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.

My guess is the cylinder you have wil work perfectly on an LT2 bottom end.  

If I had to guess on how to make it work with what you have, I'd say if the above turns out to be true,"maybe" an LT2 crank will fit into HT cases? (It seems the rod big end bearing changed so I'm not sure if you could just use an LT2 rod?)

My theory is based on my observations that Yamaha put a TON of effort into reusing parts and designs as much as possible most of the time and the fact that the JDM production routinely had parts, features, and even paint schemes one model year before the US models.

Stroke stayed the same between the HT and LT.  On the 250's the rod length changed when they went to reed valve in 72.  So it seems plausible that the little 90/100cc design had a rod length change with the reed valve version too.  

Some countries had taxes on displacement and/or restrictions on riders based on displacement.  Here in the US, we never did.  The 100cc class was pretty popular here in the early 1970's.  Yamaha was at a disadvantage with a 90 when other brands almost all were 100's and the racing classes here were for 100's.  It appears to me Yamaha basically did a factory "big bore" kit to turn their reed valve JDM HT into the LT for the US market? 

He first tried that JDM cylinder on an LT bottom end. Same thing 2.5mm shy. Ill double check with him just to be certain.
I think you are right about when the reeds came into play and they lengthened the rod on the 250s. Im betting thats what they did on this little 90.
Its hard to say what all they did regarding the JDM bikes. Theres like nothing out there! im really hoping somebody in this group from overseas has a JDM HT90 rod laying around or the bike itself and willing to pop just the head to see if that piston does come all the way up or not.

I was wondering about the all aluminum and chrome bore.  I did some research and that's technically a 1973 "MX90" cylinder.  Not HT.  

global.yamaha-motor.com/showroom/library/detail.php?id=240316

Yamaha tended to use the MX models as the precursor to the Enduros. 

Since it's from a 1973 MX, bottom end and cases are probably what we got here on the 1974 DT/MX 100.  The pictures of the MX90 support this theory as the MX90 engine clearly has the redesigned 1974 DT100 architecture.  

I remember reading a post many years ago that said that even though the 1973 LT3 and redesigned 1974 DT100 share the same stroke and base gasket, the cylinders are not a direct swap.  That might be what you're experiencing?  

It was so long ago.  It could have been that the 72-73 LT and 74-on DT100 pistons were not interchangeable.  

I wonder if the piston pin height changed sightly when they went to the 1974 engine design?  I have an old piston catalog someplace, I wonder if it shows these pistons and pin heights.

Correct i should have been more clear on that. It is a 73 MX90 cylinder and piston. I posted a link for the same site you just showed me and i dont see that specific bike like the one you showed when i browse the site. The 73 MX90 im looking at has a silver tank with a black decal like the 72 RT360. Anyways though you are probably right about the 73 being like the 74 design.

I cant say about the LT and DT 100 enduros but i was told in the LTMX facebook group that my 72 LTM has a 74 MX or was it the DT top end? i have not started my restoration on that bike to confirm but i do know the head on it is not the LTMX. Not sure if the LT enduro and MX top ends will swap with a 74 DT or MX 100 top end. I believe so but until i tear into it i wont know for sure.

He has the correct piston to go with the cylinder in question. He has 2 actually with the 403 code which is 73 MX90. He also has a 71 HTMX piston and the only difference between the 2 is that on the 403 JDM pistons the skirt is cut away on the intake side from the factory.

Another friend pointed out about your previous post regarding the rod length change for when reeds were introduced. the LT is reed and the HT is not but both rods are still the same length. I dont think Yamaha would have made the rod longer on the JDM 90s. I see the 74 DT100 is the same stroke. Perhaps the cases are machined further down allowing the cylinder to sit lower? who knows at this point.