crankshaft to gearbox distance

Hi All / Bonjour,

Can someone tell me the distance between the crankshaft axle and the gearbox mainshaft in case of "fully front" and "fully back" positions of the gearbox on its cradle?

Not sure I'm quite clear, but hope you see what I mean.

Many thanks!

Forgot to mention: on a 750 Commando, not a 850 (don't know whether it matters...)

I don't have the numbers you seek, but your explanation is crystal clear.

laurentdom said:

Hi All / Bonjour,

Can someone tell me the distance between the crankshaft axle and the gearbox mainshaft in case of "fully front" and "fully back" positions of the gearbox on its cradle?

Not sure I'm quite clear, but hope you see what I mean.

Many thanks!

Click to expand...

Just wondering, are you having trouble with belt drive fitment?

For what it's worth, I recall an issue I had some time ago. The stock belt was 890mm and I could not get it on. I purchased a 920 and could not get it to tighten up. I found that with the second gearbox adjuster, I was not getting it all the way back to the end of the slot. The 890 was right and the 920 was returned.

Not that this is your issue, but you leave us to assume.

I am in the process of fitting up my primary. I don't have time this morning but I can get those measurements this evening. Send me a PM if you don't get 'em in the next 24 hours.

Russ

Maximum travel with a bolt through the top of the gearbox but no adjusters in place is a half inch. Minimum distance, center to center of the crank to mainshaft is 9-1/8. Maximum is 9-5/8. Or about 232 mm to 245 mm.

Hope this helps. The bike is an 850, but unless there were changes in the cradle cutouts the distance should be the same, as the inner primary is the same for both engines. Now, if you need these measurements with the primary chaincase mounted on the bike, you will have to wait for tonight!

Russ

Thanks a lot Russ!

It will really help me in sorting out the problem I have.

Not (yet ???) one with the lenght of the belt, but with the primary chaincase.
I'm trying to
a) Fit a 750 Commando engine into my slimline featherbed frame. Not really straightforward, but well documented and thks to good advice got from this site, it's progressing well
b) Re-use the Dominator primary cases, that I'll have to re-work a lot. That's the reason why I need that dimension to decide whether it wouldn't be easier to use a Commando' primary case if I can still use the Dominator footrests.

So many parameters to combine! I was not prepare to that chain reaction: Engine -> Gearnox > Prmary Case -> Belt -> footrests .... but not inclined to give up anyway !

Glad I was able to help. It sounds like a great project.

Russ

Hi, a Commando engine fits straight into a twin Slimline frame. Use Atlas or 650ss engine plates, eBay, standard head steady on the rear bolt thread. Then all the normal Atlas Primary chain parts fit. You need the Atlas Stator Carrier, and Atlas balance factor for the crankshaft, the Commando crankshaft vibrates too much. You can use the commando box with Atlas primary shaft, or use the commando primary drive, and space out the tin chain case. Have fun. Best regards, Paul

The chains and sprockets on my Seeley come in two sizes - too big, and - too small.

jagracer47 said:

Hi, a Commando engine fits straight into a twin Slimline frame. Use Atlas or 650ss engine plates, eBay, standard head steady on the rear bolt thread. Then all the normal Atlas Primary chain parts fit. You need the Atlas Stator Carrier, and Atlas balance factor for the crankshaft, the Commando crankshaft vibrates too much. You can use the commando box with Atlas primary shaft, or use the commando primary drive, and space out the tin chain case. Have fun. Best regards, Paul

Click to expand...

The Atlas engine is vertical, the Commando sloped as we all know, but the OP does not say which way he intends to mount the engine, if he chooses the Commando route obviously Atlas or 650SS plates won't be much use to him. And he will be able to stay with Commando shaft centres. I am pretty sure someone out there makes Commando to featherbed plates, they should be as easy to come by as Atlas plates these days.

For info, the Rickman frame was designed originally to house an Atlas engine, vertically. I can tell you from measurement on a Rickman you can readily set shaft centres at 260mm, somewhere around middle of adjustment range....so I would expect Atlas centres to range around that....

Discussion of belt lengths can also be misleading as well since that also depends on the primarydrive ratio you choose to run, how many teeth on each pulley, and you will need that data and the centres before you try to calculate belt length...

Original Commando triplex chain ratio is 57 over 26, 2.292:1, belt pulleys range from typically 36 to 40 and drums typically 68 or 70.....with much more variation available if you want it.....

jagracer47 said:

Hi, a Commando engine fits straight into a twin Slimline frame.

Click to expand...

That is only partially true - the later ones with the edgy sump casting from 1973 onwards do not, I just tried this a few weeks ago with some crankcases and both Slim- and Wideline. The lower oil gallery casting or whatever that is fouls the lower frame tube when used with Atlas engine plates. It either needs to be mounted a few mm higher (which might lead to problems with the head, I did not check this) or be mounted at an angle. Up to 1972 cases are bolt-in.


Tim

I intend (and will) install the engine tilted forward by 15° as in a Commando chassis. Doing so, I don't have the problem that Tintin mentions. The bottom rectangular oil gallery is located perfectly parallel to the lower frame lug and 3-4 millimeters above it.
But I have a problem with the primary case dimension: a Commando one fits easily into the frame but is too short and does not leave me with enough room between engine and gearbox to use the same footrests mounts as on a featherbed,
a Dominator one is too long (because the crankshaft is some 20 millimeters more backwards than with the engine in a vertical position) and it's enough to make the case touching the frame rear vertical lug.
So I'm in the process of adjusting my Dommie crankcase to its new home: shortening it by about 10 millimeters by removing them in the middle of the inner and outer cases and re-welding the two halves.
No major issue so far, but quite a long job.

L.