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MG MGB Technical - mk1 gearbox issue

i have a mk1 67 mgbgt and first gear is very noisy i read this is normal as its a straight cut gear is this correct, i have the chance of a gearbox that is supposed to be out of a 65 but th owner of the box seems to think it is the same as a 4 synchro box any thoughts
m gill

First gear shouldn't be that much louder than the rest of the gears on a 3 synchro trans. I replaced the trans in my '67 with another 3 synchro trans when the layshaft started to fail and became quite loud. I eventually converted over to a 4 synchro/od trans, since I was getting tired of fighting with trying to engage first gear at a stop light. The trans, from the '65, should be identical to the one in your car, unless it is a very early car with a 3 main bearing engine. The difference is the diameter of the transmission input shaft where it enters the crankshaft. The two crankshaft pilot bushings are a different diameter. RAY
rjm RAY

what are the differences to look for to check this is the correct box it has a dipstick on it does this mean anything i won the box on ebay for 20 gbp, what are the sizes of the input shafts on the different ones please
m gill

I can give you sizes tomorrow - err, later today - as I have both in the garage, although the first 1.5" of the earlier input shaft is noticably smaller. The later type is basicly the same diameter as the root of the splines.

All 3-synchros have a dipstick, as do early 4-synchros. Later 4-synchros have a filler/level plug in the R/H side.

The 4-synchro is easily identified as the side cover is basicly rectangular and affixed with hex-headed set-screws. The 3-synchro side cover is narrower at the bottom and is affixed with a mixture of slotted countersunk screws and hex-headed screws.

3-synchros do have a reputation for being noisy in first gear, although that is normally due to wear and damaged teeth. A first gear in good condition shouldn't be unduly noisy.

As the change from 3 to 4 synchro took place in '67, you could actually have either, depending on when your car was made.

Where in Notts are you? I'm due to be at QMC on Thursday and Friday, so I might be able to help with identification.
Dave O'Neill 2

"A first gear in good condition shouldn't be unduly noisy."

Indeed, it shouldn't be straight-cut.

Dipstick means it is chrome bumper, rubber bumper had fill and level plug on the side.
PaulH Solihull

First gear in a 3-synchro box is straight-cut, as it's a crash gear.
Dave O'Neill 2

The 3 synchro first is straight cut and are sliding gears. These are inherently noisier than helical gears. And most of these and their laygears are worn and are noisier than when new. The more slop the noisier. Since the time you are in 1st is pretty small it is not a big deal.

I think almost all 3 synchros unless they've been fitted with new gears and reclearanced are going to be noisy in any gear compared to a modern gearbox. I used Red Line synthetic to quiet mine down some. With the top off you don't notice the noise. With the top up it's another story.

Oh yes, the 4 synchro boxes have a helical 1st gear. The gears don't slide as the synchro dogs do the engagement. Should be relatively quiet compared to the straight cut version.
Robert McCoy

Here's an early 3-synchro (small diameter) first motion shaft...

Dave O'Neill 2

This thread was discussed between 16/05/2011 and 18/05/2011

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