Thanks, yes getting bore measured first.

 

 Steve just doesn't answer calls or email so will go the In Motion route.

 

 Did you remove piston skirt yourself?

I believe Marlimar isn't his living and he has full time job elsewhere, so it can take him a while to respond, but he's a great bloke with an Aladdins cave of parts, those of us with old Ossas need him.

 

I think it's been explained about trimming the piston skirt.

Gauge first if it restricts the flow.

ie, is the gap smaller than the carbs bore with the piston at TDC.

Then file it down if needed, clean up the edge and wash it off well.

 

I seem to remember the OE pistons came with a smaller length skirt on the intake side and the pattern 73mm I fitted didn't, it just needed a small adjustment with the hammer!

 

Let us know how you get on.

 

Edited November 10, 201510 yr by goudrons

Here is my 1974 OSSA Explorer.

Engine has never be touched.

Piston here now, from InMotion, it is a Spanish one from Ital, they come in 72, 72.25, 72.5 (mine), 72,75 and 73. Skirt is full length but IM said they could have had it sorted.

I have been informed by a few that full length skirt made absolutely no difference to bikes performance. It would of course be a better balance like that. My re-borer is very experienced, shall ask him his opinion.

You've both the original piston and new pattern piston.

So after it's been bored, fit them both up with the reed cage out and judge the difference in gaps yourself.

These engines don't have reed valves.

So the intake side piston skirt is the intake timing. It is important.

Oh yes they do. Many were converted back in the day and mine is one. Romat

 

Rough history:

 

http://www.trialscentral.com/forums/topic/16878-ossa-modifications/

 

http://www.trialscentral.com/forums/topic/56275-ossa-reed-valve/

Update: Re-bored and as quiet as a mouse

 

What tolerance did you go for in the end?

1.5 thou. Popped head after an hour, all good and buttoned up again.

 

Edited November 27, 201510 yr by nickyburnell