Advice needed on making a one piece shaft for UN1

[Chris Hale]

Hello all,

Well I was out driving the GT on Monday night and I managed to break my gearbox input shaft again for the second time, this time it was the second motion shaft that has let go.

The gearbox itself is a Renault 21 Turbo UN1 – 13 and the engine it’s hooked up to is a Chevy 355 putting out around 430 HP and 430 TQ (has not been on the dyno yet), and at present I have three options open to me.

1) – Replace the broke second motion with a standard Renault one, but this is not fixing the problem and it’s only a matter of time before it happens again.

2) - Replace the main input motion shafts with an upgrade one piece shaft that is available from GTO / Derek Bell or Chris Cole, but to be honest I cannot justify the cost.

3) - My third option and the one that is most appealing to me is to make my own one piece shaft, I have been talking to a company that can make me one for a fraction of the cost that I can buy one of the other kits for, but this would only have the standard gear ratios and not the taller 1st and 2nd that the other kits have, but this is no big deal for me also I can have it made to the length I require and the right spline for my clutch plates.

I’m looking at making the shaft from EN36 steel and case hardening it to 0.030” the main idea when getting this shaft machined is to eliminate any weak points that exist on the original Renault shaft i.e. the collar that joins the first & second motion shafts & also to reduce the depth of the oil grove on the five gear as well eliminating the step just before the fifth gear spline, I have marked these on the picture below.



I would appreciate any ideas and advice that anyone could give me on any further bits that I may have over looked.

Best Regards,

Chris.

[Cliff Beer]

Chris, you obviously know what you're talking about so I don't have much to add except one thought that might be helpful - a lot of the hardcore porsche racing guys swear by cyro-hardening as an improvement to the durability of critical metal parts - take a look here at one of the vendors for this process: Cryogenic Treatment, Cryogenic Processing, Cryogenic Metal Treatment - Cryogenic Institute

[Andy Green]

Hi Chris,
As Cliff has pointed out, you’ve summed-up the issues around the main shaft for the UN-1 transmission. You’ve picked up on the weakness around the 5^th gear under-cut, as I’ve seen these break on Esprits here before. However with this point addressed it would be worth having you 5^th speed gear teeth shot-peened to improve the fatigue resistance, as stripped teeth are common on high mileage/torque transmissions, if the shaft doesn’t fail first!

This company have a good reputation and more importantly have a UK operation.

http://www.metalimprovement.com/index.php

Regards

Andy

[bigtommy79]

Its only my opinion, spend your money the way you want but i think you'd be better of buying a new shaft from Chris Cole or Tornado. You know you would get something that would work without getting all the hassle of getting new shafts turned up and cryogenically treated etc, you would have a modified 5th gear and they are tried and tested. Whats the cost of getting a shaft turned up anyway??

[40 Bud]

If you have not read the history of Derek Bells one piece shaft development, you should. He had trouble with the first design. The material,on paper, was very strong. In the real world it turned out not to be able to handle the torsional shock loads. He tried another material and treatment and had success. I believe he spelled out some of the difficulties in an old GTD club article.

[BruceLambo]

Well Chris, I had my one peice shaft made for one of my UN1 boxes. I decided the price was too expensive to get the Bell one and also they wouldnt make it longer than what I needed either. Mine was made for less than a 1/3 of the price of the Bell unit.
Yes get the right steel and the right heat treatment. I could have had any ratios made for the 1st/2nd gear that I wanted. Heres something though that you may not have thought about when making the shaft bigger. The bearing centres are actually machined out! If you machine out 1.5mm from the bearing centre, it gives you a 3mm larger diameter for your shaft etc. The orginial shaft is 24mm by the 5th gear, not including the oiliing undercut or step for the bearing. Mine is now 27mm minimum over the whole shaft between the bearings. Also remember you will either have to get new gears made for 1st and 2nd on the output shaft as well. Mine were done with welded billets. They machined off the teeth then welded on a new billet, cut the new teeth then heat treated them. Works fine.
Where did your output shaft break then? Was it the 5th gear at the end or somewhere else? What were you doing when it broke?

[Mark McCaughran]

Oh no......I have that sinking feeling...........

My gearbox is at Chris Coles right now - as the input shaft wasn't long enough to make it into my spigot bearing (I didn't know this as I didn't put it together the first time), so when the car was running it chewed my input shaft splines, trashed the second motion shaft and my thrust bearing. I'll need to check with Chris to see if my shaft is a one-piece - he didn't actually mention this but I am getting a new one as part of the gearbox repair. Is this only likely to go on higher output engines (don't laugh, but I'm only about 200bhp, but I will want to upgrade to FORD V8 in the future, hopefully around 350bhp).

Also, is it worth having the taller 5th gear mod done? It's about £500 to have it changed and Chris tells me it will take me from 3.88 to 3.44. I haven't driven the car yet, so it is hard to judge whether I should go for this or not.....can anyone give me some advice?

I guess with fuel prices the way they are it will pay for itself in a few miles! There is a garage over here charging £1.99 for unleaded.....aaaaarrrggghhhh!