Unhappy First and reverse, 2synchro gearbox

[Graham O]

I've done my first ever gearbox re-build and it seemed to go well except that first gear and reverse are not at all smooth - you can feel each tooth moving or rubbing against something.
This is one of the boxes that has shims on the 1st/2nd selector shaft, and when I dismantled it there were no shims at all.
Apart from a general clean out and new bearings, the only changed part was the reverse gear wheel from another gearbox as my old one was going through the hardening.

I put a shim in the laygear to bring the float on the layshaft to be 5 thou.

I adjusted the 3rd/4th fork to put the selector mid way between the synchro cones and then made up a shim to bring the 1st/2nd forks in line.  From memory, the shim was about 20thou.  I then adjusted the reverse fork to match.

Initially first gear was at times locking against something, but after I added the oil it does not snag so much, but clearly (and audibly) all is not right.  My motor and the rest of the car are far from ready for a run on the road, so what I am reporting is what I notice when turning the first motion shaft when the gearbox is set in first or reverse. All the other gears select ok and the box feels really smooth. 

Any suggestions please! I'm an electronics engineer, so mechanical stuff is delightfully new to me.

[Terrytuned]

If it has a shimmed 1st/2nd selector rod it is a late 3syncro box, one of the bolts in the rear cover is shorter as it will foul the reverse gear if a longer bolt is fitted check this first as it will not allow reverse gear to go fully back. Terry edit, I have also had a replacement reverse gear that was a tight mesh with the original gears.

[Graham O]

Thanks Terry, I tried backing off all bolts on the rear in turn, but nothing changed.
It could well be a tight match.  I can feel each cog struggling to mesh or rub against something.

[Chris KC]

I don't have the immediate answer to your question Graham but I would urge you to investigate even if it means taking it all apart again - don't run a 'tight' box.

I rebuilt a 4-speed over the winter and it felt horrible ("crunchy" was the word that came to mind) when first assembled but with a bit of persistence I got it sorted and now have no qualms about running it.

[Bob Culver]

Hi Terry
Did you persist with the "tight" gear? Was it evident in use?

[Terrytuned]

This is the rear cover bolt that can foul reverse gear and no Bob it was a very tight mesh the second gear I fitted was OK. I have had a set of close ratio 1st motion gears that I had to relieve by running with grinding paste in a spare case.  Terry.

[daveg]

If you used new bearings did you use the cir-clip of the old bearing it is thinner than the one supplied
with new bearings and not changing it can cause the symptoms you describe.

[Chris KC]

I'm just reading back what you said you have changed Graham.
I guess one obvious thing to try (bit of a pain though) is popping the original reverse gear back in, assuming you now have it back.
Have you tried adjusting the selector position back and forth? It can be a bit of an art finding the 'sweet spot' where nothing drags.
Have you checked the layshaft end float again since re-assembly? The shim is one thing but presence & correct thickness of gaskets is also an influence (as others have observed, the carefully planned clearance has a habit of mysteriously disappearing once it's all done up).
Whether these things could cause the symptoms you are describing it's hard to say, but they are nonetheless worth checking I suggest. Difficult to diagnose these things on the telly I'm afraid.

[Graham O]

Many thanks Guys for your suggestions. It is getting clear that I will be taking it apart to check things out.
First, I must give a BIG TICK to Terry for his suggestion about the danger of a long bolt.  I was convinced that it was not a problem as I backed off all bolts in succession and there was no change to the poor mesh of 1st and Reverse gears.  HOWEVER, I was impressed with the photo as it reminded me that there was a recess for the idler gear and so the bolt has to be shorter that I thought.  I removed that bolt and bingo- it had been interfering with the idler gear and was now shorter.  See photos of the size of bolt initially, the one that has been trimmed by the idler and the type I finally put in it's place. Thinking back, I was having problems with the box completely locking up, but that problem went away.  I guess it stopped when I had finally ground off the tip of the bolt, and I was left with a noisy gear set.
By the way, Dave, I had used the old circlip on the replacement bearing, and the laygear float came out as planned.


       

So I am now left with gears that are a little happier and I have tried adjusting the position of the idler by adjusting the reverse selector.  They now  go through a cycle of relative quietness to becoming a bit "coggy". Definitely not as smooth as another rubbish gearbox I have.

I washed out the gearbox to flush out the bits of bolt and got a small amount of fine steel dust as expected.  However the additional bronze/brass dust and minute filings I also got was a shock.  Looking at the selectors there did not seem to be any additional wear to see, and all I can now think of is that I had to stake two of the synchro cones to lock them back in position.  Maybe I made cones swell oversize and they are now being trimmed to size.

So it may be that I have three problems to check out and their aftermath.
- oversize bolt  - fixed
- possible mismatched or damaged reverse idler- try the old one and check if the new one was damaged when it was in lathe mode.
- synchro cone distortion - inspect.

I also need to clean it all out thoroughly to make sure that bearings etc are clear of any of the bolt shavings. Check that my bolt did not bend the idler shaft or reverse selector.
Then re-build.  Electronics seems a lot easier...

[David.H]

Pedants rule! ..... The shorter one you are using is a set screw (threaded to underhead) The others are bolts with plain shanks which may be intended to laterally locate better in the hole than relying on thread crests....Just a thought.
Great result though, I have not attempted any delving into gearboxes, they seem a bit magical to me...
David