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MG MGB Technical - 2nd Gear Synchro Questions

1. Why do the brass 2nd gear synchros wear so fast on the 3 synchro gearboxes? The ears especially wear down.

2. Why don't I get the nice detent click when going into second? Third and fourth are a pleasure with a very positive feel. Gearbox never jumps out of gear and is reasonably quiet for a 3 synchro.

My 65 roadster has the early version gearbox with brass 2nd synchro and 3 bearing laygear. Years ago I replaced the layshaft, bearings, seals and 2nd synchro. On teardown the 2nd synchro ears had worn down significantly. The friction surfaces seemed okay. 3rd and 4th synchros were fine and reused. Of course the layshaft was worn. Clearances were a little out of spec but with selective washers unavailable there was nothing for it.

Less than a year after the rebuild the 2nd synchro became very marginal especially on rapid upshift or any downshift. I've sort of learned to work around it with double-declutching and slow upshifts but I still crunch now and then.

The steel synchros are available but Moss lists the appropriate improved gear not available (NA). In fact no individual gears are available unless through sources like eBay.

The replacement synchro came from Moss.

I'm planning a rebuild after the summer driving season so I am just fact gathering now. My car doesn't get a lot of miles but they're fun miles here in Northern New England.
Robert McCoy


I don't know if this is your problem, but you did mention rapid upshifts from 1st to 2nd. Of the several British sports cars I have had, I always felt that the gearboxes do not take well to rapid shifts.

I think that all shifts, up or down, should be a two-step process. By that I mean you shift from first to neutral, and then from neutral to second. The pause is only a fraction of a second, but without the pause to "clear" the gears, it feels like abuse to me. This has nothing to do with the age of the gearbox. I remember my father explaining this to me before he let me drive his brand new 62 midget.

I have a Spridget that I built for autocrossing, and I did rapid shift it from first to second. It did not take but a single season to loose the detent feel into second.

C R Huff


I tried to avoid thrashing after the rebuild. When I wrote rapid upshifts I meant what would be considered normal for most cars of that vintage...not a drag racing type shift. Over the years I've learned to be even more deliberate as you describe. One of my old shop manuals says if you're beating the synchro you can delay a rebuild by slower shifting. Guess that's what I'm doing.
Robert McCoy

Well then, it sounds like abuse wasn't your problem. In that case I don't know the answers so I'll just watch with you to see what others say.

C R Huff

It is, indeed, wear on the second gear that is causing premature failure of the synchro ring. Being slightly worn, it allows for the synchro to engage at a later stage . This eats up the synchro at an alarming rate. Your best bet is to look around for a good used trans that you can use for spare parts. RAY

Thanks Ray,

What should I inspect for on a used second gear?

The gear I reused didn't seem to have wear on the cone portion. At least I didn't feel any grooves. The surface was smooth and shiney as I remember. The gear teeth were okay and the engagement dog teeth were okay. I didn't disassemble the synchro hub as I feared getting it together again.

I did get a lot of brass wear bits in the first oil change which I found disconcerting. Sort of like the synchro didn't match the gear and had to be worn in. There has been some talk in the archives about bedding synchros.

Maybe a 4 synchro swap would be a long term solution.

Robert McCoy

What today's drivers all cosnider to be normal shift would constitute unusual punishment on old British cars.

Snick...snick. The first one out of the gear the next one into the next gear is what is called for.
Bill Spohn

Hi Folks:

If you do a 3 synchro rebuild, use the steel synchro and the improved 2nd gear. Parts are available if you look around, e-bay, Chris Betson (UK), Auto Gear (UK), Brown & Gammonds (UK), etc. Also try John Esposito at Quantumechanics; located in Monroe CT, John has been rebuilding these gearboxes for many years and has assorted parts for sale.

It is very important to have correct clearences between 2nd and 3rd speed. I have had a few assemblies that couldn't clean up with max size thrust washers; so I had the interlocking ring (brass/bronze) flame sprayed at the aerospace company where I was employed (now retired) to build-up tolerances. Flame spray service is very hard to find now days.

Try to buy NOS parts, and stay away from the aftermarket junk that is now being sold. For tech info. check Barney Gaylords MG Guru web, and University Motors videos.

Good Luck: Rich Boris 67 B roadster
Rich Boris

Swithing over to a 4 synchro od trans in my '67 B was one of the best mods that I've made over the years. Almost no alterations were needed. The other great mod was the addition of a supercharger. RAY

This thread was discussed between 03/06/2009 and 05/06/2009

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