Lotus Elan

Metallic rattle

PostPost by: Harvey » Sun Mar 31, 2019 12:23 am

To Urs, Andy, Richard and others following this thread I just got my gearbox back from the local shop that rebuilds gearboxes on high end German cars but also has some experience with the Ford 2000e four speed as well. The gearbox was turned over to the shop to see if they can figure out why I was getting a chatter/clatter from the gearbox on overrun. The gearbox was also locked in first gear.

What I am going to relate is what I was told by the mechanic who worked on the gearbox. Even though I have watched the gearbox being disassembled and have studied the exploded parts diagram in the workshop manual I am still mystified how it all fits together and the relationship between its various parts. Maybe that's why I failed engineering drawing in college.

The mechanic found that the output shaft had been pushed forward displacing the circlip from its groove on the shaft thus enabling the output shaft to be forced into contact with the input shaft. Looking at the slip yoke on the front of the prop shaft he concluded that the yoke was sliding too far into the tail housing and exerting force onto the end of the output shaft. His remedy was to shorten the prop shaft by 1/2". He said I should have at least 3/4" of yoke extending beyond the tail housing when the gearbox is in the car. Since then, I had an opportunity to measure an Elan and he is correct: the yoke extended .850" beyond the tail housing. In his view, shortening the prop shaft will cure not only my shifting problem but also the gear chatter/clatter.

Next week I will be putting the engine/gearbox back in the car and will do a trial fit to see how much slip yoke extends beyond the tail housing when the prop shaft is bolted to the diff. If I don't have sufficient clearance between the end of the yoke and the tail housing it will be off to the prop shaft specialist to get it shortened. If I do have the requisite clearance then it's back to square one. What I still don't understand is why I am having this problem now after driving the car over 30,000 miles in 20 years? I'll get back with you next week.

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PostPost by: rcfurse » Sun Mar 31, 2019 11:44 am

Has the diff moved forward or the engine moved back? Change the engine mounts and see?
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PostPost by: Craven » Sun Mar 31, 2019 11:57 am

Interesting explanation from the mechanic, but the mainshaft/outputshaft is held to fairly fine limits by the rear bearing. End float of the mainshaft is dependent on the tolerance of the bearing its carrier and, and this is often overlooked or not understood, the thickness of the gasket between the gearbox case and the tail housing.
As spoken about on this forum too long propshaft length usually results in the welch plug in the yoke being pushed out of place, on an Elan setup the distance between the diff flange and gearbox is more or less fixed unlike a live axle were suspension movement causes this distance to vary requiring a sliding yoke.
It’s a pain the amount of work required to prove the mechanics hypothesis ie refitting to the car.
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PostPost by: Harvey » Mon Apr 01, 2019 1:22 am

Richard and Craven,

The diff has not moved and is securely held by the torque rods whose rubber bushings are sound. I've been running a Spyder chassis for over ten years and my engine mounts are also in good knick.

I can only say that the mechanic checked the end float of all bearings and shafts and says the gearbox is as it should be at the moment and if no force is applied to the mainshaft circlip then I should not have any further problem with it. I didn't know the closed end of the slip yoke is a welch plug. I saw no witness marks on the inside of the plug that would indicate it was coming into contact with the end of the mainshaft. It does seem as if the plug would be displaced if this were to happen.

I'll know more when we get the engine/gearbox back in the car on Thursday night. I can slip the prop shaft out of the car with the gearbox in place by dropping the gearbox a few inches if I have to have the prop shaft shortened. I do hope this is the last time I have to pull the engine to address this problem.

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PostPost by: Craven » Mon Apr 01, 2019 10:51 am

Your mechanics reference to a circlip indicates you have a later type gearbox where the mainshaft rear bearing is retained by a snap ring directly in contact with the bearing inner race, however IIRC this is then followed up by the speedo drive gear that is held in place by a second, this time a circlip.
Reading the post it just seems hard to visualise how both snap ring and speedo gear and circlip has been displaced along the shaft.
Good luck with re-fitment and a quiet ride in future.
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PostPost by: rgh0 » Mon Apr 01, 2019 12:34 pm

Craven wrote:Your mechanics reference to a circlip indicates you have a later type gearbox where the mainshaft rear bearing is retained by a snap ring directly in contact with the bearing inner race, however IIRC this is then followed up by the speedo drive gear that is held in place by a second, this time a circlip.
Reading the post it just seems hard to visualise how both snap ring and speedo gear and circlip has been displaced along the shaft.
Good luck with re-fitment and a quiet ride in future.



If the wrong retaining clip is used to retain the bearing on the tail shaft i.e. thin circlip versus thicker snap ring then the whole tail shaft can come loose as the circlip can become displaced more easily than the thicker snap ring.

