Crank Thrust Surface Trouble

[DaveD]

The surface facing the clutch should be the only side worn. This is the side to repair, you will want to keep the crank located correctly.

[clshore]

Any repair or remediation must result in proper crank location.
There's nothing that says the front TW have to be the same as the rear.

[clshore]

I'm a late commer to this question but I build and raced an 1148cc triumph Spitfire since 1972. I've noted that the question of WHY has been glanced over ever so lightly. I ran into this in the middle '70's while I was doing an normal rebuilt on the motor for my next race. At the race I had a problem shifiting and noted the clutch was not releasing. When I drained the oil I saw metal, Not good. The the thrust washers on the rear of the crank were destroyed. This had never been a problem before. I decided that the clutch was the problem, but a visual inspection of the clutch did not show any problems. So I made a test stand. The results were interestng the "brand new" clutch had a small deformation which caused the clutch fingers to not release. Result: new clutch, no more problems.

As to the fix of the thrust washers. The photo of the"bronze alloy" washers with screws was close. My engine machine shop pinned them using the same material so any wear would not be a problem. We used the "bronze alloy" for the rear washer, both halves, the front washer does not see the clutch pressure so standart washers/spacers work just fine. If you need to repair the surface on the crank. Spray welding is OK. I've used these "bronze alloy" washers ever since.

Anyone involved with Spitfires (or GT6 or TR6) knows about the Thrust Washer issue, it's been there since day one. (BTW, it's 1147cc, not 1148cc).
The stock TW are about 0.093" thick, the 'ledge' formed by the lower bearing cap that keeps them in place is about half that distance.
Stock end play is 0.007"
It's common for the TW to get installed backwards during rebuilds, with steel facing the crank thrust surface instead of bearing material.
Resulting rapid wear ensures that eventually the TW gets thin enough to drop off the 'ledge' into the sump, and then the real damage begins ...

Pinning them with brass flathead machine screws absolutely solves this.
Only disadvantage is inability to swap TW with crank in place.
But if you use solid bearing bronze TW, you never "NEED" to swap them.
Spray welding followed by grinding can rebuild the thrust surfaces, but since the solid bronze TW are CUSTOM anyway, you can just grind the thrusts surfaces,
and order the TW in whatever thickness you need, eliminating the need for spray welding.

Other cars don't seem to have this issue as badly, perhaps their TW use better materials or mounting methods.

[MadBill]

Another vote for custom TWs; 0.250" at the flange root is more than adequate to withstand clutch spring forces and avoids the process-sensitivity of welding .

[Dan Timberlake]

A ways back in 2001 Clevite was talking about upcoming "Z" bearings that would include some thrust bearing grooves etc that would raise the load capacity.

Page 5 here -
http://www.stealth316.com/misc/clevite- ... arings.pdf

More 2001 Clevite tech -
http://www.stealth316.com/misc/clevite- ... arings.pdf

Fairly modern discussion of the importtance of crank thrust face surface finish on page 23 here -
http://www.hankhinton.com/home.html

[chimpvalet]

With regard to crank thrust flange capacity, what is the narrowest dimension ( thickness at base ) expected to survive street clutch loads reliably? Clutch specs are either 8 1/2 or 9 1/2 inch, diaphragm type.

Steve

[MadBill]

<Caution: Technobabble alert!>
Short answer; "Don't worry, be happy!"

Long answer:
Hopefully if there's a metallurgist in the crowd he will come forward, but my best shot at it is that assuming the ultimate tensile strength of the shaft material is 50,000 psi and ultimate shear strength is ~ 65% of same, and yield point is ~ 60% of USS and 'allowable' stress is 25% of same, then : 50,000 x .65 x .6 x .25 = 4,875 psi.

Assuming the maximum engine torque is 120 lb-ft. and that the clutch has 2X engine torque capacity, that the average diameter of the disc is 8" and that its friction coefficient is 0.6, then the maximum clutch axial load is: [(120 x 12)/8 x 2] x 1/0.6 = 600#.

At 4,875 psi, that would require 600/4875 = 0.123"² of steel in shear, which, if the ID of the thrust surface was 2.5", would be a shim-like 0.016", resulting in a safety factor of ~ 16 for the quarter inch flange.

Be happy.
Q.E.D.

(Notice a mere nine ASSumptions; maybe you should shoot for twenty thous minimum, JIC...)

[clshore]

Did you miss the lever ratio of the clutch diaphragm?
If clamping pressure is 600 lb, and lever ratio is 2:1, then it would be only 300 lb, giving a 16x safety factor

[MadBill]

Good point! One more ASS. required.