Crank to case interference

Walt Fricke · 05-24-2010, 10:47 PM

Well, I was all set to button up the early 930 case with its 6 bolt 66mm crank.

Put the orange Loctite on, put the other case half on, all the washers and case nuts and the througbolt assemblies. Got things snugged up but not torqued. Let's see how the crank moves.
Oh, oh. It isn't moving. Much consternation: did I once again fail to get the #8 bearing dowel in its proper hole? No. Is the dowel too long (this is the first assembly of most of these parts as a single engine)? No. Try this and that. Take the other half off and remove sealant. Sleep on it.

Did I send first under mains off to be coated? No. Why is it notchy when I turn crank without other case half on? Really notchy, as in I have to force things even with the crank and IS shaft just lying there in an open case half. Why does the IS shaft seem to have zero play at times? Hmm - this is a 0 intermediate to crank centerline case, and the fancy gears are also 0s. But why is it notchy?

Hmm - some of the nice new anti-friction coating is worn on the thrust sides of the #1 bearing. What could cause such a side load?

Well, some of the crank throws are hitting the flanks of the case bearing webs in the area outlined (a before picture, so the slight scoring isn't there yet).

I guess the counterweights on the 66mm crank extend farther from the centerline than those of the 70.4s for which this case was manufactured?

I've had this case and crank for four or five years now. I should have taken some of my stock of used bearings, put them in place, and at least spun the crank to see how it went.

So now I am contemplating how I can clearance this (only some throws hit).

Two steps forward, two back.

Walt

tadd · 05-25-2010, 09:05 AM

Walt:
Sorry to hear the troubles... looks really great though. I'd like to be in your shoes (well, not exactly

) in another month.

Makes sense to me that the CWs would be bigger on the 66 than the 70.4 since the journals are bigger on the 66.

t

kenikh · 05-25-2010, 04:36 PM

Paging Henry...

BURN-BROS · 05-25-2010, 04:53 PM

Walt, Are you sure it has nothing to do with your oil pump? Are you sure you have sufficient clearance? This would be the first time I have heard a counterweight causing an issue.

Henry Schmidt · Henry Schmidt's Garage

On the short stroke 2.8 where a 2.0 fully counter weighted crank is used you will always encounter flyweight to case interference.
You can machine the case or my favorite is to knife edge the crank.
2.8 SS engines are about high RPM so the lighter weights promote spin and yet can easily balance the reciprocating weight of the rods and piston. An RSR flywheel is also helpful.

Aaron you never saw this issue because we always knife edge the crank. I taught you allot but maybe not everything.

Walt Fricke · 05-25-2010, 11:03 PM

Well, two or three hours of hard manual (literally) labor has clearanced things without making a huge mess.

At least it was something I could find, and then fix. I learned that there are six full counterweights. One on each end doesn't interfere. The pairs which straddle main bearings 3 and 5 (I think - it is late) are the ones which interfere. They would have to be knife edged on the main bearing side to do any good this way.

At first rebuild I'll have the crank knife edged. When I mentioned doing that to a 911 crank one time to a guy who ground cranks for NASCAR, he just laughed: it is so thin, why bother? But this will give me an excuse.

If I wasn't going to Watkins Glen starting tomorrow I'd have the short block together by then.

Thanks all

Walt

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by Walt Fricke

Well, two or three hours of hard manual (literally) labor has clearanced things without making a huge mess.

At least it was something I could find, and then fix. I learned that there are six full counterweights. One on each end doesn't interfere. The pairs which straddle main bearings 3 and 5 (I think - it is late) are the ones which interfere. They would have to be knife edged on the main bearing side to do any good this way.

At first rebuild I'll have the crank knife edged. When I mentioned doing that to a 911 crank one time to a guy who ground cranks for NASCAR, he just laughed: it is so thin, why bother? But this will give me an excuse.

If I wasn't going to Watkins Glen starting tomorrow I'd have the short block together by then.

Thanks all

Walt

I have encountered the same response (chuckles) from crank guys for quite some time. Part of the problem is in terms or definitions. Knife edging to the American V8 guys refers to a sharpening of the leading edge of the flyweight to reduce frontal area. Hence the comment "it's so thin why bother". My reference to knife edging the crank really refers to a lightening of the flyweight by tapering the weight to an edge.
Where the V8 guys are reducing frontal area we are attempting to reduce rotating mass.
The reduction of mass has two useful purposes.
First : reducing the mass will reduce crank twist on decel. If the twist is pronounced enough it can cause an imbalance that can create a harmonic that robs horse power and can cause reliability issues. This twist in the crank is what causes 2.4/2.7 crank failure in high performance engines.
To resolve this twisting issue Porsche made a special crank with thicker flyweight for the 2.8 RSR.
Second: although less important a lighter crank will accelerate quicker and the reduced windage will promote a freer spin at higher RPM. This equals slight performance gains with little or no downside.