Europa Twincam spiral pins problem

[mwstobbe]

I've been a Twincam owner for about ten years now, but a problem has cropped up that I've not experienced before.  Both transaxle to U-joint yolk spiral pins have started backing out.  The pins (new pins) are about 1/4" longer than the yolk diameter and I'm careful to drive them in so that an equal 1/8" protrudes at each end.  Recently, after about a 50 mile drive, I noticed that both pins had moved to flush at one end sticking out 1/4" at the other.  I reset them, and 50 miles later the same condition happened.  I dare not drive further to see if the movement would continue, but I must assume it would.  The pins aren't loose. It takes a moderate force with a hammer and drift to move them.  I've never had this happen before.  Does anyone know what's causing this?  The only user-adustable variable is the shim pack behind the yolk, which can only be either too loose or too tight.  I have tried to set it up tight, but it's not easy to know how well you've done, so it's possible I've got it wrong. I'd appreciate some insight on this before I tear into it again.  Thanks, Mike.

[EuropaTC]

Hi Mike,

As you say, these pins are normally driven in and usually take some effort to get out again afterwards so they shouldn't be moving on their own.

My first thoughts were "do you have enough shims between the gearbox & yoke ?". 

Assuming the pins are the correct size, when I've got the driveshaft shimmed up tight it takes a real effort to get the pins in place so they won't move at all.  I don't think you can "over-shim" and I remember being told by Chris Foulds (UK Lotus specialist) that not having the driveshaft rock solid with shims would lead to problems later.

My first take would be to see if I could get another shim in the pack and try again.

Brian

[jbcollier]

Double check the shim pack as suggested.  Once that's good, drive the pins back in and trim them so both ends are flush.  Then lock-wire them them in place.

[lotusfanatic]

Double check the shim pack as suggested.  Once that's good, drive the pins back in and trim them so both ends are flush.  Then lock-wire them them in place.

I agree with JB, wire-locking them is a good shout, that should stop them migrating but also add a visual check to your daily 'pre-flight' checklist? 

[EuropaTC]

I'll don a flame suit here and say that you shouldn't need to lock wire these pins.  Why ?

Well, you've ran for 10yrs without problem. There's no well publicised problem about migrating roll pins and I've never seen it in over 35yrs with the same car. 

If the pins are moving then there's something wrong, either the wrong pin assembly or something else as yet unknown. By lock wiring them in place you might be masking a bigger problem that comes back to bite you later as it gradually worsens.

I do lockwire some bolts and things that I rarely see (trailing arm front bolt for example) but that's because of the difficulty in keeping an eye on them and how critical a failure or even loosening there would be.  But a roll pin in a driveshaft shouldn't be moving on a road car and if it does I need to find out why.

2c's worth, YMMV......

Brian

[4129R]

May I suggest using a drill bit of the same diameter as the hole in the yoke, to measure if there is any gap between the yoke and the gearbox flange which needs shimming.

A drill bit of the right diameter should go in easily, through the yoke and the diff splines.

It is also a good way of holding the driveshaft against the diff when the time comes to drive in the roll pins.

[jbcollier]

Using a drill bit is a good idea.  It should go in hard though.  You want the shim pack tight.

[4129R]

Using a drill bit is a good idea.  It should go in hard though.  You want the shim pack tight.

5mm was the biggest I could get through the hole. 5.5 would not fit. If you can get drill in 0.1mm steps, may be worth trying a 5.1.

[mwstobbe]

Thanks, all, for your thoughts.  The general consensus I'm getting seems to be 1.) this hasn't happened to anyone else, and 2.) its more likely that the shim pack is too loose than too tight.  I'm getting the impression that there's no downside on being too tight; that I should strive for as tight as I can make them.  OK, I'll see what I can do.  In future, I will safety wire them, but first I want to correct the cause, not mask the symptom.

[Bainford]

Good luck Mike, and welcome to the forum. Please report back with results once you've had a chance to attack this issue. I'm curious to know what you find. Cheers.

[mwstobbe]

OK, I've added 0.010" to the shim pack on each side, and reinstalled the pins.  It took about twice the effort to drive in the pins as compared to before, which would not be the case without an interference fit.  So I can be certain that I have an interference fit now.  I've just done a 75 mile test drive and the pins have not moved.  I believe the problem has been solved.  Thanks for everyone's help.

[EuropaTC]

Hi Mike,

Yep, that sounds about right, it should take some effort to get those pins in place.  Ten thou might not sound much but as you have found, it makes all the difference. If you had to drive them in then I can't see them coming loose again.

Well done for getting to the root of the problem.

Brian

[lotusfanatic]

OK, I've added 0.010" to the shim pack on each side, and reinstalled the pins.  It took about twice the effort to drive in the pins as compared to before, which would not be the case without an interference fit.  So I can be certain that I have an interference fit now.  I've just done a 75 mile test drive and the pins have not moved.  I believe the problem has been solved.  Thanks for everyone's help.

Hi Mike,

that must be a relief! 

Mark