RET

All of the flywheel-to-crankshaft bolts in my car had the first two threads somewhat flattened because the threads in all of the crankshaft holes consistently taper inward a little towards the bottom, causing some interference as the bolt is tightened.

Is this normal for a 928? Otherwise it would seem that the holes had not been fully tapped. They are deep enough, and the bottom thread taper seems consistent with a starter tap, so a plug or bottoming tap had not been used, but I don't know if this is intentional or not.

I do not want to take any action without getting advice (either tapping the holes deeper or installing the new bolts when they arrive), as this is the only 928 that I have ever worked on and I do not always know what is correct (and the WSM's are not often helpful).

Maybe this is a lot of concern without reason as the flywheel had not parted company with the rest of the car in all of the years since it was assembled, but I have nothing better to do until the parts arrive.....

Thanks,

Bob

Mike Simard

Something must be wrong, that should never happen.
It's very bad because it prevents the bolt from being properly stretched and that will lead to failure.
Are these the stock triple square bolts? Measure the bolts and crank thread depth and we can figure out what's amiss.

RET

Mike - this did not seem right to me either, but whoever bolted this flywheel to the crankshaft had to notice the increased torque required before the bolt heads even touched the flywheel surface, and there have been one or two other things on this car that did not quite meet my expectations.

For all I knew, this might be some new example of Teutonic over-engineering, and since this was almost certainly done by Devek, I assumed that there had been a very high level of competence, and that anything out of the ordinary would not have been tolerated..

I have no idea if the bolts are stock: this legend is on the top of each bolt (as best I can make out): 1N8US 12.9 MZX. They have an M12 twelve point triple square head, 10mm by 1.25 thread size, with a grip and thread length of .916" (23.26mm), and a thread only length of approximately .549" (13.44mm). The crankshaft hole is .727" (18.46mm) deep, but I do not know how deep the total threaded portion is, or where the taper starts. The GTS flywheel is .510" thick.

Thanks

Bob

Mike Simard

Your measurements are what I see when I check a bolt and flywheel that are here and that sounds like a stock bolt. I can thread that bolt into it's full thread depth freely. Are you sure yours are actually binding?
I would order new bolts and see if they thread by hand freely and take it from there. That would be very odd to have to tap the crank deeper and I would suspect there something else going on first.

RET

Thanks Mike - my new bolts arrived today and they match the old bolts...

How about the thickness of the flywheel? It is .510", but I suspect that it is alright. FWIW, this is a Scat stroker crankshaft.

Even the old bolts with the crushed end threads will not thread into the crankshaft to the last thread without a significant amount of torque.

Anyway, my old HSS tapered tap refuses to do its duty, and a new HSS bottoming tap (the only kind I could get locally) grudgingly turns a little until the effort becomes too much. It looks as if it got dulled in the process of cutting one and one half turns more too. I think that the crankshaft is harder than the tap, and I am afraid of breaking it in such an awkward place.

I have ordered a TICN coated vanadium steel tap from McMaster-Carr in the hopes that it will work. I don't see any other way other than deepening the threads that would be suitable: adding washers or shortening the bolts seem to be very bad ideas.

Thanks,

Bob

Mike Simard

Well that explains it! There was just some discussion about Scat cranks here in another thread, I better be careful not to hurt anyone's feelings

The area where the crank bolts are tapped is very tight and in order to keep the hole from breaking through, it must be tapped with consideration of the space involved. Scat was careless by simply using a plug tap and sending it out but that's obviously what happened here. You need to have it tapped to full depth which should be simple if you find someone with a tap and basic skill.

BTW, my flwheel is .515.

Mike Simard

BTW, I would use a plug tap and grind the pointy tip down. It needs to be a good HSS one and wont be cheap. It sounds like you already have a good handle on that and I'm sure you'll do a great job and your car is lucky to have you.

As you know, "old" taps in the shop are always unusable because nobody can bring himself to throw them away. That one of yours will be around 100 years from now and still be dull.

RET

Hello Mike - actually the tap on order is TiCN (Titanium Carbonitride - whatever that might be) coated cobalt vanadium steel, not HSS, which was the composition of the last two taps that were defeated by the crankshaft... and you're right, it was not cheap: $35.24 for one, and for that much money, it better tap the holes all by itself and then get me a beer when done. However, I will buy several if I have to, as long as they do the job.

Thanks again for all of the responses.

Bob

RET

Conclusion, FWIW: the rather expensive TiCN coated cobalt vanadium steel tap worked where the HSS taps failed. The holes are finish tapped and the flywheel is installed with new bolts.

The new tap was a nice tool, it almost looked too good to use and was very hard and sharp, but threading those nine holes pretty much finished it off. That crankshaft is hard.

The first couple of holes were satisfying to thread, particularly after the previous attempts: I could feel the tap cutting the hard metal. I could also feel it getting dull. I think that I pretty much roll formed or simply wore threads into the last few holes. I should have bought at least three taps..... it took too long, but it is done.

Hopefully I will get the clutch back in tomorrow.

Bob