Ring wear ledge in cylinder

[Scott Adams]

I have chosen to rebuild the top end of a 1991 KDX 250 without machining the bore. The condition of the bore is good and is within spec. My concern is that after I install the Wiseco piston kit and cometic gaskets some minor dimensional variations could cause the top ring to contact the prexisting ring wear ledge in the cylinder. I considered adding a thin paper gasket to the cylinder base gasket to lift the cylinder a few thousandths but without knowing how the new base gasket will compress and what its final thickness will be compared to the factory gasket these dimensions are difficult to obtain. Any thoughts? Am I worried about something that I should not be?

 

[Ol'89r]

I have chosen to rebuild the top end of a 1991 KDX 250 without machining the bore. The condition of the bore is good and is within spec.

Scott. If the bore is in good condition and within spec, there should be no ledge. Did you hone the cylinder?

Mike the bore with a bore gauge and it will tell you how much of a ledge you have. If there is a measureable taper in the bore, you should have it re-bored.

Just my $ .02

 

[Scott Adams]

Thanks for the $0.02, The bore diameter above the ledge measures approximately 0.0014" less than the diameter just below the ledge. Therefore a ledge of approx. 0.0007" must be present. I have not honed it yet but only wiped it with some 250 grit paper, The bore is Nickasil treated thus it is not machineable with standard metal cutting tools but certainly abrasives would have some effect. The bore above the ledge is only approx. 3/16 tall and if there was a way to grind it out without cutting into the underlying worn bore I would. The meaurement of the ledge sounds minor, but if it is sharp I could see it causing damage regardless of it depth. Your opinion?

 

[DEANSFASTWAY]

I gotta agree with 89 er on the bore wear issue but sometimes on an olsd bike you like to just keep running . You could either measure the gasket thgicknesses or maybe bolt the cyl on and use a contrast dye or like a magic marker on the cyl wall then turn the motor over after a few turnd the rings will scrape away the ink ansd leave a comparable impression. oil will leave a mark where the rings pushed it away. if you need gaskets you could getr cometic to make different thixckness to your spec . For some bikes Pro X makes very slightly oversized pistons for slightly worn cylinders. also the pro X rings are thicker than a wisecos in most cases which wont etch into the cylinder as badly. If I were you id have a shop mic the bore real good as you might bew better off Replating now instead of trying for another piston go round. the Kx/KDXs also wear the bore at the bottom of the cyls near the intake and when the pistons go slappety slap slap so does the bottom of the cyl and down into the cases it goes. I can understand your point on the old bike though . I ve seen quite a few brand new pistons that were in worn out bores and broken because the cyl was thrashed anyway. Its too bad we cant just bore out the cyls and plate the piston instead.

 

[DEANSFASTWAY]

If you hone it get it rigid honed Sunnen AAmco etc. if you ball hone it it will wipe the little grrove impressions away but they will still be there you just wont see them i wouldnt hone too much on that old KX cyl anyway as theres not much there anyway with stock plating. maybe use all the same stuff as youre replacing same brand parts etc. If the bike has alot of time on it you might also want to take a peek at the KIPS turnstyle gears as they have a way of wearing out and the P/V wont open all the way and the thing will detonate on top end , that wiil be your next problem . Dont laugh we see it on 2001 bikes . Theyre the gears that run up and down and turn like a carousel opening the exhaust secondary ports. Good luck

 

[Scott Adams]

Thanks Dean and 89'er for your comments, I will try the ink when building, I have completely disassembled the exhaust valve and all the components. Got rid of all the carbon and they look ok, a little wear on the two exhaust valve gears that contact the rack. The cycle does not have many miles on it for its age and little shows little bore wear, you are correct its shows more wear down low in the porting but still in spec. Its hard to dump a bunch of money into it considering its age and value. Certainly if I fire it up and the moter shells out, I will have wished I spent the extra money. Just interested in hearing people's opinions and maybe it will bring up something that I have not considered, you know the "simple" fix or check that one can easily overlook.

 

[Rcannon]

Could the ridge be that deep without the other parts of the bore being damaged?

I love the new cylinders. Sure, you could bore them in the old days. They ALWAYS needed it. At least the modern bikes give you a chance!