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PostPost by: joe7 » Mon Apr 01, 2019 1:50 pm

Looking closely at the mainshaft note the recess for the rear snap ring is beveled toward the rear. The snap ring for the mainshaft has a taper to fit. If installed incorrectly the snap ring isn't seated properly and it will come off. I've rebuilt two where the snap ring let go. I guess either some rebuilders know this and the others that don't have a 50/50 chance of getting it correct.
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PostPost by: Harvey » Tue Apr 02, 2019 8:38 pm

Craven, Rohan, and Joe7,

Thanks to all of you for offering reasons why my mainshaft might have been pushed forward into contact with the input shaft other than a prop shaft that is too long. I will know more about this when we replace the engine/gearbox and prop shaft Thursday evening

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PostPost by: Harvey » Sun Apr 07, 2019 9:15 pm

On Thursday night a couple friends helped me fit the engine/gearbox back into the car. When we inserted the slip yoke into the gearbox extension housing the yoke had .750" exposed from the end of the housing to the square shoulder at the rear of the yoke. The mechanic who worked on my box wanted .850" of exposure and an Elan I measured also had .850 of yoke exposure. So my yoke exposure was a tenth of an inch short of that which the mechanic wanted and that which the Elan had.

Just because the outside of the yoke had .750" of yoke exposed does not mean there is that much clearance before the yoke would come into contact with the end of the mainshaft. Because the mainshaft extends beyond the extension housing and because of the way the yoke is splined internally, the yoke has only .450" of travel available before it would begin to apply pressure on the mainshaft. In theory, this should be sufficient travel because the distance between the diff flange and the mainshaft should not change during operation of the car if the diff torque rods and bushings are tight and in good shape as mine are. But, in reality does it? The fact that the forward U joint on the prop shaft which is almost new on my car was damaged means something untoward is happening to my prop shaft.

I am going to see my mechanic tomorrow and report my findings and also discuss the circlip/snap ring concerns raised by Craven, Rohan, and Joe7 above. In an abundance of caution I am probably going to have the prop shaft shortened and balanced even though the difference between my yoke clearance and that which is desired is only .100". I will report back after I speak to my mechanic.

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PostPost by: Craven » Sun Apr 07, 2019 10:28 pm

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PostPost by: 2cams70 » Mon Apr 08, 2019 12:13 am

I thought you adequately answered the question concerning propshaft length earlier. No witness marks from the gearbox output shaft striking the inside of the front propshaft yoke means the propshaft length is not an issue. What was the issue with the front U/J? Do you have any pictures you can post?
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PostPost by: Harvey » Mon Apr 08, 2019 8:07 pm

When I said I did not see witness marks on the inside of the welch plug indicating it had come into contact with the end of the mainshaft this should not be interpreted to mean the yoke did not come into contact with the mainshaft and pushed it forward. The design of my yoke is such that it will "bottom out" against the mainshaft before the mainshaft reaches the welch plug. This may be because of the way it is internally splined.

I just returned from speaking with my mechanic and he was puzzled by the fact that I had almost 1/2" of movement available to the yoke when the propshaft was in place before the yoke would contact the end of the mainshaft. He said this should be sufficient to prevent any contact between the two if the distance between the diff and the end of the mainshaft while the car is in operation remains constant. However, he still recommends shortening the prop shaft to remove all doubt about the prop shaft being at fault.

After speaking with the mechanic it appears that i misled those who have posted replies to mine. I was confusing a circlip with a snap ring. The mechanic said it was the red snap ring that was pushed out of its groove and not the circlip that sits rearward on the mainshaft. I apologize for my ignorance of what sits where and what was pushed out of place. Thanks to Joe7 for offering to send me a snap ring of the correct size.

With respect to the damaged U joint, a picture would not be helpful even if I had one which I don't. I can only say that instead of moving smoothly through one of its axis it was binding about midway through its rotation. So now it's off to the drive line specialists to shorten and balance the propshaft. I will report again when I get the car back on the road. I am hoping I can say that the car now shifts smoothly and that the chatter/clatter/rattle on overrun is gone.

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PostPost by: joe7 » Mon Apr 08, 2019 9:31 pm

If the red circlip was pushed out the more than likely it was installed incorrectly. I'm not sure what you are referring to about the snap ring rearwards on the mainshaft.
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PostPost by: 2cams70 » Mon Apr 08, 2019 10:18 pm

If you really want to be certain about the propshaft length put some plasticine or modelling clay in the yoke (not too much though) assemble things and see how much it compresses. I would very much doubt an issue with the propshaft length unless things have been changed at some time so they aren't original. Maybe see a mechanic who doesn't specialize in German cars! I have to say I disagree with the popular notion that Germans are the best mechanical engineers. I think history shows the English were better and certainly more innovative - especially in the automotive world!

Maybe someone on this forum can measure the propshaft length on another car for you or take a picture/measure the length the front yoke protrudes from the rear of the gearbox. Once it's cut you can't easily put it back.
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PostPost by: Harvey » Mon Apr 08, 2019 11:14 pm

The mechanic, whom I trust, said the red snap ring sits in a groove/recess on the mainshaft and a circlip sits behind it near the plastic gear that drives the speedometer. He read all the posts on this thread including how the snap ring is fitted. The Lotus parts house that provided the snap ring verifies it is the correct size.

Even if it is assumed that the snap ring may have been incorrectly fitted previously, I am assured it is now oriented correctly. Somehow, force was applied to the rear of the snap ring to push it out of its groove. Because I've had the engine and gearbox in and out of the car three times in the past 15 months I'm taking no chance that the propshaft might be the cause of this force. Besides, it wouldn't hurt to have it balanced in the process of shortening it.

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