Hello, can anybody help? I have a montesa engine number of 51H00221 .  I've looked at engine and the serial numbers and the 348 should start with 51M. Im Confused! Any help appreciated as I need engine parts, thanks.

Yep, should be M. How about a photo?

Hello, thanks for your quick reply. You can just about see the whole number. Thanks

It doesn't look like a 348 motor. Clues being the shape of the clutch cover and the bolt-on inlet connector. I'm thinking Cota 349 motor. That doesn't solve your mystery though because 349 is 51M too.

Here is a photo of the engine number on one of my 348 motors and it is located on the top of the front engine mount lug. I don't know where 349 numbers are stamped but it might be worth a look at the front engine mount lug

I've just noticed that the frame is also Cota 349 not Cota 348

Here's a photo I lifted from the North Atlantic Trials Assn webpage that shows a 349 motor up close and it has the engine number where yours is stamped

It would help with reading the stamping of your engine number if you scraped the paint out of the stamping

Hello, thank you for your help. I brought the bike thinking it was a  349 frame with a 348 engine. I guess it is a complete 349. My wife has confirmed the engine number as being 51 H 00221 . It is a sidecar outfit! 

It was running spot on, and then started seriously back firing. I think we've traced the problem to being a worn keyway ignition side. I've got a couple of ideas how to fix it, but fear it will need a new crankshaft. Do you know where I can get a crankshaft from , also, anybody to fit it as i don'tI have the skills/tools to do the job. A picture of the beast.  Thanks

I forgot to mention , there is no number near front engine mount. 

That large frame lug on the side was a give away that the frame was married to a sidecar at some stage of the bikes life. Could be that 51 H #### is a moto cross engine...just guessing not looked on the Montesa ident site. Just looked on Sheldons site and AMS Racing both do not show 51H so my suggestion is wrong, but your engine needs a good clean off in the number area. Some one on this site may be able to give some more detail as to the H .

Edited November 6, 20205 yr by section swept

A better picture of the number. 51H ? 0221. 

There's the number

Hmmm that stamping looks a bit dodgy for a few reasons.

They didn't make a bike model called a 51H.

The number after the 51H is too low to be on a 349 motor. 51M series started in 1976 and there were many thousands of M51 Cota 348s made before your series 1 Cota 349 was made but yours is stamped number 00221.

The appearance of the metal surface after the 51 is smoother than at the 51 meaning the original letter and number after the 51 could have been ground away and re-stamped with a different letter and number.

Another possibility is that Montesa made trials sidecar-specific motors that had a different engine number series and this has not become widespread knowledge yet.

What does the frame number stamping look like?

Another interesting thing about the bike is that the swingarm looks non-standard. Maybe it's a stronger design to cope better with sidecar use