KT 100 tricks


Hi Pete, I have sent you a few pics of a KT100 barrel I managed to get my hands on. This has been tweaked by a guy who claims to be the Guru on these engines? What are your thoughts?


Dawg 89
As long as the ring dosen't catch . Let er rip .


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Biggest tricks? --> Don't overlook the mechanics of the engine:

- main bearings aligned?
- main bearings have correct interference?
- con-rod has correct clearance on both ends, and checked for parallel ends and in twist direction as well?
- crank running *dead* true?
- cases dead square?
- bottom of cases perfectly flat? Top of motor mount perfectly flat? (Yamaha cases bend/flex easily)
- correct end play in the crank?
- bottom of cylinder dead square to the bore?
- is the piston you're using square? (wrist pin bore to the sides)
- check taper of piston (more radical tuning requires more taper, milder exhaust and lower gears less taper)
- is the cylinder deformed correctly for honing so it will be perfectly round when engine assembled?
- DON'T over-tighten the exhaust header (deforms bore) - very low torque and double-nut is best (same torque as when honing)
- does the ring *perfectly* fit the bore size? (light checking is best).
- shoot for the correct ring tension, end gap not critical as long as it has enough
- choose an assembly sequence and torque increments on every step, and try to always do it the same way

- give the top of the exhaust a slight bit of an "arc", don't make it straight across (way easier on the ring)
- when it comes to cutting cylinder or head, I'd personally cut the head if I can get to the squish I want
- combustion chamber shape can definitely change the power band:
----- small diameter deeper dome for for short/tight tracks
----- larger diameter shallower dome for tracks where a lot of time is spent at higher revs

A ole racing buddy of mine had a kt100 from deis and it had a heck of a groove in the cylinder where i guess a wrist pin came out.i seen it in his garage and i thought man how are ya winning races with that thing.and he won quite a few with it.several 500$ to win races at our local slick bullring track.i guess the piston travels so fast that groove wasnt hurting it


Dawg 89
I had a 9 port Mac . it had milled grooves that opened up when the piston was on its way down .
Sam Wesley did them .


Hi Pete, I have sent you a few pics of a KT100 barrel I managed to get my hands on. This has been tweaked by a guy who claims to be the Guru on these engines? What are your thoughts?

Two observations:

- I can't tell from the angle the photo was taken, but when the exhaust port gets that wide, it's good to have some "arc" across the top of the port. That makes life a lot easier on the ring.

- I have found that opening up transfer passages on a KT100 doesn't do nearly as much as the exhaust port. A good example of this is that the KT100 can be run with the cylinder rotated 180, and performance doesn't measurably change. To me, that's an indicator that the engine design has an abundance of transfer capacity. Of course if the exhaust is opened way up, then I suspect more transfer could help, but I don't think it takes all that much.

Hey flattop, that's my cylinder. I still have it, bored for an .080 over weisco I coated. I ported it in the late 80s. Pete , I based my ex port width on a percentage of the bore that was tested and proven over time for a bridged exhaust. So, I made most of the ex port hit all at once. Going for the max.
Put exhaust at 90 degrees, transfers at 115 to 116 degrees, intake at 90 degrees, squish at .030 14degree angle 10CCs . Jon Brogden has a post about boring the carb bigger follow that along with intake passage to fit carb, you can play with the timing, doesn't make that big of a change with these basic changes on a stock bore and stroke. if you try to put a TT75 or any crank that's 50 mm bore the engine then use Buller's 54mm piston for a Yamaha put exhaust at 88 degrees the rest the same this will make your engine 115cc ?