New piston with Keystone rings

Searching the net I came across a new 0.25 oversize piston set (336-11635-10) for the Yamaha LS3. Original NOS 1st oversize for a reasonable price. The set consists of the piston (52.25 mm), piston rings, piston pin and piston pin clips. The original piston was still "running" but the compression was at the lower end of the tolerance range.

Original Yamaha LS3 piston 336-11631-00-98

NOS first oversize 52.25 mm piston set Yamaha LS3

A look inside the new original Yamaha piston

One big improvement of the new piston set is the so called "Keystone" piston ring set, which is better than the conventional one, because the keystone rings reduce blow-by and minimize the change of the ring sticking in it's groove.

Plain piston rings vs Keystone piston rings

You can recognize Keystone pistons by the "K" stamped on top of the piston head. Conventional/plain piston rings and Keystone rings are not interchangeable as the shape of Keystone rings is different and thus need a different shaped groove in the piston.

NOS piston with the letter K stamped on top

 If you take a very close look (you almost need a magnifier) at the piston rings you will see the 25 stamp on top of each ring which indicates the first (0.25) oversize.

Oversize piston ring number (25) stamped on top of ring

Close up of the "25" mark on the second piston ring

I brought the LS3 barrel with new piston to Milton Engines so they could give it a bore and hone after measuring the new piston.

Bored and honed two stroke cylinder Yamaha LS3

As the piston came with NOS piston rings and no further instructions I had to find out which one of the rings was the top compression ring. Except for the "25" marks on top of both rings there were no other signs/marks on the rings. So I had to look for more differences; The ring with the green outside (also called parkerised ring) was a bit wider (measured width of the ring material) than the silver one. The grooves both had the same depth. When I took a closer look with a magnifier I discovered the silver ring was ribbed horizontally while the green one was smooth.

Close up of the second "ribbed piston ring

The piston ring set came with a very thin expander ring. Normally this expander ring is put under the second ring.

Expander piston ring two stroke Yamaha LS3

The expander ring will give you a smoother ride because it makes the piston "rock" less in the bore. Most people don't use them, especially not in race bikes with bigger ports. If one of those expander rings brake and finds it's way out of the groove.... well you get the picture..

In order to find out which ring was the top piston ring, I measured the end gap for both rings in the freshly bored and honed barrel. The green one measured 0.30 and the silver one 0.25. Both within the specs of 0.15-0.35 mm. But I read on the net  most of the time the end gap of the 2nd ring is a bit bigger than the top piston ring end gap. If the second ring end gap is smaller than the top piston ring end gap you'll get gas build up between the top and second rings, This can cause the top ring to flutter or bounce and lose its seal. So the silver must be the top ring (See pics, first one is with the silver faced ring).

Piston ring en gap silver ring (0.25 mm)

Piston ring en gap green parkerised ring (0.30 mm)

 I didn't use the expander ring either also because it was probably over 40 years old and a bit rusty.

NOS Yamaha piston, needle bearing and wrist pin wrong ring on top

Silver top compression ring - Yamaha LS3

Spark plug after the first mile

After the first mile test drive in a back street I took the bike for a spin across the city for an ID check in order to get a number plate. It's a ca. 24 km trip over mainly 60 km/h roads with traffic lights etc. Because the engine is still running in I tried to keep the revs a bit lower than I normally would.

The bike ran pretty well, no unusual sounds and no excessive smoking. In the pic below you can see the colour of the spark plug after the trip.

Spark plug after 30 miles on 60 km roads city traffic

After the ID check and 234,- dollars I finally got my number plate :)