Gearbox Crunch 3Rd To 4Th

[daveygravey]

I have a rather annoying gearbox crunch when changing from 3rd to 4th on my GT6 mkII Overdrive box.

 

If I change gear fast it crunches

If I change gear slowly it crunches

If I change gear in a smooth 3rd->Neutral->4th motion it doesn't crunch

 

I've got the gearbox out and on the bench.

 

Basically as I am not too familiar with gearbox internals and would appreciat some help regarding what would be making it crunch.

Visually looking at the box everything looks ok, but I suspect there is some wear?

I am guessing that it could be the Synchro Rings at fault but would like to know what to look for from the experts.

I've posted a pic pointing out the syncro rings around 3rd/4th - perhaps someone could point the wear/problem out?

 

 

It would be great if someone could point out which bits of this kit (or not) I need to change to fix my 3rd/4th crunch problem:

 

 

Many Thanks,

Dave

[Aar0sc]

Hmm. I get a crunch 1st - 2nd - could it be the same thing?

[Nick Jones]

The 4th gear synchro baulk-ring is the left-hand of the two brassy coloured things your lines point to. Normally they are the last to wear, but it may have been swapped around in a previous rebuild as this is quite a common trick. You can get an idea of its condition by pushing it as far left as it will go (using finger pressure), onto the 4th gear cone. It shouldn't go right up to it but leave a gap (30 - 40 thou from memory maybe). If it does go right up to it or almost, this is an indication that its inside surfaces are worn.

 

You should also give the tip of the input shaft (the bit that goes into the clutch) a wiggle. 3 - 4mm of movement could be considered normal, much more than this is not and means that the spigot bearing between input and mainshaft is knacked (not uncommon on these) and the excess movement produced can affect the operation of 4th gear synchro and, if really bad, actually wear the baulk ring.

 

Cheers

 

Nick

[daveygravey]

The 4th gear synchro baulk-ring is the left-hand of the two brassy coloured things your lines point to. Normally they are the last to wear, but it may have been swapped around in a previous rebuild as this is quite a common trick. You can get an idea of its condition by pushing it as far left as it will go (using finger pressure), onto the 4th gear cone. It shouldn't go right up to it but leave a gap (30 - 40 thou from memory maybe). If it does go right up to it or almost, this is an indication that its inside surfaces are worn.

 

You should also give the tip of the input shaft (the bit that goes into the clutch) a wiggle. 3 - 4mm of movement could be considered normal, much more than this is not and means that the spigot bearing between input and mainshaft is knacked (not uncommon on these) and the excess movement produced can affect the operation of 4th gear synchro and, if really bad, actually wear the baulk ring.

 

Cheers

 

Nick

 

Hi Nick,

 

Thanks for the quick reply - I've checked the input shaft and there is some play but it is minimal (1-2mm) so I think this is OK.

I've done what you suggested and pushed the far left synchro baulk-ring against 4th gear (I am assuming the largish cog on the far left is 4th gear as the exploded diagram does not name it). Here is a picture with it pushed against it while 3rd gear was selected:

 

 

I've also taken a shot of the synchro baulk-ring on the other side of the 3rd/4th gear selector for comparison while 4th gear was selected:

 

 

As you can see the far left synchro baulk-ring is very close to the 4th gear and looking at all the other synchro baulk-rings it appears to be too close.

 

What do you think?

 

Thanks,

Dave

[Nick Jones]

That looks like the smoking gun then. I suspect it was the 2nd gear synchro in a former life. There's a slightly more technical version of the process you've just done here

http://www.buckeyetriumphs.org/technical/Gearbox/GearboxOverhaul1/GearboxOverhaul1.htm The dimensions are not necessarily correct as that's a TR/Saloon box. The whole article is a good read, although again, your box has some detail differences.

 

There is also a previous thread on here where I did the same exercise with my PI box but the photos went when the forum went south.... (as it were). I'll return the photos and post a link when I can find them on my PC

 

If that is the only issue with your 'box then it cuts the work down a bit and you'll just need one synchro baulk ring. However, some words of caution on that subject.

1. There are two sizes of baulk ring. If yours really is built of original single-rail parts then you'll have the smaller ones. The single rail box ones are slightly bigger. Single rail parts can be built into 3 rail cases and quite often are as they are a bit stronger so assuming may lead to frustration.

2. The new baulk rings on the market are poor. I have no direct recent experience of the small chassis box ones, but the saloon/TR box ones come in two flavours: The cheap (not really) ones are christmas cracker quality, utter shite and unfit for purpose. The one I had wasn't even round. The more expensive ones, so called "race-quality" are alot better but still not as good as original standard parts. If you have an old box in the corner, now is the time to strip it (good practice) and check out the baulk rings. Even buy a cheap non-OD box specially (note remarks about synchro sizes though).

 

Be aware that you'll need to do alot more dismantling that it first appears. The input shaft does not just come out as the gear you refer to as 4th gear (there is no 4th gear as such in this type of box, the input shaft and mainshaft are simply locked together to give "straight-through" gear) drives the front big gear on the laygear cluster underneath and gets in the way. This means that you have to remove the bell housing and tail-shaft housing (or overdrive and adapter plate), then remove the layshaft (push through with a piece of clean dowel the same diameter as the shaft and a couple of mm shorter than the inside length of the casing. The layshaft will then drop down out of the way and allow you to tap the input shaft forward with a brass drift. Once that's out, you can swap the 4th gear baulk ring and reassemble.....

 

Or you might prefer to have someone do it though many "pros" will insist on doing the lot. Whereabouts are you?

 

Cheers

 

Nick

[daveygravey]

Hi Nick,

 

Thanks again for the very thorough reply, much appreciated.

I'm going to try and strip it all down this week, I do have another box, so I will assess the clearances and compare.

The layshaft 'dropping' should like 'fun' so I suspect it will take me some time.

 

I'll let you know how I get on/run in to trouble.

 

Cheers,

Dave