So i have a friend who has a cylinder, head and pistons for the HT90 JDM. 403 code stamped inside the pistons, single ring, chrome bore, aluminum casting, huge intake port! it actually uses a 125 intake boot. The issue we have is the piston does not reach the top of the cylinder. its 2.5MM short. The LT and HT have the same stroke. We put this cylinder on both the LT and HT lower ends with the same result. 2.5MM shy. He also has the 71 HTMX piston and i have the cylinder for that bike as well. We thought maybe the piston height was the difference but nope thats not the case.

The only difference between the HTMX and HT JDM pistons is the JDM has the skirt cut away on the intake side by the factory. We have done all sorts of measurements using the HT enduro, HTMX and his JDM cylinder not to mention an LT enduro cylinder. The HT JDM cylinder sits 2.5mm taller and the cooling fins are further apart. Our best guess is the rod is longer on the JDM HT90s.

I know its a long shot but im hoping somebody in here either knows or has a connecting rod for an HT90 thats stamped 403? Maybe someone has a manual for the JDM HT90 that tells us the stroke? Not much info out there regarding JDM bikes. Hes looking to sell these to another friend of mine whos turning an HT90 enduro into the MX.

Wildly guessing but HT1 if that's an HT90 is piston port so i wonder it you have a yz100 cylinder & piston. Or if there's such a thing as a race kit for an LT2 which is likely to have a rod length change going to reed valve so gudgeon eye in the piston could be different. As i said--just a wild guess. On reflection 2.5mm isn't much & any rod length i've found is usualy 6mm but i'm talking bigger motors.

Yes the 70-71 HT enduros and 71 HTMX are piston port but from what i can tell by the images the 72 HT90 JDM has the same reed set up as the 72-73 LTMX bikes. The 73 HT90 JDM changed to what you see on a 73 ATMX125 here in the states. He has a NOS 125 MX intake boot and it fits perfectly on that JDM 90 cylinder hes got.

I have a 71 HT90 enduro, both LT 100 enduros and both LTMX 100s plus the service manuals. All HT90 and LT100s are the same stroke here in the states. Im thinking that 73 JDM HT90 has just a slightly longer stroke. Somebody out there must have a rod or a piece of literature with the information.
Im leaving a link so you can see the 2 bikes im talking about, hopefully it works. They actually made the 72 in a gold and red color.

global.yamaha-motor.com/stories/history/products/

I should note that you can find those bikes under years 71 and 72 in the link. I believe they listed them there because Yamaha would start building next years bike in the early fall the year prior.