, reinstalling and doing the clearance check, then seeing the white grease on the valve pockets.

If I set the valve in 1mm and do an entire revolution it's ok, no binding, but at about 1.2mm I get binding. On the other bank I get binding at 1mm. The exhaust clearance is fine (over 2mm).

My question is, how much material can be removed from the pistons? Can I have the shop safely remove .75mm for safe tolerance?

I am running TI retainers with Supertec racing springs.

That's cutting it pretty close. I don't remember offhand what expansion rates are for these but I think at operating temps you'd be too close for comfort.

I can't tell you what the thickness is in this area, but I can tell you what I did for a customer recently.

He ingested a piece of debris into a hi-strung race motor (running no air filters) and dinged up a very expensive piston pretty good. THere was no time to get new ones made. We sent the piston to Peter Dawe who used some sort of ultra-sound/x-ray/sci-fi thing to actually measure the thickness of the material under the ding to make sure there was sufficient metal under it. You simply could not get a reliable measurement with large calipers.

You might try large calipers first as the Max Mor. pistons have a more standard "regular" shape but short of that, try calling Peter.

The pistons are off to the machine shop. I think the depth can be measured at the valve pockets, I'll soon find out.

Should BOTH exhaust and intake pockets be machined to maintain the balance of the piston?