100cc Revenge

[dogger315]

I am building a bike to race in the Marty Tripes 100cc Revenge Series.
After a lot of research, I have settled on a sleeved down 1978 Honda
CR125.  It will be loosley based on the FMF package racers that were
all the rage back then.  Since this series is pretty much a "run what
you brung" deal (unlike the AHRMA), I am free to go all out. 

I'm using a stock 1978 CR125M frame, FMF "Godzilla" aluminum swingarm,
Girling gas/oil shocks, modified stock front forks, RC125 fuel tank, and lots
of other goodies.  The engine will be sleeved down to 50mm and then ported
by Eric Gorr who will also do the headwork. 
 
If anyone is interested, I can post progress pictures of the project as
it comes together.  Should be a fun project and a fun racer.

dogger

[JETZcorp]

Hey, can't hurt anything by posting pictures!

[Coop]

I am very interested in progress reports and pictures!

[2T Institute]

Best we got from 100cc air cooled engines was about the 25HP mark. Be careful using very thick sleeves heat soak get to them and the drop HP the longer they run. A small tip is to use Yamaha KT100 s or J kart pistons cheap and readily available the size jump is 0.02mm or 2 thou the cylinder will never need reboring just honing to size. Keeping piston to bore clearence is important for off the bottom drive.  Tightest clearence we ever run was 0.015mm or 1 and 1/2 thou on a tight track.

[dogger315]

The frame is due to arrive this week.  I will post some progress pictures once it is
finished.

Be careful using very thick sleeves heat soak get to them and the drop HP the longer they run.

The cylinder will be sleeved down from 56 to 50mm.  The pistons being discussed
are a Suzuki RM85 big bore for piston port application or a YZ100 piston for reed
valve.  Thanks for the input, I'll check into the KT100 pistons. I'm not too
concerned about horsepower loss because it will probably coincide with loss of
suspension dampening due to fade, and be welcomed.

Best we got from 100cc air cooled engines was about the 25HP

I'll be happy with 20 if I can get a broad (for a 100) powerband to go with it.

dogger

[2T Institute]

There is no excuse for poor twin shocks these days. Very good and reasonably price twin shocks provide a level of suspension performance we only dreamed of back in the day. Supplemented with fork emulators in the front suspension is no longer on the radar

Yamaha KT100 S/J pistons are designed for the piston port kart engine, both can handle 16,000rpm. They are far cheaper and lighter than the RM or YZ 100 pistons.

Get an aluminium sleeve machined up and plated, heat will affect them more than what you envisage, especialy after the suspension is sorted

[JETZcorp]

You could always pursue the redneck solution to the heat problem.  Mount a plastic water bottle on top of the cylinder, and when it gets hot enough to melt the plastic, it'll automatically cool your engine!

[Coop]

You could always pursue the redneck solution to the heat problem.  Mount a plastic water bottle on top of the cylinder, and when it gets hot enough to melt the plastic, it'll automatically cool your engine!

LOL, outside heat maybe, but that wouldn't do anything for internal heat...

[JETZcorp]

I wouldn't be so sure.  I've noticed sometimes when a bike is running hot and then you go splashing through a stream, the bike has a little bit more get-up-and-go for a little while.  For an air-cooled bike, outside heat and internal heat are pretty closely connected; that's why the fins are there in the first place.  Cool the fins down and the heat will naturally spread from the motor to the fins, that's how they work with air moving over them, and a nice splash of water does the same thing, for a very short time of course.

Of course, if you've got an engine that's hot as hell (as a result of high revs and low speed) but now idling (and therefore not generating much heat anymore) and then you dump a bunch of water on it, I've heard it's possible to do some minor warping from the extremely quick temperature change.  And of course, a minor warp for a normal thing can mean serious consequences for something with the precision of an engine.  That's the theory anyway, I haven't heard of this actually happening to someone, but it sounds credible enough that I've passed up opportunities to throw snowballs at my cylinder in the past.

[2T Institute]

You could always pursue the redneck solution to the heat problem.  Mount a plastic water bottle on top of the cylinder, and when it gets hot enough to melt the plastic, it'll automatically cool your engine!

No doubt your an 'ideas man'

[dogger315]

Very good and reasonably price twin shocks provide a level of suspension performance we only dreamed of back in the day. Supplemented with fork emulators in the front suspension is no longer on the radar

I will need 13.5" eye to eye to fit this application.  I started out leaning toward Ohlins,
but am definitely open to suggestions if anybody has alternatives.  I plan to run Race
Tech emulators, Progressive springs, and any other upgrades I can come up with, I
was even thinking about having the legs ti-nitrided to reduce stiction but I don't know
if the seals used on these right side up forks even have a stiction problem.

Get an aluminium sleeve machined up and plated, heat will affect them more than what you envisage

So you recommend Aluminum over a Steel liner due to better heat disipation.  I'm having
the cylinder sleeved by Northwest Sleeve, I'll have to check to see if they even offer a
blank Aluminum sleeve.  The sleeved down engine may lose power but I believe it will
still be more than a match for the other twin shock competition - Green tank YZ100s,
Hodaka (98) Super Rats, other sleeved down 125s, etc.

Thanks again for the info.

dogger     

[2T Institute]

In the US you can get Works Performance or YSS has a dealer in the US, we also get GAZI here in Australia. I doubt you would need  the Ti Nitride fork legs but will certainly psyche out the opposition. The emulators/springs will have you grinning from ear to ear.

I know LA Sleeve has Blank alumiium sleeves, might be NorthWest one of them does anyway, they have it on their website.
The beauty of the plated sleeve is you can go to max bore size and stay there.

http://www.worksperformance.com/html/vintage.html

http://www.suspensionshop.org/index.php

http://gazisuspension.com/products.html

[dogger315]

Thanks for the info and the links.  Looks like the KT100 pistons are a non
starter.  Those pistons start at 52mm and go up from there.  Since I am
using a 125 crank and rod at 56mm, I need a 50mm piston to make 100cc.
I can't justify changing the stroke because that would open up a whole
new can of worms with port timing, exhaust pipe shape, etc., so I will
need to stick with the original plan.  Too bad, because like you said, those
pistons cost less than half as much as the others.

Just checked the Wiseco catalog and there are many choices for 50mm
pistons (even Honda for an over bored CR80).  Some of them are the
coated single ring GP style ultra pro-lite.

dogger