Mecatecno - Making Pistons Fit

[gasgas249uk]

2 questions ....

I have a Mecatecno 325cc which i am in the process of getting into tip tip nick.

Although the engine is built by Mecatecno , Bultaco 325 pistons fit , just after a bit of wet and dry.

1.What piston clearance should i have ?

My only problem is that the bultaco piston is not so tall and on TDC sits 5 mm from the top of the barrell. In order the get the piston up to the top of the barrell i'm going to have to mill down the bottom of the barrell by 5mm.

My only problem then is that the skirt of the barrell at the bottom will not fit and will need grinding down by the same amount and some of the ports may not line up.

2.Has anyone any experience of doing this and am i over looking anything?.

I'm well aware that once i've done it theres no going back. The old piston is shagged.

Cheers

[charliechitlins]

Basic rule is about .001" clearance for every inch of bore.

Any possibility of milling the top of the barrel so the ports will stay where they belong?

Honestly, I would try just living with the extra 5mm before you start going crazy.

See how it runs before you go cutting hard-to-find parts.

You could lap the head to the cylinder so you could run with no head gasket and close up the squish area a bit.

I have a 4-stroke bike (Harley), where the piston misses the top of the barrel by a MILE, and it runs great.

Kicks easy too!

[baldilocks]

I might be wrong but I think if you grind the base of the barrel your ports will line up correctly as the top of the piston is 5mm lower wherever it is in the barrel ? 5mm is a lot though. You could try talking to Nigel Birkett or Shirty as the 270 conversion for the air cooled yams required a similar adjustment as did the short stroke versions, from memory this was only about 2mm.

[gasgas249uk]

Charlie Thanks for your informed reply.

I feel that i cant mill down the top of the barrell of the head as its quite a technically machined arangement with recessed head studs in the top of the barrell which would all be badly affected. Secondly when the piston matches the bottom of its stroke im told by my dealer that the bottom of the intake ports should line up with the top of the piston. If i mill the top of the barrell down the piston will sit 5mm below the bottom of the port(fully open) ....i dont see the problem here myself.

However milling off the bottom of the barrell will leave the top of the piston exactly in line with the bottom of the port entrance (as per he standard piston) and what the dealer advises.

Overall one thing jumps to mind ....

Your right , it is a big step to complete when i will never ever get a replacement barrell if it screws up . You advice sounds good to me .I'll see how it runs first and then go from there. The dealer seems to think it will run real flat with little compression. One step at a time me thinks

These old bikes need to be kept alive ... and the keep they grey matter working.

Cheers James

[martinm]

James

Have you still got your original piston?

If so, find a set of calipers and ring this man....Steve Reilly, Pistonbroke Engineering in Bristol 01179 412300.

He wont have a clue what a Mecatechno is, but if you measure the diameter, pin size, skirt height, pin/crown distance...Steve has a computer data base with about 2500 different pistons on it, you may find that something else will fit.

Worth a try before you go past the point of no return on the barrell.

Martin

PS I did this exercise on a Rotax piston and found they used the same one in a Kawasaki Jet Ski !!!!!!!!!

[charliechitlins]

There are also people who make pistons.

It's a little expensive, but probably not as bad as you'd think.

[charliechitlins]

However milling off the bottom of the barrell will leave the top of the piston exactly in line with the bottom of the port entrance (as per he standard piston) and what the dealer advises.

Cheers James

<{POST_SNAPBACK}>

Also...if you have the 'standard' arrangement of 4 base nuts at the 4 corners of a square base, be careful about the integrity of that base.

Consider making washers as large as you can to distrubute the pressure of the base nuts. And make sure to re-torque after break-in. Any loose-ness could snap an ear off. I deal with a lot of this with Indians.

You may even want to consider welding a 2-piece collar on top of the whole base, thereby putting the 5mm you took off back on top.

[ask greeves]

What your considering , needs loads of thought before you attempt anything. The piston on a two stroke, not only opens and closes ports on the top side of the piston, but also on the underside, so the length of piston is very very important. Example you wouldn't want the exhaust port opening up into the crankcase at top dead centre or maybe the inlet port opening onto the top of the piston at top dead centre also, Martin M's suggestion would probably be the best route, get the right length piston, with the correct gugeon pin height.

[marlintec]

Mecatecno's engine has rotative valve, so the piston skirt has no effect. You can effectively mill the barrel base to use sherpa's pistons. Be sure to let the squish clearance at least to 1.5 mm not less.

The engine is suspended on the chassis, bolted in the head. So if you change the barrel height maybe you'll have a tight fit in the chassis, be careful.

I owned one of these beatiful machines in the eighties. 'Dragonfly' was her name (had an iron sleeved barrel) the last model was 'Skywalker' (nikasil barrel).

Cheers, JM.