Fit RM125 pistons

So far I have only put approx 200km on the Wossner 2000-03 RM125 pistons.

All I can say so far is that the bike seems to run fine and hasn't seized or damaged itself to the point where it is noticeable from riding the bike. The Wossner piston assembly is about 16% lighter than the standard piston assembly, and only 2.5% heavier than the HRC piston assembly, but much cheaper.

Wossner website link.

I have now completed 3 x 15 minute racetrack sessions on the bike and all seems fine.
also see 2TRaceLabs section on fitting RM125 pistons.

UPDATE 2021-03-22: The bike has been running for 6 years now. It has done multiple track days and 2 dyno run sessions with no real problems. It did blow a LH Top cylinder headgasket at one point. Still puts out 62 RWHP. Admittedly I do not use it all that often. About 1 track day per year on average sadly.
___________________________________________________________________________

I came across this post in the NSR-World forum which states that the HRC F3 piston assembly weighs 162 grams, but won't fit on the standard crank (well, conrods really). I guess this is due to a reduced diameter small end?
The Wossner 2000-2003 RM125 piston assembly weighs 166 grams. This is with single ring, gudgeon pin, 20mm wide small end bearing and circlips. And it fits the standard Honda crankshaft.

I measured the standard Honda MC18 piston assembly to be 197 grams.
The link above shows the standard Honda MC21/28 piston assembly to be 201 grams.
So 2% difference between MC18 and MC21/28 which is probably just margin of error anyway.

___________________________________________________________________________

A) It appears that the Wossner pistons work fine, but it is extremely important to get the piston to bore clearance correct. Note: to date I have only put 200km on the engine.
My "A" barrels measured 54.02mm diameter at the top and 54.01mm diameter near the bottom of the barrel.
My man getting the Wossner pistons spoke to "Luigi" at Wossner in Germany. Luigi suggested going with a clearance of 0.05mm at the 54.02 diameter top of the bore. So the pistons will be 53.97mm nominal diameter.

For what it is worth, the Honda manual states that the piston to bore clearance should be 0.035mm to 0.044mm with a service limit of 0.08mm, and that the piston OD is to be measured 15mm up from the bottom of the skirt on standard Honda pistons when working this out.

B) Also, the deck height of the Wossners is 22.0mm, standard Honda is 22.6mm.

C) The dome angle of the Wossners (8.5 degrees) is different from standard Honda pistons(10 degrees).

D) Given B) and C) above, you need to do some machining of the heads (and possibly barrels) to get the squish band on the head to match the new piston dome angle and deck height. I would suggest setting the squish in the range of 0.8mm to 1.0mm for pump gas.
But you need to make sure that the combustion volume isn't reduced too much as a result. I had my heads modified to give a combustion volume of 12.8cc to the top of the spark plug hole. This was done after setting the squish.

E) The standard small end bearing is 17mm wide. You must get 20mm wide small end bearings when fitting the Wossners.

F) I drilled holes in the pistons to help lubricate the exhaust port bridge.

G) One thing that I am not happy about with these Wossner pistons is the cavity in the side of the pistons. This cavity is just there to make the piston as light as possible I guess. But the cavity lines up with the transfer ports and possibly?? causes a less than optimal flow through the transfers??

I guess the hole through the centre of the gudgeon pin has an even worse effect?

___________________________________________________________________________

Anyone thinking of fitting these Wossner pistons to their NSR250 should probably have a thorough read of my blog from this page on to this page just to get a good understanding of things.

Wossner website link.

NSR250