rebuilding a top end ...

Not on an XJ, but the same things should apply. I have never done this type of work before and need some information.

This is a Yamaha YZ125 and I am told it needs the top end rebuilt. From what I understand, that basicially means replacing piston and ring.

In researching parts, I have found that there are 4 different pistons/rings. There is the regular, then three more with an O/D of .25, .50, and .75.

What does the O/S stand for? and if you have an OS piston, do you also have an OS ring and vice versa?

How do I evaluate if the cylinder needs to be rebored? How do I determine if an OD piston is needed, etc.

Thanks a bunch folks,

 

AFAIK the o/s is for Over Size. so if the bore is sufficiently worn you'd
go one o/s above the original. for piston and ring. I would assume you could
do compression check (check in other posts for with/without oil).

I would be surprised if you had to replace the rings AND piston. thought the rings would normally wear out first.

As for re-boring the cylinder. Check the I.D etc against spec. and thereshould be listed tolerances in the workshop manual.

If cylinder checks out then it may just need a re-hone for bedding in new rings. (not 100% certain, others here will know better).
Again, be sure to check manual, haynes or otherwise. You'll likely need something like that for specs anyway.

 

What you will need to do will be determined by the Condition of the Cylinder Bore.

If the Cylinder Bore has not suffered any kind of damage, it is possible to DEGLAZE the Cylinder Wall and keep the Engine STOCK.

As the Cylinder wears ... the Inside Diameter INCREASES.
To accommodate for slight wear ... O/S Oversize RINGS are available.

When the Cylinder Bore has sustained damage beyond what can be repaired by using the HONE ... the Cylinder is BORED-OUT.
The Inside Diameter INCREASED beyond what O/S Rings will correct.

In this case, the PISTON needs to be replaced with an O/S Piston.

(( Check for the availability of Oversized Parts BEFORE you have any Machining done ... You may be limited to needing to find a Cylinder Block in good condition ... if Oversized Parts are no longer available. ))

 

I think the YZ cylinder is a nikasil plated bore which needs re-plating if it its damaged / scored.

Plated bores are pretty hard, so the piston and rings will wear out before the bore does (well, that's the theory).

If your bore looks / measures OK a new piston and rings should be all you need.

 

Thanks a bunch guys. I really appreciate the input. I'll cross my fingers, Hillsy, and hope fore a nikasil bore that doesn't need repair ... <grin>