Top Cylinder Issues

[david88]

hey there,

I am rebuilding my 70Hp johnson engine so that i can sell my boat with no worries of unhappy customers. but i have a bit of a querry.

The top cylinder has a bit of light scorring and the big end bearing has completely welded solid and broken up. the head gasket was blown at the top.

I thought about honing the cylinder slightly and realised that the top piston is already larger than the rest so i assume this was a problem that has happened before.

The rest of the powerhead is in perfect condition apart from a bad salt buildup which i presume caused the problem with the gasket letting the water in originaly.

My query is that apparently it is unusual for the top cylinder to go, and for this to happen twice (at least that i know of now)is there a possibility that there is some other problem that i dont know about. the piston fits snugly still so if i just honed the cylinder sleeve slightly, got new rings, new gasket kit, new big end bearing (and cleaned up crank surface til smooth) should this solve the problem for good??

also how do i find out the size of piston ring for a non standard piston?

Thanks heaps guys. I just want to make sure that the future owner has no issues.

[david88]

oh also does anybody know anywhere in sydneys inner-west that bores cylinders? i know of the many reputable places outside of this area but if anyone knows a place in the inner west it would be really handy. cheers

[Guest Aussie007]

the cylinders can have different size pistons within a limit whats the limit? i donno with jetskis its .5 what do your pistons read?

[The Incredible Hull]

Yer I would go with what Obey1 said...

I rebuilt the top end of my Aprilla, and you definitely need to go to a mechanical reconditioner to get it measured.

Best thing to do is take the piston and the top end and get them both measured, and he'll tell you what size piston and rings you need.

We're talking about 1/100th of a Millimetre, something the naked eye cannot see.