The A, B, C, D's of a top end

[BSer]

O2' YZ125, in the process of doing the first top end job. I installed a "D" piston into a "B" cylinder on the Yami' dealers recommendatioin. The reasoning is that the letter designations are for varitiations in machining and once the cylinder is broken in a "D" piston works in any size cylinder. This seems to make sense, what do you guys think?

I know the answer is piston to cylinder clearance...but my measurements with a vernier caliper where inconsistent and the dealer didn't see a need to mic anything up. Is this just common practice?

Thanks in advance.

 

[Rcannon]

Is the "D" the smallest piston? If so, it sounds like the dealer is being cheap and not wanting to stock all the sizes.

 

[whyzee]

Originally posted by BSer
... and the dealer didn't see a need to mic anything up. Is this just common practice? ...

NO.

"D" is the largest.

 

[BSer]

Yes, D is the largest, and it wasn't in stock they ordered it. These guys know how I ride, and how often, so I'm sure they were pretty comfortable nothing was too worn. I'm putting it all together today and if when the weather warms up I try it out.

 

[mackay]

I was under the impression that the A-D pistons represented a variation to the old boring/oversize pistons in iron sleeves. As the Nicasil cylinder wears there are incrementally larger pistons that cas be used, since nicasil cylinders cannot be bored (I believe) So as your cylinder wears, measure it and install the corresponding piston size (see owners manual). I would be worried about automatically going to the largest piston (D) as you might not need it and run the possibility of having a seized piston.

 

[mackay]

P.S the rings don't come in A-D sizes they are a 1 size fits all thing

 

[Studboy]

On my 02 YZ250 I have a C cylinder, which states that the piston size is supposed to be 66.360~64.363 mm and the one size fits all Wiseco piston I have sitting here is sized 66.370 mm, seeing that it can fit the A cylinder which has specified a 6.352~66.355mm piston if you use the specified piston there must be too much clearance? Maybe similar to the infamous CR125 death rattle?

 

[marcusgunby]

Wisecos do sometimes rattle a bit at start up-however it wont match a std cr or 03 kx piston for a rattle war;)

 

[SMMICK]

Originally posted by BSer
I know the answer is piston to cylinder clearance...but my measurements with a vernier caliper where inconsistent and the dealer didn't see a need to mic anything up. Is this just common practice?

Thanks in advance.

Is this common practice.... yes, but it shouldn't be!

Most spare parts guys are clueless, you should always mic the piston and bore before assembly, especially when dealing with plated bores.

You may get away with it 9.5 times out of ten but if the piston is too big you can cost yourself a piston/barrell/crank.....I know because I have felt the pain!!