Slipping clutch

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Slipping clutch

Postby johnmcguire » Thu Jan 22, 2015 10:52 am

After rebuilding my 948 the new clutch started slipping badly after only 350 miles. I thought it must be oil on the clutch from a leak at the rear seal on the crank. On pulling the engine there is no evidence of oil comtaimination but the nearly new clutch plate is slick and worn. The clutch surface on the flywheel is very slick and polished especially compared to the surface of the pressure plate which is also nearly new.
Should I resurface the flywheel clutch face and if so how?
Thanks again, John
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Re: Slipping clutch

Postby Charlie Tolman » Fri Jan 23, 2015 12:58 am

If you recently replaced the clutch disk and also the clutch pressure plate, and the flywheel was used previously, then with a slipping clutch I would expect the surface on the flywheel to be the most worn of the three components.

Somehow, compare the spring pressures of the old and new clutch assemblies. Nine springs ?

Compare the thicknesses of the old and new clutch disks.
Compare the distance from the mounting surface of the clutch housing to the surface of its pressure plate, of the old and new clutch assemblies.

Check for "free play" in the clutch linkage. If there is none, the mechanical and hydraulic linkage might not be releasing the clutch pressure plate all the way. Lubricate the bushing for the clutch pedal.

Is the clutch cylinder, mounted on the transmission, have an activation rod that is too long ?
Is the correct throw-out (release) bearing being used on the end of the clutch arm ? Should be short for 948 engines and clutches.

Charlie T.
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Re: Slipping clutch

Postby johnmcguire » Fri Jan 23, 2015 12:40 pm

Charlie, Thanks once more for your expert attention. Should I resurface or roughen up the clutch contact area on the flywheel? It is really slick. Any advice on how to make it grab a little better?
Meanwhile I'll check on your other points of error as best I can. The pressure plate, disc and bushing that were on the car when I got it at approx. 90,000 miles are long since gone but the disc that failed at 350 miles is indeed thinner than the new replacement. What I put on 350 miles ago was a Borg and Beck kit. The throw out is completely off the pressure plate when the clutch is not engaged.
Thanks, John
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Re: Slipping clutch

Postby admin » Fri Jan 23, 2015 1:04 pm

I agree that somewhere the new parts are "amiss"...

Another thought is that the pressure plate may not be putting enough pressure on the clutch plate--that would let the clutch disk spin--wondering if you have a mismatched disc/pressure plate?

Might help to have the flywheel resurfaced too--if that's warped, then the clutch plate might not have enough "flat surface" to grab enough...if you've got it all apart, I'd resurface the flywheel, and put in a new clutch disc and pressure plate
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Re: Slipping clutch

Postby Charlie Tolman » Sat Jan 24, 2015 12:59 am

The thinner clutch disk might be the key to the problem.
A thicker disk would increase the clamping force.

If you are using a 948 clutch assembly, how many springs does it have ? Nine ?

I had competition 948 and 1098 engines that would generate about 100 hp each. Neither the 948 clutch nor the 1098 clutch were adequate, with slipping in top gear. I had the aluminum flywheel drilled to accept a standard 1275 clutch, and with the 1275 throw-out bushing, there were not any problems.

As long as everything is apart, refacing the flywheel is a good idea. I do not have any information about the process or a recommended surface roughness.

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Re: Slipping clutch

Postby Charlie Tolman » Sat Jan 24, 2015 2:11 pm

What is the thickness of the new clutch disk ? Thanks. CT
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Re: Slipping clutch

Postby Charlie Tolman » Wed Jan 28, 2015 2:21 am

Some information from Internet websites:
A stock 1275 clutch disk, nearly new, has a thickness of 0.28".
Two Borg & Beck Sprite clutch disks (unknown engine application), old and new, have a thickness of 0.295" and 0.34", respectively.
This information is not complete. However, I hope to have a 948 dimension from Moss Motors in a day or two. Again, what is the thickness of your new disk?

Possibilities:
Has the clutch disk been installed backwards? The central hub is offset, and if installed
backwards, the longer portion of the exposed hub might touch the mounting bolts for the flywheel to crankshaft, and this might not allow full clamping of the clutch system.

Is the clutch disk warped? Is the wear uniform around all the new friction surfaces?

More ramblings.

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Re: Slipping clutch

Postby Charlie Tolman » Wed Jan 28, 2015 11:37 am

Frogeye Spares in England, says their 948 clutch plate is
roughly 8.5 mm ( 0.335" ) thick.

CT
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Re: Slipping clutch

Postby Charlie Tolman » Wed Jan 28, 2015 6:32 pm

Victoria British says that their 948 clutch disc is about 5/16" ( 0.312" ) thick. CT
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Re: Slipping clutch

Postby admin » Thu Jan 29, 2015 1:00 pm

Can't hurt (other than in your wallet) to try another clutch disk...

Shouldn't seem to be this hard
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Re: Slipping clutch

Postby johnmcguire » Fri Feb 06, 2015 3:49 pm

The old clutch plate that failed at only 350 miles is down to 8.25 mm or 0.320 inches. The replacement I got from Moss measures 8.70 mm or 0.340 inches.
What little wear there is on the pressure plate appears uniform. I will resurface the flywheel but wonder how best to do this.
To introduce a little humor here, do you imagine that boring out to 1050 cc and increasing the compression to 12.0 has " overpowered" that little clutch?
John
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Re: Slipping clutch

Postby Charlie Tolman » Sat Feb 07, 2015 1:22 am

With an engine that is developing a little more power than a stock 948 unit, I would not expect a clutch slipping problem.

I have not used a 948 clutch, but I know of people that had slipping problems with a six-spring clutch housing for a competition application. A nine-spring clutch is an upgrade.

With my 948 and 1098 competition engines, each developing near 100 hp, I had the flywheel drilled to accept a standard 1275 clutch, and never had a problem with slipping.

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Re: Slipping clutch

Postby johnmcguire » Mon Feb 16, 2015 12:51 pm

Charlie et al.,
I think I found the problem. The peripheral third of the clutch plate is worn and the inner part appeared untouched. The flywheel surface is nearly evenly worn but perhaps a little more heat scoring peripherally. The machine marks on the nearly new metal pressure plate are essentially untouched except for about 90 degrees and the peripheral third. In this area there is heat scoring and no machine marks.
I know that the metal pressure plate is spring loaded and should therefore exert even pressure but not in this case. This was a new Borg and Beck kit 350 miles ago. The springs in the pressure mechanism all work but the frame that attaches to the flywheel was uneven. Three of the attachment feet had metal stamping edges that held this part of the thing further from the flywheel. I hope a little filing and sanding will make things flat this time.
I was temped to get a new kit but then I could be chasing down a different problem. Pulling the engine for this has been fun but involved. At least I am fixing the oil leak at the same time.
Hope this works, John
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Re: Slipping clutch

Postby Charlie Tolman » Tue Feb 17, 2015 1:30 am

John,

It appears that you have found the problem. Good work !

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Re: Slipping clutch

Postby johnmcguire » Tue Jun 02, 2015 10:15 pm

Perhaps the uneven pressure plate was the problem. I put the engine back in and the clutch works fine so far. I drove it 200 miles in the mountains last week without a hitch.
John
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