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Old 10-12-2015, 06:56 PM
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69427 69427 is offline
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Default Engine vibration: Pressure plate imbalance?

Last year I swapped out the five speed for the much lighter original M21 four speed. I had to put a new clutch disc in because the splines were different between the two transmissions. While I had everything apart I thought I'd put in a new, lighter pressure plate too. The old PP was a 16# L-88 part, while the new PP is a 13.5# piece. After putting everything back together I noticed a slight imbalance feel as the RPMs increase. I'm going to pull the clutch assembly apart this winter and inspect everything to see if anything wrong is evident, but obviously I can't check the balance of the PP and clutch disc. I'll probably take these parts and the flywheel to an engine shop and see if they can spin them up and see if there's a problem there. In the mean time, am I missing anything, or am I on a wrong track? The engine always seemed smooth with the old clutch and transmission setup.

Thanks for any input.
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Old 10-12-2015, 07:00 PM
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Twin_Turbo Twin_Turbo is offline
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You could try repositioniong the pressure plate. Does your bell allow access to the PP bolts? That way you could rotate it maybe without taking the whole thing apart
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Old 10-13-2015, 12:43 AM
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So, what else changed?
Different drive shaft / angle?
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Old 10-13-2015, 02:06 AM
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Originally Posted by Twin_Turbo View Post
You could try repositioniong the pressure plate. Does your bell allow access to the PP bolts? That way you could rotate it maybe without taking the whole thing apart
I'm using a stock aluminum bellhousing, so unfortunately it's an all or nothing kind of deal.
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Old 10-13-2015, 02:25 AM
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Quote:
Originally Posted by Bullshark View Post
So, what else changed?
Different drive shaft / angle?
Same (stock) driveshaft. When I swapped out the 5 spd and put back in the 4 spd I had to fabricate another transmission mount bracket to attach to the crossmember. I spent a boatload of time trying (via shimming) to get the angles matching, but given how much PITA it was to try to get an accurate angle measurement at the pinion yoke I honestly can't say that I was entirely successful in matching those angles.

I'm trying to get some more definitive symptoms/information once I get the car running again. I ran a track day last Friday (other than the vibration, damn the car ran sweet) and after a crapload of laps bottoming out the car (onto the exhaust system) at the apex of one of the bumpy corners I broke (and almost ground through) part of the exhaust system near the LH muffler. I need to weld/patch it back up before I can run the car again.
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Old 10-13-2015, 12:25 PM
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time for a Quartermaster triple disc, you won`t believe the difference
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Old 10-13-2015, 02:50 PM
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Quote:
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Quote:
Originally Posted by Twin_Turbo View Post
You could try repositioniong the pressure plate. Does your bell allow access to the PP bolts? That way you could rotate it maybe without taking the whole thing apart
I'm using a stock aluminum bellhousing, so unfortunately it's an all or nothing kind of deal.
Dumb ??? guys, auto trans has the Bellhousing cut back to the pan, with some tin as a dust guard that I always leave off....WHY can't the stickshift Bellhousings be modified for a similar install?? make bolts easy to get to.....

same old 6 bolts holding to the block in either case, seems to be to be an obvious mod.....

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Old 11-04-2015, 10:50 PM
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After some exhaust repairs I was able to start the engine again and run up the RPM. I ran it up to about 4500, the RPM where I noticed the vibrations the other week at the track. The engine was obviously under no-load (which if it was purely a balance issue I would expect somewhat similar results as under WOT conditions), and the vibration level was nowhere near what it was at the track. It seems entirely possible that I misdiagnosed the problem, and it might actually be a driveshaft angle issue, as Bullshark inquired about. This driveshaft angle vibration would be a new experience for me. I never experienced this when I had the five speed in the car, and I'll confess I spent less time trying to get the angles correct when I installed the five speed a couple decades ago than the significant amount of time I spent trying to get things angled correctly when I put the four speed back in last year. Measuring the angle/position of the trans yoke was pretty damn easy, but trying to get a clean accurate measurement of the pinion yoke is damn near impossible (for me) with the crossmember in the way. If anyone has some suggestions on how to get the pinion angle measured accurately, I'd greatly appreciate the help. Thanks.
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Old 11-05-2015, 11:57 AM
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HF sells a magnetic digital tilt gage for under $40. Great little device, got my diff in to within 0.5 degree of the trams yoke.
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Old 11-10-2015, 03:37 AM
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Got a smart phone? Try this.
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It's pretty cool. Walks you through the process.

Tom
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