1976 RT

[zaz]

All,
Is it possible to fit a piston port cylinder and piston to a 1976 rt 100 engine? If so, which year and model cylinder should I be looking for? Thanks.

John Zazella

[Bullfrog]

A very interesting question John! And one I hadn't really considered before. The short answer is that No, you can't bolt on a piston port cylinder to your Road Toad. ALL of the alloy 100cc cylinders (which is what you'd need for it to be a "bolt on") for Hodakas were reed valve. Alloy 100cc cylinders started with the Model 98 Super Rat (reeded), then came the Models 99 (Dirt Squirt-reeded), 01 and 02(both Road Toads-reeded).

However, there is a solution to your request. While some may say it can't be done . . . you can convert your Road Toad cylinder back to piston port induction with JB Weld epoxy and a piston without reed holes. I don't know where to guide you for advice on "how to". I've done it . . . and have operated the cylinder on both the track and the trail . . . and have contemplated doing an article for the Resonator Revisited . . . but that hasn't happened yet.

Ed

[zaz]

Great info Ed, and thanks. The purpose is to be able to race it in the euro 125 class. It would not need to be a rocketship, just get around the track and make it through a season.

John Zazella

[Bill2001]

Wouldn't it be easier to get a 125cc piston-port engine and bolt it into the RT frame?

[racerclam]

Zaz, You can use an iron wombat cylinder but would have to open up the cases for the spigot and use a 125 crank a 125 . Or I can easily modify your cylinder and handle all the poet work as well as carb mods if your interested let me know .

Rich

[Bullfrog]

If you go the route of full engine tear down to machine for the larger 125cc cylinder spigot, you could use the Combat Wombat Alumiferric cylinder.

All-in-all . . . the easiest route to what you want to do is the JB Weld conversion back to piston port on your existing cylinder.

But there ARE several ways to get to piston port on your engine.
Ed

[zaz]

Great info all, thanks. Since the rt 100 motor is fresh and runs great, I may go the 125 motor route if the $$ isn't too much. If anyone knows where an engine, or wombat cylinder/crank sits, please let me know.

John Zazella

[zaz]

Here is another question, with regard to the 125 crank. Is the 125 a longer stroke, or just a longer rod mounted to the same crank as the 100?

Edit - I found the stroke is the same, 50mm.

[dirty_rat]

As you noted, both have a 50mm stroke. The 125 rod is approx. 5mm longer than the 100 rod, and the crank halves are somewhat different.

[zaz]

This project is coming along nicely. I decided to take Ed's advice and convert the reed 100cc cylinder to a piston port 100. Question - Which piston to use that doesn't have holes? Stock 76 rt 100 cylinder and head.

[Hydraulic Jack]

The piston port 100's are the older bikes with an iron cylinder. I think the 96 Dirt Squirt was about the last of the piston port 100's. The wrist pin part numbers for the 96 (92C) Dirt Squirt and your 02 are the same, so I am going to assume the older piston would mount on your rod assembly. The part numbers for the rods, however, are not the same between the Dirt Squirt and your Road Toad, so it is possible that while the bore and stroke are the same, the rod length is not, which suggests that the piston skirt lengths may not be the same, or the piston crown height may not be the same. This would be something that you would need to work with during assembly to verify the skirt does not hit the crank on the down stroke and that the piston does not extend out the top of the cylinder on the up stroke. Perhaps someone else has already done this and can comment on possible issues mixing iron bore pistons and alumiferric cylinders.

Edit: Port heights between RT and DS compare well, so I am going to guess that the older DS piston will work in the newer cylinder. If so, then likely any of the earlier 100cc pistons would work, depending on whether you want a single Dykes ring as used in racing engines or more normal two-ring piston used in the 100B series.