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Identifying M20 v M21 V8 Gearboxes Externally?

Started by _LXChev366_ , Jan 06 2010 07:06 AM

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7 replies to this topic

#1 _LXChev366_

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Posted 06 January 2010 - 07:06 AM

as the topic heading says, how can you tell one from another?

TIA

Pete

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#2 yel327

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Posted 06 January 2010 - 07:41 AM

Often the grooves on the input shafts helps but not 100% correct. As the boxes are identical bar a few parts the best bet is to pull the side cover off and read the cluster gear numbers. If it's a 6cyl it'll be unlikely to be an M21 unless someone has made one up.

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#3 76lxhatch

That was easy!

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Posted 06 January 2010 - 08:30 AM

You can always put it in gear and count the turns to find the ratio if its on the bench

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#4 _Mid Life Crisis_

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Posted 06 January 2010 - 09:09 AM

I have had the same question and it appears not to be an easy question to answer, have a read of this http://www.gmh-toran...showtopic=41812 Thread. It might help.

Cheers

Dave

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#5 _LXChev366_

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Posted 06 January 2010 - 09:38 PM

Thanks Dave and others...
it looks like I have a M20, as it has one ring. I will slide the yoke in the back of it and do a ratio check to see what it is tomorrow. I was too busy today.

Thanks all.

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#6 yel327

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Posted 07 January 2010 - 08:25 AM

It doesn't really matter unless you want M21 ratios as they are all worth about the same, except maybe a V8 box worth a little more for the longer front shaft cover. The bits that wear and normally need replacing when they are as old as they are today are the main bits that are the difference between the boxes anyway (input shaft and cluster). You could turn your M20 into an XU-1 ratio M20 by simply changing the input shaft and cluster with new repro bits, or into an M21 by changing out 3rd gear as well.

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#7 _LXChev366_

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Posted 07 January 2010 - 11:21 AM

I did a revs count on the gears and I come up with,
1 = 2.5
2 = 1.8
3 = 1.5
4 - 1.0
so it looks to be an M21.

I just bought an LX car and it came with the box (not fitted) and he wasnt sure what it was.

Thanks again

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#8 yel327

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Posted 07 January 2010 - 01:34 PM

Sounds like an M21. Funny thing is most V8 boxes I find nowdays are M21. There are about 15 in the shed and they are all M21's bar one from memory. Every $50 V8 box I find at swap meets seems to be an M21, and the last couple of 6cyl boxes have been M22's.