I recently became the owner of what I think is a and#39;70 Cota. Its in fairly decent shape and I think a previous owner was refurbishing it a bit. They were doing an ok job but a but were lacking in some of the details.

 

 

The engine wasn't turning over at all. I assumed it was going to be a stuck piston from sitting. I have no idea how long it had been sitting. I pulled the head to find a brand new Tarabusi piston in the bore. I then pulled the engine covers off to see if something was amiss. No problems there. I then put a wrench on the crank nut and was able to turn the engine over with some difficultly, once it reach the upper half of the stroke it turned easily. I then removed the cylinder and the cranked turned easily. Nothing in the bore to cause any problems. Put the cylinder back on (didn't bolt it down) and the engine turned easily through the entire stroke. Put the bolts in, tightened them up evenly in increments and again the crank wouldn't turn. Loosen the bolts and it turns. At that point I needed to call it a day.

 

Will be getting back to it again but does anybody have any ideas? Been going over it in my head but haveing a hard time thinking of a reason why this might be happening. This is my first Montesa. I have had a couple Bultacos and an Ossa.

 

Thanks

 

 

sounds like something out of alignment poss bent conrod ,,,, something to do with small end thrust washers ? i presume theres nothing sat in crankcase bottom , or is there a problem with main or big end bearings ,,, is cylinder sitting square to eng cases 

It is also possible that the crankshaft is not sitting in the middle of the engine, causing the conrod to rub when the cylinder is held square by tightening the cylinder bolts.

I would also check that the crankshaft halves are not being squeezed inwards by incorrect shimming under the crank seal carriers.

just thought of another possibility - maybe that rebore was done out-of-square

Thanks for the ideas so far.

 

Had a few minutes to take a quick look at it. If I shim up the left side of the barrel with 2 business cards (one wasn't enough) and tighten everything down, nothing binds. So now I just have to figure out where the binding is.

 

I think the best possibilty might be a bent rod. The piston pin is a pretty tight fit into the piston. The use of a heat gun was needed to ease the removal. If a previous owner got a bit crazy with a hammer trying to drive the pin in or out its possible that they might have tweaked the rod a bit.

 

Now I have to figure out how I can determine if there is a bend in it.

So I think I have figured out what it is. The cylinder liner is bottoming out on the case. The opening for the cylinder spigot tapers down and when I tighten up the bolts that hold the cylinder it presses in on the liner making the piston stick. The base gasket that was in there was quite thick actually but not thick enough. What should the base gasket thickness be on these? Possibly a new liner and its to long? I get a 17.5mm measurement from the base of the cylinder to the end of the liner. I would need a base gasket about 1.25mm thick to keep it from bottoming out.

 

Thanks

maybe someone shortened the aluminium part of the barrell (where the base gasket goes) to achieve the right deck height with a different piston or conrod, but forgot to also shorten the liner.

Unusual to have a tapered recess for the liner

Very interesting problem you have there

Taper was a bad word to use. Its not tapered.

The bottom of the liner is chamfered, as many are.

The chamfer on the bottom of the cylinder liner bottoms out in the case.

cant you just get a tad machined off the liner 

cant you just get a tad machined off the liner 

 

That is what I ended up doing. After that it went together without a problem.

 

Moved on to other issues. Checked out the points. Gap was ok but timing was 6mm BTDC. Should be 3mm from the info I have. If its something different let me know. Turns out  I didn't have that flywheel puller in the assortment so ordered one and waiting on that.

Timing should be 2,5 mm BTDC. 

Sorry my reply is a little late (but it may help if anyone else finds the same situation)

I took a picture of my barrel, the distance is 32.9mm from the gasket base to the edge of the liner.

But you now seem to have it sorted!

Your cylinder is very different than mine. I should have taken a picture before I out it all back together. Yours must extend into the case much more than mine does. I had to cut mine down to about 16mm to get it to fit.