Fitting new Pistons and Rings

As mentioned earlier the pistons were in bad shape and needed to be replaced with new ones. The cylinder walls were also scratched and in need of some boring and honing.

Partly melted piston Yamaha RD125 A 1974

Partly melted piston Yamaha RD125

Damaged piston Yamaha RD125A

So I brought the barrels to Sam Milton, and after measuring they told me I could get away with 1st oversize (43.25) pistons. Original ones are pretty hard to find these days but luckily I found a new set on Ebay and bought the piston rings in France.

Yamaha RD125 466-11635-01 pistons

Yamaha RD125 466-11635-01 piston inside

Yamaha RD125 466-11635-01 piston top

Later Yamaha RD125A 307-11601-11 rings

When searching for new piston rings make sure you know what you're looking for; The earlier AS3/RD125A pistons (part number 396-11635-00 ) go with the 396-11610-00 rings (so called Dykes / "L" section rings) while the 307-11601-11 rings for the later 466-11635-01 piston are "square" section. The RD125A models did use 396- piston and rings but these were superseded to 466- pistons and 307- rings.

One of the heads had some of the piston top "glued" to it, but with a high pressure cleaner and steel wool I was able to remove it.

Piston material "welded" onto cylinder head

Yamaha RD125A cylinder head after cleaning

Fitting the new pistons rings is always a bit of a challenge because they tend to break when you bend them to much. I used a couple of feeler gauge blades and a rubber band to slide them in the piston ring grooves.

Fitting new piston rings Yamaha RD

Fitting new piston rings Yamaha RD125

New piston rings fitted

I also used new needle bearings, clips (ALWAYS use new clips) and good second hand piston pins.

"Inside" piston clip fitted - yamaha RD125A

Small end rod needle bearings 93310-11281

Before I put back a piston I have already fitted the inside clip, it's a lot easier then doing this when the piston is already on the rod.

"Inside" piston clip fitted - yamaha RD125A

Make sure the arrow on top of the piston is pointing towards the front of the engine and the piston ring ends are at the right spot. Fitting the barrels over the pistons is a matter of compressing the rings back in their grooves and carefully lower the barrels. The bottom of the barrels is a bit wider then the actual cylinder to make live a bit easier. Grease the cylinder walls, rod bearings, pins etc with engine assembly lube so the don't wear in the initial start up.

First cylinder back in place

Cylinders back in place

New head gaskets fitted - Yamaha Rd125A

Before I fitted the cylinder head I cleaned, sanded and nickel plated the cylinder head bolts. For more info about DIY nickel plating see http://yamaha-ls3.blogspot.com.au/2015/02/diy-nickel-plating-bolts-and-nuts.html

Nickel plating cylinder head bolts Yamaha RD125

Nickel plated cylinder head bolts Yamaha RD125

First cylinder head back in place