Gearbox removal and fitting with engine in place.

My question is can i fit the gearbox back into my Mk 11a Commando with the engine already fitted? I have rebuilt the engine and it is back in the bike but the gearbox is still on the bench as it is jumping out of first gear and I have not discovered why yet! Thx Pete.

Hi Pete:

Suggest you search up the following thread I started some months ago:

I thought It was clutch slip, but I think it actually may be a gearbox, (1st gear) issue?​

I had a - perhaps - similar 1st gear problem with my 850 MKII, that this forum was very helpful in resolving. I also had to pull the gearbox with the engine still in place. Essentially you have to remove enough stuff so the gearbox can be rotated 90 degrees and withdrawn from the timing side. The fun part is that the rear of the engine has to be dropped some to allow the gearbox to be withdrawn!

Here is a quote from Commando expert Greg Marsh, (Greg, I am assuming you do not mind me re-posting your most helpful comments):

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If you've never taken a gearbox out there's something stupid required. The cradle is cut so it looks like it will just twist out, and it will except that the bottom rear of the engine is in the way. So, there are a few ways to do it.
1) The clutch needs to come off, and while not required it's easier with the drive sprocket off especially if 21+ teeth. Keep track of the clutch location spacer and any other spacers but you don't need to remove the clutch location circlip.
2) Remove the bottom and top gearbox bolts and notice the thick spacer at the back of the gearbox that the top bolt goes through.
3) To actually get the gearbox to twist out of the cradle, support the frame or put the bike on the side stand (i.e., not on the center stand), remove the head steady, remove the bottom and middle cradle to engine bolts and loosen the top one. Usually the engine and cradle will drop enough to get the engine out of the way. If not, you might need to loosen but not remove the front and rear iso through bolts (one front, on rear).

Greg...

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Kind regards,

James

Peter Waller said:

My question is can i fit the gearbox back into my Mk 11a Commando with the engine already fitted? I have rebuilt the engine and it is back in the bike but the gearbox is still on the bench as it is jumping out of first gear and I have not discovered why yet! Thx Pete.

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No. The bottom/back of the engine blocks the cradle cutout needed to put the gearbox in. You can remove the head steady and top and middle rear cradle to engine bolts and loosen the iso to frame bolts and probably get the engine and cradle to drop down far enough to get the engine out of the way.

Ok thank you for your replies. The engine is in loose anyway without top mount and front iso mount not attached nor bolted in. Also all the primary is off. Just need to sort first gear jumping out. Will look into the thread recommended by Britstuff.

Can you please tell me how to find the Old Britts article re shimming to stop popping out of first gear. I am in the UK and the links are all "dead"

Peter Waller said:

My question is can i fit the gearbox back into my Mk 11a Commando with the engine already fitted? I have rebuilt the engine and it is back in the bike but the gearbox is still on the bench as it is jumping out of first gear and I have not discovered why yet! Thx Pete.

Click to expand...

Yes you can get the gearbox in and out without removing the engine
My commando was jumping out of first for a while
At least I thought it was but it turned out it was not completely going into first
And it needed a new hairpin selector spring and just for good measure I shimmed the layshaft a little to give better engagement from where I'd previously fitted a roller bearing conversion
This is all from memory which is not very reliable these days but either way it worked and the gearbox has been excellent for many years since

Old Britts walks you through shimming ( rear Iso. shim) the layshaft with a new bearing , to prevent popping out of first . A new lay first bushing is critical ( delicate item ) . Good luck .

The link to the Old Britts website does not work here in the UK...do you have a working link to it please?

L.A.B. said:

Old Britts, Assembling the Commando gearbox

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Thank you!!

I thought the easiest way to move the engine out of the way was to pivot it on the upper rear cradle mounting bolt although as I own a Mk3 this isn't necessary as the Mk3 has a different shaped cutout in the cradle not that I've ever had to remove the gearbox.

L.A.B. said:

I thought the easiest way to move the engine out of the way was to pivot it on the upper rear cradle mounting bolt although as I own a Mk3 this isn't necessary as the Mk3 has a different shaped cutout in the cradle not that I've ever had to remove the gearbox.

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That works too, but if the bottom 5/16" cradle to engine is a bolt - you can't get it out easily to allow the pivot on the top bolt. If it's a stud, then it's the way to go.

I think the 72-74 had the bolt and prior to 72 the stud. He will have a bolt since it's a 2A unless someone changed it. With the bolt you must raise the engine and cradle to get the head of the bolt past the frame. I have no clue why they switched to a bolt (06.3211). When I build pre-MK3 bikes, I use the stud (06.0624).

At any rate, a silly PITA - the cradle is made for it, but the engine is in the way! At least it was fixed for the MK3.

marshg246 said:

I think the 72-74 had the bolt and prior to 72 the stud.

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Yes, note that the original bolts were not high tensile so it's likely the previous studs weren't either. The bolts became high tensile from serial 207776.

L.A.B. said:

Yes, note that the original bolts were not high tensile so it's likely the previous studs weren't either. The bolts became high tensile from serial 207776.

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The current 06.3211 bolts from AN are marked grade 5 so AFAIK, not high tensile. Where did the info on 207776 come from?

marshg246 said:

Where did the info on 207776 come from?

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From the Norton twins modification list.

L.A.B. said:

From the Norton twins modification list.

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Is that somewhere on Dyno Dave's site? Any factory info? AFAIK, there is no service release. Not saying it didn't happen, just curious.

This isn't going to be all that helpful as it's been a while and I'm not fully remembering, but I did it on my 73 Mk 1 850. I believe I had to remove one engine bolt (stud?) and lift the frame to pivot in some clearance. I do NOT think I touched the headsteady (pretty sure I did not) but as I say it's been a while. FWIW....


Given how regularly I turn to you folks when I run into trouble, there MAY be a thread on here with my travails in this regard, which might provide a more definitive answer to the question.