Cracked main shaft

[spitfiredriver]

I broke my spitfire 3 rail gear box last week and after stripping it down I have cracked the main shaft.

I have read a few posts about uprated  mainshafts but do not really know what else I will need to change. I also have a J type overdrive.

It is a 1300 with a tr5 profile cam and fuel injection. The car was going really well but the Borg and Beck clutch did struggle if I was driving the car hard. Not sure if I should just rebuild it as standard or upgrade, I need to buy a new mainshaft anyway.

Any recommendations?

 

[Nick Jones]

5/8" mainshaft tip size (as single rail gearboxes) would be a worthwhile upgrade and I think it's possible to buy new mainshafts like this for only a small extra cost.  This would mean either getting your existing input shaft machined out to suit or (I think) using the input shaft from a 1500 single rail box.  The latter will need a fine spline clutch plate (late Toledo/Dolomite 1300).  Also worth noting that the later single rail gearbox input shafts had oil holes drilled from the root of the gear teeth through to the pilot bearing bore so that the meshing action would provide forced lubrication to the tip bearing.

Regarding the clutch..... I supposed using a 1500 flywheel and clutch is one option, though heavier and probably wrong bolt sizes.  Alternatively you'lly need to see if anyone still does and uprated cover.  I've had good service from Helix in the past - not cheap but good quality.

Nick

[GT6Steve]

We have repaired that in the past.  First time we used a hardened bearing sleeve to mpress over the turned down tip and it worked very well.  If your's is actually cracked it may not be viable  Next time we bored the shaft and pressed in a dowl pin which also lasted fine.  I think I have a drawing some where of the first solution if you have a machinist you trust

 

Finally, I have a drawer full of mainshafts for Spitty's (I recall) if it's worth the shipping from the US.  a hundred Dollars I'd estimate.  I have tons of Spit gearbox parts...sadly ;-)

 

[RedRooster]

Had a similar problem with mine due to lack of lubrication, ended up getting the machine shop to fit a bronze bush & drill an oil way, its on the old CT site under 'lost 2nd gear' if those threads are still available?

RR

[RedRooster]

Can't see that on the CT site, but did manage to find an old photo

[Nick Jones]

We did something a bit similar with our Spit 'box (3 rail 4 synchro with D-type) using an oilite bush.  It's still working but it wasn't intended as a very long term solution and the engine is pretty standard.  Story here

http://sideways-technologies.co.uk/forums/index.php?/topic/7107-chriss-mkiv-basket-case/&page=3

 

[spitfiredriver]

Thanks for the advice.

After looking at another gearbox main shaft It looks like the one that has cracked has been repaired before. The bearing surface ( Tip) looks like it has been replaced before.

[oldtuckunder]

On 6/1/2018 at 6:49 PM, GT6Steve said:

 Next time we bored the shaft and pressed in a dowl pin which also lasted fine. 

 

I used the oilite bush pressed onto turned down tip approach many moons ago which worked fine. However I like the bore and dowl pin idea, will try and find a brain cell to log that one in for future reference.

Alan

[Nick Jones]

16 hours ago, spitfiredriver said:

Thanks for the advice.

After looking at another gearbox main shaft It looks like the one that has cracked has been repaired before. The bearing surface ( Tip) looks like it has been replaced before.

I'd think that means time for a new mainshaft.  While the tip can be replaced by boring and inserting it has to be done just right.  I've seen the aftermath of one that wasn't.  Bucket of shrapnel just about describes it.  The case might have been savable........  And with the new mainshafts costing under £200 I doubt a repair would be much less.

Nick