1981 248 Montesa, same bottom end as 349. I’ve owned since new, got rebuilt about 15 years ago, done little since. It’s been a few years since I rode it and can’t remember if it always sounded like this but I think not. Any thoughts as to what this rattling is from those of you with better memories or bikes that do/don’t make this noise? Thanks.

Video of bike running: https://drive.google.com/file/d/1hDHj3LvyNAvQO7q4dQYDrzyU5u1NtZ_K/view?usp=sharing

 

From here it sounds like a very poor fitting piston and rings and I would be concerned with the bottom end seals because of the bikes age and that sound.  Compression testing would be a good start followed by inspection of the top end and if you don't find stuck piston rings or serious damage there you might be looking at a major overhaul that requires splitting the cases.

No it should not sound like that.

To me it sounds like a top end noise. Probably stuck rings (because you said it had been sitting for ages) or a failed little end bearing

Pretty easy to pull the cylinder off and check it out

Edited July 9, 20242 yr by feetupfun

Little end bearing was my first thought. Not too bad a job if you can get the barrel off OK. Inmotion Trials should have a new bearing.

Thanks all. Looks like the head and barrel are coming off when I get the time, it'll not be for a month or so but I'll report back. Small end bearings are available from In Motion but it seems that OEM piston rings are all but impossible to find. So far have found a second oversize bottom (L-shaped) ring in the USA, and a couple of leads for specialist ring suppliers here in the UK. Fingers crossed!

If you haven't pulled a Montesa cylinder off before, you may not know that you will need an (uncommon size) 7mm hex drive tool for the (recessed) cylinder nuts.

If the rings are stuck, they may still be fine after cleaning out the ring grooves and the rings.

Thanks for the heads up. Remember filing an 8mm down literally decades ago! Wonder where it is now…