RT1 project

I got a NOS first oversize piston for the RT3 engine.

Here it is with the old piston. The 25 is the marking for a first oversize, which is .25 mm.

I took the piston and cylinder to Banes Machine Shop in Lafayette, Indiana for boring and honing. I've had them do a few cylinders, and they do a very nice job. I then did some light porting, just smoothing and blending, and next bead blasted the cylinder and painted it with Dupli-Color gloss black engine paint. I've got a little more detailing to do on it and a final cleaning and it will be ready to go.

Here you can see some of the port chamfering, which prevents a ring from hanging up on a port edge. Smaller openings like the transfer ports don't require as much, just filing the sharp edges off, but larger openings like the exhaust port need more of a chamfer, especially in the center of the ports. You want to be able to run your finger over the port openings and have them feel smooth, without any sharpness. 

With the engine closer to being done I went through the rest of the bike to decide what parts I'll be using. I see a lot of sandblasting, painting and polishing in my future. This will be a street ridden bike, so that figures into what parts I'll be using. I'll be using the same Bassani pipe and SC500 forks from mock-up, but I'm thinking of going with a stock low front fender. An interesting thing about the frame, it is titled as a '70 RT1, and has the (correct for '70) key switch bracket under the left front of the gas tank (it went up by the gauges in '71), but the VIN falls in between '70 and '71 for the US market from the list I have. It also has a fork lock on the steering head, which I don't think the US market got until '73, and that points to it being a JDM (Japanese Domestic Market) bike, probably bought overseas and brought to the US by a military serviceman. That doesn't really matter, since all that I have of the original bike is the frame and swingarm, but it adds to the story of the bike.

For wheels I'll be using stock DT/RT hubs, but with Takasago shouldered aluminum rims. The rear rim is from a MX250 and the front a MX175. Most of the Yamaha shouldered 21" rims are for conical hubs, so wouldn't lace to the DT/RT hub very well, but the MX175 front hub is close to the DT/RT hub, so should lace up without issues. I got a decent set of Boge Mulholland shocks at the Boone County Swap Meet, and I'll be rebuilding them to use on this bike.


MARSHALLovercloth said...

Boge shocks!!! this bike is gonna be perfect. the wheels and pipe and the 1970 tank and the Bassani pipe.
I assUme you will use trials tires. I hope you have a great set of bars.

Joe C said...

Yes, trials tires for sure. It should be a super neat bike when done.

Felix said...

Hello Joe C,

i often check your blog and like what you do to all of the old DT's and RT's. I come from Germany, i restore a old DT2 from 1972 and i need your help please! I would like to change the 250cc to the bigger 360cc one, but i dont know what i really have to change to win. Is it only the bigger piston (pistonrings) and the bigger cylinder to reach the 360cc. or i also need a other con rod? Can I use the stock 250cc head for this project or is there a difference between the 250cc to the 360cc heads? I know I need also a bigger carburetor and actually I got the orginal mikuni carburetor with the 32mm bore from ebay! So now I also search the bigger 32mm carburetor snorkel to fit it to the airbox!

Can you help me to get info’s what I really need to reach the more power and to run the bigger cylinder because in Germany this mod. are totally rarely and I cant find any info’s!

Kind regards Felix

Joe C said...

Felix, really the only practical way to go from 250cc to 360cc is change the entire engine, which will fit right in the bike. Both the bore and stroke are different, and the cylinder stud spacing is different also, so a 360cc cylinder won't fit a 250cc lower end. You can do a big bore 250 with a custom sleeve and end up around 325cc, but you'd spend more money than just getting a 360cc engine.

Be sure to check out https://yamaha-enduros.com if you haven't already. It is a great forum and very helpful.

For reproduction rubber parts like the airbox joint go to http://www.kdirepros.com and http://www.claussstudios.com. I've used products from both of them.