Crankcase Compatability

danfr · Aug 29, 2021

The bottom end of my 71 Commando engine has a couple holes chipped out and a small part of the case where the bottom bolt runs through to mount to the cradle. So i've began to look for a replacement bottom end. The cases are stamped 20M3S 1437XX.

I've noticed the 70's Commandos use the 20M3S as well. Would I have any issue fitment wise using a crankcase from the 70's as long as it's stamped the same?

ashman · Aug 29, 2021

Cheapest way to go is get it welded up and re drill the hole, you have nothing to lose if it don't work.

kommando · Aug 30, 2021

A good alloy welder wil sort that out but the cases would need parting and degreasing.

Fast Eddie · Aug 30, 2021

Definitely repairable.

olympus · Aug 30, 2021

Hi guys, I'm confused.....This 71 crankcase here has what appears to have the larger drain plug.
My 72 has the smaller plug???
Am I mistaken or is the attached photos showing later cases .. not original engine

Matchless · Aug 30, 2021

The large drain/screen was deleated for 72 & reinstated the next year. Don't know the exact date or engine number but LAB will probably know.

Fast Eddie · Aug 30, 2021

Matchless said:
The large drain/screen was deleated for 72 & reinstated the next year. Don't know the exact date or engine number but LAB will probably know.

‘Probably’ ?

L.A.B. · Aug 30, 2021

Matchless said:
The large drain/screen was deleated for 72 & reinstated the next year. Don't know the exact date or engine number but LAB will probably know.

Yes, the large drain plug was deleted from 200000 and reinstated on 850 crankcases but not '1973' 750 models except the very last '230' series 750s that had '850' crankcases.

http://www.classicbike.biz/Norton/Parts/1970s/Commando_Parts_73-74.pdf

Fast Eddie · Aug 30, 2021

‘Probably’ my arse !

kommando · Aug 30, 2021

Bit more complicated

Fitted up and including 71, dropped for 72 and subsequent 750;s including 73's or MK5's. Added back in to the first 850's and any 750's using 850 cases eg the 235XXX series short stroke 750.

danfr · Aug 30, 2021

Oookkkkk sooooooo no one knows about compatibility between 70 and 71 crank cases?

cheers

olympus · Aug 30, 2021

L.A.B. said:
Yes, the large drain plug was deleted from 200000 and reinstated on 850 crankcases but not '1973' 750 models except the very last '230' series 750s that had '850' crankcases.

http://www.classicbike.biz/Norton/Parts/1970s/Commando_Parts_73-74.pdf

Does anyone know the reason why the change away from the larger plug to the smaller one & then re instating the larger one again??? .... or at least have an educated guess

concours · Aug 30, 2021

ashman said:
Cheapest way to go is get it welded up and re drill the hole, you have nothing to lose if it don't work.

It will work well. A good choice for this repair.

concours · Aug 30, 2021

Is that loading/ramp damage?

danfr · Aug 30, 2021

concours said:
Is that loading/ramp damage?View attachment 81676

Not that I’m aware of. I’ll have another look when I get home this evening. I would imagine something would strike the drain plug first?

kommando · Aug 30, 2021

olympus said:
Does anyone know the reason why the change away from the larger plug to the smaller one & then re instating the larger one again??? .... or at least have an educated guess

No idea why it was dropped, but as 72's are closer to being a grenade than any other year whatever the idea was it was stupid. Confirmed by the rapid re introduction on the 850's.

baz · Aug 30, 2021

danfr said:
Oookkkkk sooooooo no one knows about compatibility between 70 and 71 crank cases?

cheers

Yes

danfr said:
Oookkkkk sooooooo no one knows about compatibility between 70 and 71 crank cases?

cheers

Yes they are compatible

jbruney · Aug 30, 2021

Don't you just love the wandering tangents?

olympus · Aug 30, 2021

kommando said:
No idea why it was dropped, but as 72's are closer to being a grenade than any other year whatever the idea was it was stupid. Confirmed by the rapid re introduction on the 850's.

Interesting... and the reasons being??

L.A.B. · Aug 30, 2021

olympus said:
Does anyone know the reason why the change away from the larger plug to the smaller one & then re instating the larger one again??? .... or at least have an educated guess

I'd have to take an uneducated guess that if it wasn't done to save a few pennies or increase the efficiency of the scavenge by moving the pickup further forward (that had the opposite effect at high engine speeds) then I don't know as there doesn't seem to have been an official explanation for it that I'm aware of.
I'd guess that debris now unfiltered (cam thrust washer tangs etc.) being drawn into the scavenge pump and the damage it caused was the reason for its reinstatement on the 850 crankcases which have both strainer plug and drain bolt.

Crankcase Compatability

danfr

ashman

kommando

Fast Eddie

olympus

Matchless

Fast Eddie

L.A.B.

Moderator

Fast Eddie

kommando

danfr

olympus

concours

concours

danfr

kommando

baz

jbruney

olympus

L.A.B.

Moderator

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