1275 clutch on 948

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1275 clutch on 948

Postby Asolepius » Mon Feb 07, 2011 7:24 am

I am fed up with 9-spring clutches on my tuned 998 and have ordered a 1275 clutch kit. I have someone lined up to drill the flywheel to suit, but are there any other potential issues? I have a 1098 gearbox so not sure if there will be a clearance problem. Also do I need the 1275 clutch fork?

The latest problem has been the slave cylinder not returning (it is a new one) so it has worn out the release bearing. The master cylinder was sleeved and rebuilt 6 years ago and I have only recently had this problem. I am thinking about a roller release bearing but it's expensive.
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Re: 1275 clutch on 948

Postby hpmowog » Mon Feb 07, 2011 11:31 am

Yes, you will need the 1275 clutch fork, as well as the 1275 pushrod. You may be able to get by with keeping the early slave cylinder, but I can't confirm since I have used only 1275 slave cylinders with this set-up.
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Re: 1275 clutch on 948

Postby Charlie Tolman » Tue Feb 08, 2011 1:32 am

I had a 1098 engine in my '63 Sprite, with a rib-base transmission. The stock flywheel was drilled for fitment of a 1275 clutch. I assume that the clutch fork was a 1098 version. Using a clutch release bearing for a 1275 worked well. The clutch slave cylinder might have needed a slightly longer push rod.

I just noticed that the 1098 and 1275 clutch forks have the same part number in the VB catalogue. Oops.

A new 1275 clutch, the same clutch fork and release bearing, were also used with a 948 engine with a light-weight aluminum flywheel.

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Re: 1275 clutch on 948

Postby Asolepius » Tue Feb 08, 2011 8:00 am

Thanks guys - this weekend I will attempt a world record time for a clutch swap! Actually I was offered an ally flywheel last year but the seller wanted GBP500! Still have to check the hydraulics. On another board someone said I should use the 1098 backplate (I have a spare) but I am not convinced. The thin backplate will just bring the release bearing slightly closer to the clutch, which I can adjust out with the pushrod (mine is adjustable). In any case after numerous attempts I have got the rear main bearing seal working and I really don't want to disturb that.
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Re: 1275 clutch on 948

Postby Sprite33 » Tue Feb 08, 2011 3:15 pm

Adjusting the push-rod to make up for the thickness of the back plate can change the arc of the clutch fork. If all is not right you could get the release bearing too far off to one side. You should take some measurements of the position of the release bearing when the clutch is disengaged and engaged to see if the fork arc is good. You may be able to compensate by trying the different release bearings (948 vs 1275).
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Re: 1275 clutch on 948

Postby Charlie Tolman » Wed Feb 09, 2011 1:26 am

My 1098 engine had a thick back plate, and my 948 engine had a thin back plate.
Both engines used the same transmission, and both had 1275 clutches and release bearings.
The clutch slave cylinder might have been fitted with slightly different push rod lengths. ?
Both arrangements worked well for many years.

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Re: 1275 clutch on 948

Postby Asolepius » Tue May 17, 2011 6:46 am

It is all assembled now. 1275 clutch on re-drilled 948 flywheel, 1275 release bearing, 1098 gearbox and release fork, 948 slave and pushrod, thin backplate. All works perfectly. I wish I had done this years ago, as the over-centre action of the diaphragm is so much less tiring. Engages nice and sharply. With the new big valve head it is a joy!

The old clutch was largely destroyed. The release bearing had been constantly pressing against the plate, so that all the carbon had disappeared and it was steel on steel. What would cause that? I dismantled the master cylinder to see whether it had been sticking, and it looked perfect. It's as if the fluid was not returning to the master, but the slave was operating OK. Very odd.
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Re: 1275 clutch on 948

Postby Charlie Tolman » Wed May 18, 2011 12:34 am

Your new clutch combination should work well for several years. The only difference between your combination and one that I used, is that my slave cylinder was from a 1098.

A few years ago, I had a problem with one rear brake draging on a Chrysler Cirrus. Brake shoes would last only a few weeks. The problem was a flexible brake line, from the chassis to the wheel cylinder. Apparently, the rubber brake line was deteriorating internally, resulting in a nearly blocked line for fluid flow. Fluid would flow when the brake pedal was depressed, due to the high line pressure. However, the internal line blockage would not allow all of the brake line pressure to be reduced when the brake pedal was released. The wheel cylinder was not retracting all the way. In an electrical analogy, this system was acting like a diode.

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Re: 1275 clutch on 948

Postby Asolepius » Wed May 18, 2011 3:10 am

Interesting - but there is no flexible hose in the clutch hydraulics. I am going to keep an eye on the release bearing clearance. The only thing I did notice with the master was that the thin washer between the main seal and the piston was slightly chamfered on the edge, as if it had been too tight in the bore. But it didn't seem at all tight.

Well I am all ready for the track day on 11th June. Will try to get video and post it here.
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Re: 1275 clutch on 948

Postby Charlie Tolman » Thu May 19, 2011 12:50 am

The brakes were acting strangely, on my 63 Sprite. The problem was eventually found, where the brake pedal was not retracting all the way upon release.
Lubricating the bushing and through-bolt for the pedal pivot solved the problem.
I wonder if a similar situation with a clutch pedal might be related to your problem.?

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Re: 1275 clutch on 948

Postby Charlie Tolman » Fri May 20, 2011 12:18 am

My friend had a Bugeye that was modified for SCCA H/Prod racing, which had a 1275 clutch slave cylinder. I do not know if it was a standard 1275 arrangement, but I remember a short flexible rubbler hose attached to the slave cylinder. ??

My 63 Sprite, modified for G/P and H/P racing, had a steel tubing clutch line, which was bent into a multi-turn coil a few inches from the slave cylinder. This served as a flexible link. I do not know if this was a stock arrangement, or was installed by the previous owner.

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Re: 1275 clutch on 948

Postby HP Spridget #53 » Fri May 20, 2011 1:20 am

Charlie,

I believe the coiled steel line was stock. I recall the lines just as you described on the 3 or 4 Bugeyes I had back in the late 60's and early 70's. Of course it has been a long time... 8)
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Re: 1275 clutch on 948

Postby Charlie Tolman » Sat May 21, 2011 12:25 am

Thanks, Tim.

CT
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Re: 1275 clutch on 948

Postby Bill Masquelier » Sun Sep 18, 2011 12:43 pm

This is very late but-
it sounds to me like Asolepius should check into his master cylinder that there is NO check-valve in his clutch side which could be causing his release bearing to remain in contact with his pressureplate and wear it out (there SHOULD be NO check valve on the clutch side!!)
This could have been the initial problem
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Re: 1275 clutch on 948

Postby MagicMidget » Tue Dec 27, 2011 6:19 pm

Hello Guys.

Just have af few questions.

A 1275 clutch on a machined 1098 flywheel , using a 1275 Roller Release Bearing Kit from Peter May.

The Gearbox is an 1275 Close ratio.

Witch Fork is the right one to use , can the Bearing go all the way to push the clutch ?

Kind Regards. Gary
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