model 98 Super Rat

[olddogs]

How can you visually tell a bare, late model Rat cylinder from a early Road Toad cylinder since they are both black, 50 mm, 100 cc with reed valves? Does the Rat have a bridge in the middle of the exhaust port?

[Zyx]

Is the early road toad an alumaferic design? I suspect they both have a bridge but don't quote me. From the photos, the road toad had a black cylinder with silver fin edges but the rat was all black. Not much of a clue this far after production.

The one certain way to tell would be to measure the exhaust port.

[Bullfrog]

Both do have exhaust bridges - and yes, all Road Toads have Alumiferric cylinders. Probably the best identifier is the distance from the top of the exhaust port to the head gasket surface. On a Model 98 (orange tank) Super Rat, the stock exhaust port is 28mm from the head gasket surface. I don't have the measurement for the Toad cylinder, but it is most certainly more -- probably 30mm. Of course all this assumes the exhaust port hasn't been modified.
Ed

[olddogs]

Ed. The Hodaka book goes into detail about the retrofit and relief efforts on the exhaust bridge that was causing many Road Toads to seize. Why didn't this problem first appear in the 98 Rat as it went into production a year before the Toad?

Are the intake and transfer ports on the Toad identical to the late Rat? Wouldn't the higher exhaust port would just move the power up in the rpm range.

[racerclam]

THe Toads muffler is waay too restrictive and it causes it to run much hotter then the Rat under load the dirt squit also has this issue , there is a cost for quiet.

Rich

[Bullfrog]

The Road Toads left the factory without the exhaust port bridge being relieved, therefore the big stateside project to open the crates, remove the cylinders and relieve the exhaust bridges.

Spec Compression Ratio HP@ Exhaust Duration Transfer Duration Intake Duration
98 Rat 8.8:1 9,900RPM 181Deg36Min 126Deg 360(reed)
99 Toad 7.2:1 7,800RPM 167Deg 124Deg30Min 360(reed)

Having a higher exhaust port does indeed tend to move the peak HP point higher in the RPM band, but there is more of it (HP that is) - considered to be a "good" trade off by racers (not necessarily a good thing for trail riders). Only a detailed side-by-side comparison of the cylinders would reveal other differences in the porting - things like, exhaust port size/shape, transfer passage size/shape, transfer port size/shape, intake port/passage size/shape. Of course several other things also have a significant impact on the production of more power, including (but not necessarily limited to) -- larger carb, less restrictive air cleaner, CDI, single ring piston, expansion chamber.

Ed

[rtboone]

Greetings

Ed
I measured a RT cylinder, the exhaust height is indeed 30mm from the gasket surface as you had mentioned earlier.

Quick question. How much material should be removed from the exhaust bridge and do you take more from the middle to give it a slight arc?

Thanks

Tom

[Zyx]

Just a suggestion, assuming it has not already been done:

Most of the port timing in the various specs is expressed as degrees, which is fine and is correct from an engineers perspective. However, most of us aren't engineers and even fewer have a degree wheel and the time to verify porting by this means. A more common means of identifying porting is discussed above, i.e., millimeters measured from the top down on the cylinder.

If there is no chart currently converting degrees of timing to millimeters for port roof measurement, perhaps there should be one. Likely no one person has access to physical cylinder examples to make empirical measurements, but it should be possible to convert directly from degrees to millimeters, if I can figure out how to do it. A comprehensive chart would help folks do just what was asked about in this thread, how to tell one cylinder from another. It would also stand as a reference for those who are uncertain if their porting has been modified.

Does anyone know if such a chart exists? If not, would it be a chart folks would like to have access to?

[olddogs]

Thanks for the info. My guess would then be you could have a pretty decent running Rat if you used all the Rat accessories, CDI, carb, air box, pipe and piston with the Toad cylinder. For restoration and quick rides around in non racing conditions it seems as though it would be an acceptable compromise. I just happen to have a nice std. bore Toad cylinder, but the 98 engine was missing the top end.

[Zyx]

Should work. The result should be approximately like a good, strong woods bike instead of an MX bike.

[hodakamax]

Greetings

Ed
I measured a RT cylinder, the exhaust height is indeed 30mm from the gasket surface as you had mentioned earlier.

Quick question. How much material should be removed from the exhaust bridge and do you take more from the middle to give it a slight arc?

Thanks

Tom

Hey Tom, in answer to your question, here's the info. Click on it and it will sharpen up.

Max

PS--I just did one and I didn't have a gauge. I just used a ring and feeler gauge. Worked good.

[rtboone]

Max

Thanks for the info. It looks like an easy job and using the ring sounds like a good idea as I don't have a telescoping micrometer. Of course, this gives me a good reason to buy another tool. One never has enough tools, do we?

Tom

[BrianZ]

I made a race bike with the Toad engine. I raised the exhaust port and used a Super Rat head. I also ran a 28mm carb and HT3 pipe. The motor worked really well but I broke third gear twice. Paul's improved cushioned hub rubbers are supposed to alleviate the shock load on the transmission, but I haven't tried them yet.

I can attest to the fact that the exhaust bridge must be relieved. I found out the hard way.

Brian

[hodakamax]

Road Toads were after my time selling and racing Hodakas. After rebuilding a model 99 for a good friend I could only look in awe at the RT cylinder and after racing 90s and 100s, think, Wow, this is what I needed at the time, particularly the large exhaust port. (I'd already finished the rebuild, when I found out about needing to relieve the bridge.) Back apart, back together and ready to go. Live and learn.

Max

[Bullfrog]

Arizona,
Check your messages, OK?
Ed