RT1 project

Along with redoing my SR500 this year I want to do a complete project. I went through my Yamaha projects and picked the '70 RT1 to move on to finish work. I bought the bare frame in September of 2014, and have been collecting parts for it since then, putting it into a loose mock-up.

The engine I'm using is from the last year of the original RT series, the '73 RT3. '70 and '71 were piston port, and '72 and '73 are reed valve engines. I bought the engine on ebay, and don't know it's history, but it looks like a good one.

You can tell by the case screws that it's likely never been split before. The engine was also fairly clean, but doesn't look like it was cleaned, so all points to the engine being a low mileage cream puff, at least as 44 year old two strokes go.

The bore was clean and damage free. At first I thought the 75 marking on the piston meant it was the 3rd oversize, but once apart and I measured it, I found it is still the standard bore. Piston to cylinder clearance measured in the .004" to .005" range. You could re-ring it and it would run, but if it didn't piston slap right away, it soon would, so will need to be bored oversize.

The rod big end measured to be in great shape, so a crank rebuild won't be needed.

Digging a little deeper, dirty as you'd expect here. At this point I could just replace the crank seals, but I think splitting the cases and making sure everything is clean and right is the best way to go.

Here is the bottom end disassembled.


There really isn't too much to these engines if you stay organized.

Here are the cases with the bearings removed and the initial clean-up done. Next will be the final cleaning, deciding what finish to do on the cases, and then installing the new bearings.