Anything and everything GT6.

Clutch, gearbox, overdrive, propeller shaft.

Whilst paying a visit to my local friendly workshop, he has the gaerbox of a local members GT6 disembowled on the bench (a'noon Don!)

He quizzed me on the mainshaft, which appears to have had a new tip press fitted into it's end. It is a 3-rail + J type.

I assume this is because they break off, but is this a pukka fix?? Or is it a bodge of epidemic proportions.

It is a bit loose which was also worrying, you can turn it with your fingers.

Any ideas??

Colin

I hope you paid due defference to the soon to be replaced mainshaft! Bob told me all about it the other night, still, at least I'll have a gearbox and OD that I can thrash about.

Are we there with the 3:63 Colin?

there are more than enough parts there, though I shouldn't have dismantled the casing donor diff to find out what was wrong with it, I forgot to mark it up so it could go back together the same way.

Bob said delivery to the meet 3rd Jan.

I guess you are a few quid lighter over the main shaft now........

Colin

Yeah - I was going to get Sarah an engagement ring but that will have to wait now!

Colin,

The example you mention is most definitely a bodge! The tip of the mainshaft is supported by a needle roller bearing in the rear end of the input shaft. This arrangement is poorly engineered and the hardened surface of the mainshaft tip breaks up. I'm not aware that they actually break off very often, but I know this has happened.

It's the condition of the mainshaft tip that usually determines whether the shaft can be reused when rebuilding the 'box, as the rest of it is generally ok. In my experience, the tip is always knackered! There have been difficulties obtaining replacement mainshafts in the past, and one trick is to replace the tip on a used one. This can be done successfully, but there are some potential problems. Usually, the end of the shaft will be accurately bored out to take the replacement tip as an interference fit. The tip must be absolutely aligned and fixed properly. One that can simply be turned by hand is a complete disaster, as it will simply run in the shaft and become progressively more sloppy. Occasionally, the tip is such a tight fit that the end of the shaft cracks as the tip is fitted or soon after. The crack can be very hard to see with the naked eye.

One advantage of a properly retipped mainshaft is that it's possible to specify a more durable material. A few years ago, John Kipping had some new mainshafts manufactured. I fitted one of these when I last rebuilt my GT6 'box and it's been fine so far...

I wonder who's 'box it was that you saw in the workshop. They should be warned!

Their box is receiving a brand new mainshaft, don't worry about that!

It looked a very professional job, that may have just subsequently failed.

Colin

Hi Guys,
I had a gearbox rebuilt many years ago by a company based in Hounslow. They replaced the tip in the mainshaft with a larger diameter tip which was pressed in to the bored out end of the mainshaft. The input shaft was also madified so that a larger needle roller bearing could be used. A friend of mine with a Vitesse 2l also had one of these gearboxes. The problem was that the new larger tip would as in the case of this post gradually come loose and in the case of mine and my friends gearboxes made it very difficult to change gear. Although the supplier repaired the boxes I was never very confident about them. I sold the car soon after.

Interesting thread this.
A long time ago in the 80's My Gt6 suffered the same problem in that the mainshaft tip became badly chewed up due to the break-up of the needle bearing. I managed to source a replacement needle bearing which had an internal and external sleeve. The outside dia. was the same as the original but the bore of the bearing was of course less than the dia of the mainshaft tip. I had the tip ground down to accept the new bearing and reasoned that although the case hardening would have been removed the bearing would now be running on its own outside and inside sleeves. There is no loading on the bearing and it only acts as a location allbeit running at high revs. The bearing inner sleeve was a good 0.0005" interference fit and also located with loctite bearing fit. My ony concern was for lubrication since the original was splash lubricated and the new bearing was sleeved. I refitted the gearbox with this modified mainshaft and bearing and had no further problems for 5 or 6 years until I took the car off the road for long term lay up. I rebuilt the car in 2003 but opted for a new gearbox and overdrive. I still have the original modified box.

Geoff

Geoff

Did you get the inside of the input shaft ground out as well??

Or did you find a bearing that fitted inside it??

I found info in a Courier from about issue 90 odd giving instructions and have done that with my own spitfire box.

Colin

Colin
The bore of the input shaft was not in good condition after the original needle bearing failed, but not as bad as the mainshaft tip. The bearing I sourced (INA I think) had the outside sleeve nearly the same dia as the hole in the input shaft , but slightly less. Anyway it fitted. I did consider resizing the hole to accept a larger bearing and therefore less reduction in the mainshaft tip dia, but the machine shop where I worked had no facility for internal grinding and most of my favours had been used up so I opted for the best bearing size to fit the hole. I located it with loctite and left it for days. It was a good enough fit to be true in the bore and the loctite appeared to hold it. As I said I refitted it and ran with it for some years without apparent problems. Someday I may strip the old box and see if it's still OK.

Geoff

Section : Gearbox

Is this a bodge or a pukka repair.....

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