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Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 9:45 am
by jono
I've had a gearbox tailshaft oil leak since day one with every box I've tried.

My present gearbox had a new tail bush fitted and new seal last year but the leak persisted. I'm about to fit a new gearbox which has also had a new shaft bush and seal fitted and I want to try and cure this once and for all.

My attentions have turned to the slip yoke (the only thing which I've never changed) and on inspection this does show some wear but nothing which seems significant (no indent where the seal makes contact). However the running face where it contacts with the bush is polished and slightly ridged.

All my diff and engine mounts are good (diff fitted with TTR competition mounts) so what usually causes this oil leak from the tailshaft?

How critical is the fit of the slip yoke in the bush? I can pick up slight wear using a vernier caliper but it does not seem too bad.

I am poised to buy a new slipe yoke but have a feeling there is an underlying issue which will make this go the same way.

Any comments appreciated.

Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 10:30 am
by 2cams70
Check that the disc plug that's pressed in at the end of the yoke splines is well secured and oil tight.

Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 10:47 am
by rgh0
To some degree I think the overall sliding yoke design will always leak a little as it will be difficult to get the movement such that the lip seal is 100% effective. I dont think you find any modern gearboxes with this design due to this inherent leakage.

However to minimise the leakage

1. You need to ensure the yoke fits on the GB output shaft without any lateral movement - due to wear on the outside of the shaft spline or inside of the yoke. while not absolutely as critical as the outside fit into the sleeve you still dont want it to lose.

2. You need to ensure no significant wear on the outside of the yoke shaft and it is polished so that the seal runs and seals on the shaft properly. The seal also need to be well fitted into the gearbox tail housing with a suitable loctite sealant so no leakage around its outside.

3. You need to ensure the yoke shaft runs smoothly and is a close fit in the tail housing bushing. The bushing should have a spiral groove so that the yoke rotation ion normal forward driving pumps the oil back into the gear box.

4. You need to ensure there is no leakage past the plug in the end of the yoke end of the yoke shaft where the universal joint fits as this can be loosened or knocked out completely if the yoke is inserted into the GB to far.

The required fits are all within a few thousandths of an inch and greater clearances will result in excessive lateral movement of the yoke at the seal resulting in increased leakage.

Replacing components these days is hit and miss to get a good fit as most new components are not of OEM quality and don't have the accurate manufacturing tolerances of the originals.


Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 11:11 am
by Craven
If I recall correctly, the tail bush has an oil spiral groove and there is a cast in tail oil return channel so the design is such to provide a continual flow of oil over the yoke surface.
Oil is retained by the seal only; perhaps something is causing an out of concentric rotation pulling the lip seal out of shape.

Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 11:35 am
by jono
Many thanks for all the comments especially Rohan's usual comprehensive and informative response.

I'm going to chance a new slip yoke from Kelvedon then at least I have everything as new.

There's certainly more than just a few thou wear on my yoke and there is 'slop' when you try it in the gearbox tailshaft housing.

The new Burton bush does have the spiral oil groove.

Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 12:01 pm
by rgh0
If the bush is inserted one way it pumps oil towards the seal if the other way it pumps oil away from the seal- which way is yours inserted? I believe it was originally intended to pump oil away from the seal.

Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 12:16 pm
by Craven
In the original post, this has occurred with more than one gearbox it is unlikely to be caused by a gearbox problem.
Quote ?I've had a gearbox tailshaft oil leak since day one with every box I've tried?

Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 1:47 pm
by snowyelan
Sorry to disagree, but a spiral isn't directional. Pumping direction is dependent on relative rotation (CW or CCW). Think threaded rod with a nut, leftie loosie, righty tighty works with both ends. The bush itself may have machining that helps with oil control. I seem to recall when I replaced mine the spiral ended before it reached the seal end.

Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 1:58 pm
by 2cams70
I've never come across an extension housing seal on these boxes that doesn't leak if it's any consolation. The question is how bad does it leak! It's a concern that the replacement Burtons bush doesn't have a groove. All the OEM ones did.

Re: Slip yoke wear - causes?

PostPosted: Wed Jul 03, 2019 3:59 pm
by prezoom
The term slip yoke is a bit of a misnomer with Elans. When used with a live axle, it does indeed slip in and out of the tailshaft as the axle moves up and down. In the case of a differential being attached to the chassis, once the drive shaft is installed, slipped into the gearbox tail housing, the yoke never slips. Over time, the oil seal can wear a groove in the yoke, and replacing the bearing or the seal will never stop the leaking, because of the wear groove. There are two ways to get past the leaking. If the seal can be moved within its mounting area, adjust the position of the seal so it no longer rides in the groove. This can be a common occurrence with the output flange of a differential. Or, replace the slip yoke with a new one. I have repositioned differential flange seals so the seal contacts an undamaged section of the flange with good results. Same should apply with the slip yoke with a chassis mounted differential.

Re: Slip yoke wear - causes?

PostPosted: Thu Jul 04, 2019 3:40 pm
by promotor
This may be helpful to those interested in this thread regarding the bush and its installation.
The start of spiral / scroll of the bush (when viewed from the rear of the gearbox) sits at 6 o'clock in the housing. Just about every gearbox I have ever stripped has the spiral sat in this position.
However, the Lotus Elan manual I have states that the bush should be installed with the split (not the spiral) uppermost. This is borrowed / mirrored from the Mk1 Ford Cortina manual. Consequently, this puts the spiral at approx 10 o'clock

This information seems to me to be incorrect, and the the latest information from Mk2 Cortina workshop manuals (some use an almost identical gearbox to the Elan) explains that the bush should be fitted with the spiral start at the 6 o'clock position.

I have a new old stock gear casing (for a Ford aluminium single rail gearbox which is closely related to the Elan gearbox but not a interchangeable part with an Elan) and this came a new old stock bush installed in exactly the position as described ie 6 o'clock. In my opinion, this is the correct way of fitting it.

When viewed from the output end of the gearbox the propshaft rotates anti-clockwise.
Oil should reach the oil seal by way of the oil channel BELOW the bush - there should then be an oil bath that the prop yoke skims and in turn pulls oil into the bush which will be wound up the bush towards the front end (ie furthest away from the oil seal) of the bush.
The bush in the gearbox is exactly the same as genuine new old stock (Ford) ones I keep in my spares.
I have seen non-genuine replacement bushes and they don't fit with a proper interference fit and for this reason I won't fit them. I can't remember if they have the spiral / scroll or not.

It doesn't matter which way round you install a bush with a scroll (ie north to south or south to north) - the scroll still runs in the same direction.

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Regarding the oil leak problem - did both gearboxes have the breather bolt installed?

Re: Slip yoke wear - causes?

PostPosted: Thu Jul 04, 2019 6:03 pm
by jono
Yeah, both have the breather with the little tin hat - this was the first thing I checked and both are clear.

Interesting stuff above, many thanks.

A new yoke has arrived today and is now fitted, will see how it goes :D

Re: Slip yoke wear - causes?

PostPosted: Thu Jul 04, 2019 6:49 pm
by alan.barker
It doesn't matter which way the Bush is fitted.
Unless the spiral has been cut righthand instead of lefthand.

Re: Slip yoke wear - causes?

PostPosted: Thu Jul 04, 2019 7:09 pm
by promotor
Just found the details in the Mk2 Cortina manual :

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Re: Slip yoke wear - causes?

PostPosted: Fri Jul 05, 2019 12:39 am
by 2cams70
I'd be very interested to know anyone who has a box where it doesn't leak at least a bit here. I've always had at least a drop or two coming from here even after a full rebuild within 10,000 miles or so. Perhaps Elans are less prone because the yoke doesn't slide in and out much.