The 'E' Type Forum

Does anyone know what the pinion angle should be for the SIII IRS? Also, can anyone confirm at what angle the engine\gearbox sits when properly installed? Is it dead level or does it slope down toward the tail end of the gearbox?

Background:
I have a JT5 installed as a conversion from automatic and get some vibration at high speeds(over 65). Tires are balanced, all U-joints are new on the balanced driveshaft and the axles. I noticed in the parts manual that the gearbox spacers for the rear trans mount are different between auto and manual(c-32458/2 vs c-7493). Anyone know the difference in thickness? Seeing as how I converted from auto to manual and only had my original spacers, could the difference in thickness of those spacers cause enough of a change in the driveline angle between the engine\gearbox and the differential pinion angle to be the source of the vibration? The trans mount on the JT5 is totally different that the original, no spring. The mount is similar to a chevy, being made of rubber, similar to the engine mounts except rectangular, not round. Also, the manual indicates spacers for the motor mounts showing "as required". "Required" to achieve exactly what? Engine height? Engine tilt side to side? Which one?

I know the two angles are supposed to be in parallel(e.g. engine 3* down - pinion 3* up) and that too much of a mismatch can cause a driveline vibration.

I have not been able to find this documented anywhere. If it is, could someone please point me to it?

I plan on doing some measurements in the future but would like to know what the angles SHOULD be as a starting point so that I can set everything right.

I know, lots of questions

Thanks,
Steve

Hi Steve,
I don't recall seeing any published figures for driveline angles but, if accurate, these two technical illustrations from the original S3 press release appear to show that in the static condition the design intent was that the gearbox output shaft should be horizontal with offsets which appear to be of the order of 3deg vertically and 3deg laterally to the diff pinion shaft.
Hope this helps,
Bob

C7493 is superseded by C32458, they are the same?

For manual gearbox the mounting spring and the upper spring retaining plate are P/Ns C12299 and C34263.
For automatic transmission these are C19988 and C21932, to accommodate the heavier mass of the automatic.
Net result, the gearbox should "float" on the spring, i.e. it should find its own level. From its rest position you should be able to push it up or pull it down. If you can't do this, if the gearbox is hard up or hard down, if the spring is prevented from moving, that could very likely result in vibrations.

Note that the spring is damped, by the donut washer C29011 being an interference fit within the mounting bush C30159.

When I installed an overdrive unit I had to go the other way, due to the increased mass I had to swap out the manual spring and retaining plate for the automatic versions.

Thanks for the pictures Bob. It appears that the engine sits level but it looks like the differential pinion angle is up, but how much? Oh well, the measurements will tell.

lowact, as I stated in my original post, the JT5 uses a rubber mount instead of the stock trans mount. This is why I need to make sure that the trans tail is positioned vertically at the correct height to achieve an acceptable angle with regards to the pinion angle.


Thanks

My experience with a JT5t in my 3.8 was that I overtightened the rear engine stabiliser and this caused a rattle.

Might the vibration be due to other than tail-shaft alignment, e.g. why not half-shaft bearings?
To test the effect of tail-shaft alignment you could change it by packing out the mount with a lot of washers (easy without the spring) and test drive to see if this makes any difference, my money's on no.

Abowie: RE rear engine stabiliser, since this is a V12, there is no rear engine stabiliser. But the way the V12 twists under load when the motor mounts are hot & squishy, I have thought about maybe rigging something up along those lines. There ARE tapped bolt holes in the firewall and bellhousing. They might have originally had this in mind and then later decided it wasn't needed. And this is with the so-called uprated motor mounts.

lowact : All new bearings in the diff and the hubs. With the driveshaft disconnected it all rotates smooth as butter. CJ did a nice job for me on the IRS. Unless maybe, they didn't put back my original dog bones? Who knows?

Eventually I'll get around to measuring the angles, setting them to where they should be and see if it did any good. I was just hoping someone might have run across the design specs after all these years. Always a good starting point IMHO.

Thanks all!

Finally got around to work on my Jag once more. Moved to a new town, downsized house for retirement going from a 2 story with 4 car garage to single story with a 2, remodel, remodel, remodel; you get the idea.

I had mentioned earlier that I had questioned whether my IRS was assembled correctly.

Unless maybe, they didn't put back my original dog bones?

Now that I've been able to get underneath and take some measurements, I'm REALLY curious what the various angles in my driveline should be.

I jacked up the rear only and this allowed me to install the correct manual transmission spacers that go between the body and the mount plate at the tail end of the trans. I originally had the spacers installed that came from the factory which were appropriate for an automatic, way too thin.

NOW it's time to take measurements!



Now I have read that parallelism is important between the flanges and the measurements tell me that my flanges are out of parallel by just over 6 degrees. This is what makes me think again about dog bones (inner fulcrum shaft mounts). I would like to verify that mine are correct for a SIII since I've heard that these are what determines the installed pinion angle. With all of my measurements taken relative to 0 degrees vertical and the car jacked up at about a 5.5 degree angle, I also measured the bottom plate on the IRS cage at about 3 degrees, down in front. If my calculations are correct, that would mean that the IRS when level would have a nose up angle of about 5.55 degrees. Does this sound right? I also noticed that in the diagram provided by Bob (which I repurposed, thank you Bob), the flanges do NOT look to be parallel.

Questions, questions, questions. Anyone got any answers?

A pair of U-joints will never want to be parallel AND totally inline with each other as that means the needle bearings inside will wear in one place only and take on a set. If the U-joint rotates slightly, like one of those gyroscope toys when it is slowing down, then the grease is distributed around all of the bearings and nothing will wear prematurely as nothing ever settles in one place.
kind regards
Marek