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Since you seem handy, take Jake's online M96/M97 101 engine class which qualifies you to attend his 102 hands on engine building class where you'll actually tear down and rebuild a motor, to the point where you could then do yours at home. The courses are signed up via The Knowledge Gruppe - http://www.theknowledgegruppe.com
Your timing is actually pretty good - the online class is at the end of April with the engine building class in May and he only does one set of these each year.
It will be the best couple thousand you've spent if you really want to know about these motors, just don't drop a lifter into the bucket of oil and coolant intermix while Jake is standing right next to it
So we split the case today and removed pistons 4-5-6. No smoking gun. Taking that half of the case and the pistons to the machine shop to be checked. More to come...
Machine shop found no smoking gun, but did note that the cylinder bores get larger (taper out a few thousandths) near the top of the stroke. No lip at the top. Not sure if this is normal?
Absent anything obvious, I think we are going to do a full rebuild on this. May as well punch it out to a 3.8 along the way. Suggestions on pistons?
A few thou taper and no lip are both normal for a 70k mile engine. If you go with the LN 3.8 Nickies, the package comes with pistons, rings, wrist pins, and clips.
A few thou taper and no lip are both normal for a 70k mile engine. If you go with the LN 3.8 Nickies, the package comes with pistons, rings, wrist pins, and clips.
Interesting. The taper was the only thing that could be even remotely attributable to the low compression. The is very odd.
You sure this wasn't a way to get your wife to go along with why the motor is out to check low comp.
But only when the motor is torn down all is ok so I may as well go big bore 3.6?
What were the measurements? These need to be taken at 3 different bore positions, in 3 axes and compared.
You MUST find the smoking gun, or it WILL find you. You cannot assume anything in engine building. If you think that just swapping out the cylinders will do the trick, you might find another issue was underlying when the new engine does the same thing.
I cannot stress enough the necessity of pinpointing this problem now.
On the smoking gun, the gunk in the ring grooves makes me think that the rings may have been stuck, but the mineral oil they used to test for fluid leak down might have loosened them up. Measurements to date have been from multiple heights and angles, but not carefully recorded or compared, so they are somewhat unscientific. I guess I have more work to do there.
Check ring thickness at the edge of the od and the id. Then check the top ring grove to see if it has taper and has been hammered out at the od, flared so to speak.
i just had pistons reworked that were hammered out .020" and the ring was flopping around in the grove. It was a aircooled race motor though.
So it looks like I should pull apart the rest of the engine and take the pistons and case back to the machine shop for some more careful measurements.
Stay tuned. Thanks!
As was stated earlier there is something going on that would make the motor have a poor leakdown in the crankcase. Keep looking it's there, place a ring at the different heights to set measuring point with 3 point micrometer. While a 2 point is good, a three point is very precise with ***** at the end of each point. Make sure they have a setting ring to insure it's measuring accurately. These are very expensive but a good machine shop should have access to one.
stuck rings would definitely be something that would give give you a poor reading you would need to know what dia they were stuck at. We're the ring lands packed with sludge??
Keep at it, satisfaction will come when you know exactly what the cause was.
Cheers!
Originally Posted by 4Driver4
So it looks like I should pull apart the rest of the engine and take the pistons and case back to the machine shop for some more careful measurements.
Stay tuned. Thanks!
After taking the case and pistons to the machine shop and chatting with a few local Porsche engine shops, we are pretty comfortable in a stuck ring diagnosis. There was some sludgy build up in the grooves of several pistons, and no other issues were found. We are going for a full rebuild with Nickies. The case is at the machine shop now, and we will be doing the reassembly once it comes back. I'm hoping to find a reasonably priced wrist pin and clip installation tool, but have not had any luck as of yet.
The heads have been checked and cleaned by the machine shop, and have passed muster with no issues.
03-26-2017, 11:13 AM
