Gearbox woes

[Albervin]

After a full engine rebuild where the engine and gearbox were re-assembled using 99.9% of the same parts, I experienced a gearbox problem. The rebuild was due to a leak between gearbox and engine so no real parts problems. The only new parts were gearbox seals and bushes. This is what I found after a graunching noise became an issue. The G16 and G 17 bushes (new!) were destroyed. The G16 in particular was a mess and it looks like it was caused by G17 binding. What you see was a perfectly clean gearbox 250Km ago. So, a question; what clearance for the bushes? I thought 2 thou was enough.

 

[Len Matthews]

These bushes should be Oilite. I had the same thing happen when PB bushes were supplied.

 

[Bill Thomas]

Hello Len, How can you tell one from the other ? , Cheers Bill.

 

[Len Matthews]

PB has a solid appearance and can be machined whereas Oilite is porous and cannot (or shouldn't be).

 

[A_HRD]

Years ago, the late Dennis Phillips of Mid-Glos had his Twin gearbox bushes go twice in very short order. It transpired that his mainshaft was bent from an earlier 'incident' and the wobbly motion just ground away the bushes. Might be one to check…..

I too prefer oilite bushes in this hard-to-lubricate area.

Peter B
Bristol, UK.
STILL WANTED: Series 'A' Front Frame

 

[davidd]

Albervin,

I would think a clearance issue would have manifested itself in heating or overheating the parts first. Your clearances sound generous, which they should be with solid bronze. Peter's scenario seems more likely. It seems there was some high misalignment causing the grinding action.

Even though Oilite is an oil impregnated sintered bronze, it is often machined. It is available in rods, tubes and plates for machining. It should never be ground or honed. As long as the tool is sharp and the cutting speed is correct you will be OK. If you wash out the oil with solvent or cutting fluid, you should re-introduce oil with a vacuum. I have never had to do this, but I would think a Mighty-Vac brake bleeder and a heavy plastic bag would do. Oilite gives the machining procedures below:

https://www.oilite.com/bmp.asp

David

 

[Albervin]

As the bushes are in a very oily environment (gearbox) I didn't think oilite would have been necessary. However, I will defer to the experts on that. The shaft is certainly going to be checked for straightness but it worked fine over the previous 13 years.

 

[clevtrev]

If the gearbox were shimmed up properly, there would be miniscule oil going through a five thou. gap.

 

[davidd]

As the bushes are in a very oily environment (gearbox) I didn't think oilite would have been necessary. However, I will defer to the experts on that. The shaft is certainly going to be checked for straightness but it worked fine over the previous 13 years.

Yes, the indicators you mention are tough to explain. However, I think this has been a troublesome area for others.

Phil Irving recommended the Oilite bushing in the street bikes. For racing he was concerned that the sintered bushings would not survive the loading. I remember that Bill Hoddinot and Sid rebuilt the Black Lightning Cooper car driven by David Cooper. It all worked well, except they replaced the bushings in question with solid bronze items and they failed the first time out despite their efforts to channel extra oil to the area. Additionally, Dave Mattson said these bushings only lasted one trip to Bonneville. He replaces them after every visit.

I am not an expert on twin gearboxes, but if I look at Trevor's post above, I think I can see why the Oilite bushing was recommended by Irving. It may be that the shaft developed a wobble that did not damage already worn bushings, but could not stand the constraints of the new bushings.

David

 

[Bill Thomas]

After my Lock up, My fault, Lack of oil, I did not like the setup, So I ground a couple of nicks on the end of G4 to let more oil on to the bush, I think I have of other people doing the same, What are your thoughts Trev, Cheers Bill. I also use a thick gear oil.