Ah finally some info that starts to make sense. What you are saying about the number series sounds legit.

I think you are right about this. I have heard others mention similar concepts. I didn't ride the TT I had long enough to really get a good feel for it but I do seem to remember it was jumpier then the XT when they both had their own fly wheels. Another bonus I hear to the heavier fly wheel is that it will carry more momentum for kick starting which allows more force to be created to spin the motor. I am guessing a lighter weight flywheel will spool up & down quicker as the the motor is revved up & down. The heavier flywheel would resist building momentum and loose less momentum when the throttle is adjusted. A heavier fly wheel will also hold speed better on rolling terrain with less throttle adjustment I would assume. As you say the heavier weight of the fly wheel "helps smooth" things out.

I hope this is the case. I am more concerned about smooth reliable day to day performance then I am in high performance. Sometimes high performance options can be a headache for day to day riding it seems. I am less concerned about performance & more concerned the the bike just runs good & and is easy for me to put together. I already seem to have a good Nikasil cylinder so if there is no reliability or specific performance characteristics that are lost by not using the XT cast iron cylinder then I will just go with the Nikasil cylinder for now.

The main reason I am concerning myself with the cams & cylinder liners is I want to be sure that the parts match for consistent performance. I have horsed around with some bike tunes that were half OEM/sport grade parts & half racing parts. Such tunes are always crap it seems as the OEM sport parts will be tuned for one shape of power band & the race parts will be tuned for another power band with the end result power band just not really good for anything because half the tune is focused on one rpm range & the other is for another rpm range. This is why I am trying to sort the parts out. Its not so much the max HP or something I am after. I just want everything to be in balance.

This is what is probably the case. Following your parts theory while the 3rd set of numbers is different between the carbs this could simply be due to the jet & needle differences I would think. Since I have both carbs I will measure them with a caliper whenever I have them off again. In all honestly while I have seen multiple references to the TTs having "hotter" cams and gearing differences I have only seen one mention of a difference in carb throat diameters & it was on a forum so ???

Now here I am still kinda wondering what is up & this is what is causing me to wonder. There are multiple mentions around the internet that have stated there is a difference in first gear between these transmissions. Several weeks ago I actually made a spread sheet comparing all of the transmission parts between the TT & the XT. Everything was the same according to how you describe the part numbers with the second set being the actual part. So after looking at the chart comparing the parts I began to wonder if people on the internet claiming first gear was different were comparing older & newer models of XT & TT. The newer XTs do have a higher primary gear as I understand it so maybe that is where all this talk is coming from.

Then a few weeks ago I was messing with the GearingCommander website. Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator

On the GearingComander site I noticed it showed two different 1st gear ratios for 80s TT & XT bikes. It showed 31/12 for an 80s XT600 first gear & 30 / 13 for all TT600 first gear. Below all my writing in this post are pics showing the gearing difference according to GearingCommander. When I saw this on the GearingCommander site I was like well maybe the gearing is different for 1st gear. Now I looked it up again tonight to grab the below pics & I noticed GearingCommander is also saying that the 5th gears between the two bikes are different also. But this can't possibly be right as the OEM part numbers for 5th gear & the 5th gear wheel are exactly the same for both XT600 & TT600 bikes. The OEM part numbers for the main axle which I assume holds first gear only match in the center numbers. So if you are right about the part number system then GearingComander has the wrong data plugged in. There is all this hearsay on the net, part numbers seem to match, and GearingCommander is showing different ratios. Its kind of a mess.

Below are the only part numbers that differ between 85 TT600 & XT600 transmissions. Everything else has exactly the same part numbers, including 5th gear & 5th wheel. The main axles sold with a complete set of gears or by themselves have differing first three digits & the main axle bearings have different second digits.

TT600 Main Axle complete with all parts 34K-17410-00-00
XT600 Main Axle complete with all parts 5Y1-17410-00-00

TT600 Main Axle by itself 5Y1-17411-00-00
XT600 Main Axle by itself 34L-17411-00-00

TT600 Main Axle Bearing 93306-30540-00
XT600 Main Axle Bearing 93306-20540-00

So I am just left wondering what is the truth in all this. Are the main axles different in there first gearing? Maybe all this first gears are different is partially coming from GearingCommander having the wrong data? If you are right about your analysis of the part numbers then the main axles & corresponding gearing would have to be the same though.

Until I have the parts in my hand to compare or some kind of known proof one way or the other I am just gonna let the gearing issue rest. Its kind of a moot point for me anyway as I am gonna build on the XT bottom end & have no intention of messing with the tranny just to find out what is up. If I ever disassemble both trannys I will count the gears.