16 M gearbox advice please

segt · Post by **segt** » Tue Apr 27, 2004 12:17 pm

My 1950 16 M gearbox is regularly jumping (up) out of 3rd gear. Gearbox no. is G79 M49. Is this the Burman B52 that I read about in the Tech section. Any ideas as to what may need to be checked & replaced would be greatly appreciated before I start pulling it apart. Still great riding weather in Western Australia as we approach Autumn.[8D]

silverarrow · Post by **silverarrow** » Tue Apr 27, 2004 1:00 pm

I confess to not having to touch the gearbox in 30 years on my 18S but from what you describe i would suspect the selector may be worn and therefore the gear is not properly engaged.

Does it jump out every time you select 3rd? How long will it pull in 3rd before it happens?

Les Smith (silverarrow397) [:o)]

segt · Post by **segt** » Tue Apr 27, 2004 2:02 pm

It pulls in 3rd for a varying time from roughly 10 seconds to say enough time to change up to 4 th. On average it pops out of 3rd 75 % of the time before I can change up to 4 th. Does that help. Thanks Les for your quick reply.

lawrence · Post by **lawrence** » Tue Apr 27, 2004 8:11 pm

Hi Peter
I used to be terrified of gearboxes and never got involved in their internals. Having now stripped several i believe they are a lot easier than people suggest. If you strip it carefully and examine it, the cause of trouble usually shouts at you! (I mean the worn camplate or whatever, not the wife). Make a rough sketch of the asemblies as you dismantle it and you can't go wrong.

Good luck

Lawrence [}:)]

bjork · Post by **bjork** » Tue Apr 27, 2004 10:25 pm

When you get to strip it, take a good look at the dogs on the sliding pinion for 3rd gear. As they wear they get rounded off at the corners and wear to a slight taper shape in extreme cases. A common fix if the gear it self is otherwise ok, is to get them 'undercut'.This means grinding the dogs edges (side that engages and is worn) to a slight reverse of the taper. That is to say with the dog being narrower at the bottom than the top. That way you get almost a hooking effect and the gear remains engaged positively. Originally I would guess they were parallel sided, if they wear only a tiny bit you get the problems you describe. As said by Les and Lawrence, check the selector forks and camplate as well as the camplate plunger and spring that locates the plate in the right position for each gear.Sorry about the weather, can't help with that. You can only get the real thing here![:)]

bjork · Post by **bjork** » Tue Apr 27, 2004 10:33 pm

Just to add a little extra and bump up my scores, that dodgy dog business applies to any constant mesh gear box so wether you have a B52 or CP only matters when you come to order the spares for it. Oh yes and the holes the dogs drop into need to be unworn too,with good square edges.IMPORTANT!! don't ask about the oil to use[}:)] Just get a manual and do what it says in there.[;)]So is it raining yet with you?

segt · Post by **segt** » Wed Apr 28, 2004 2:47 am

Thanks all for the good advice. I use an SAE40 engine oil made for old motors by Penrite but I have only owned the bike for 1 out of its 54 yrs. No real rain yet in southwest WA since October last year!. It should start to rain within the nxt few weeks. Steve I left the 'real weather' behind in S Yorkshire UK 1982 and can't say I miss it that much.

Rohan B. · Post by **Rohan B.** » Wed Apr 28, 2004 7:16 am

Peter,
Your gearbox should be the Burman CP box, and is from the numbers you quote.

Be worth you also checking the rotary cam in it - there is a spring-loaded pointy piece of steel (whose name eludes me !)(the parts list calls it a 163-X-2 pawl engages in gearbox camplate) that catches the notches in the camplate. If anything is amiss here then it could be causing problems. Check also that the shifter forks (with peg that follows the camshaft groove) are not bent and are in good order.

Didn't someone on this list recently mention shimming the box to prevent jumping out of 3rd gear ?

hth,
Rohan.

hth,
Rohan.

Tolly · Post by **Tolly** » Wed Apr 28, 2004 10:31 pm

That's right Rohan, I mentioned I had to make a shim/spacer to fit behind 3rd gear to stop it jumping out. It's a bit trickey to get the correct measurement first go but I used a straight edge across the face of the main body of the box when all the gears were assembled and took a measurement with a vernier guage. This shim/spacer enables the gear to fully engage. I've had no trouble since and have lost a whine in the process. I hope this helps.

Regards, Mike T.

silverarrow · Post by **silverarrow** » Thu Apr 29, 2004 12:25 pm

Wish I could do that on the second gear of my Sturmey archer box on the Silver Arrow. 2nd gear is designed to be in gear when in fact it is half ingear on two separte cogs. here are cases when 2 halves make a whole but is not one of those. Keeping second pulling securly is a mystic rather than a mechanical art.

Les Smith (silverarrow397) [:o)]