Reading Pistons

[ford832]

To move a little past reading spark plugs,here's some decent info.By way of Eric Gorr.

http://www.smellofdeath.com/lloydy/piston_diag_guide.htm

[JohnN]

You know what I find kinda funny..... in over 30 years of riding and racing motocross bikes, I have seized exactly one motor. The reason?? I had it bored (because it was worn) to the next over-size by a mechanic with very little experience...

After the bike was re-bored to the next oversize the bike ran like a charm.

While I did not follow the factory maintenance schedule as far as replacing pistons and rings, I did not leave them in forever. Many of the problems shown in that chart require a great deal of abuse or lack of maintenance or bad machining work..

In addition I was militant about changing the oil, cleaning the air filter and carefully inspecting nuts, bolts and screws to make sure nothing loosen up. This didn't take a tremendous amount of time, but was worth it.

[ford832]

Agreed though of more interest is the top piston,a properly jetted 2t.Unfortunately,that's really not a very good picture.I've yet to blow one but have fouled two plugs.One was a dud-lasted exactly 30 seconds after being put in new,the other was from following a friends son through the woods on his 50 when I had my RM.I always reringed after one season then changed the piston the next although they always looked good with very little evidence of blowby or wear.This being the case,it always amazed me the difference in power with a new top end-even when it happened so gradually you don't realize you've lost any.

[JETZcorp]

I'm still looking forward to the day when my dad re-does the top-end on his '77 AW250.  First things first, it's got the wrong cylinder on it.  For some reason I'm not sure of, it has a gigantic 1975 barrel, which has older porting.  Maico apparently made a vast improvement on porting for the AW bikes in 1976.

Second, the piston is dented.  This happened when he started messing with jetting and points as per the Maico manual, not realizing that his '77 cylinder (which was using at the time) had been ported beyond the factory's specifications by Wheelsmith to suit a professional motocross racer.  Thus, the bike needed a different set-up from a stock '77, and this particular combination make the engine go out of control.  It revved higher and higher and higher in the shop, even after he pressed the kill button and turned off the gas!  It just kept revving and revving and revving until he thought the bike was going to explode and send shrapnel through the walls at 150mph.  I'm not sure what happened next, either the weird harmonic or whatever finally allowed the bike to rev down and die, or it just flat-out seized.  Either way, it left a small dent in the piston that's surely causing the bike to lose a little bit of power.

If he doesn't do it in the next few years, I might have to do it for him for his birthday or something.