I'm going to look at an XR100 motor on Friday. The guy says he can't find a serial number, but I bet it's just covered in gunk.
Since I'm going to look at this to buy it, I don't want to have to leave and come back or fumble through stuff on my phone attempting to identify the year or version of the motor.
My research says that all the XR100 motors will have a serial number beginning with HE03E- and I've found that -52... is 1983, for instance. Can anyone tell me what the rest of the serial would indicate years of major changes? I realize there isn't much, the XR100 was a really reliable motor, so Honda didn't mess with it.

socalxr, on 16 December 2011 - 08:18 PM, said:

Perhaps serials beginning in 91, 95 and 2001?

And is the serial always near the shift lever or did Honda stamp it in other places too?

Edited by Smacaroni, 18 July 2012 - 04:05 AM.

Oh, for what it's worth, I don't have any big plans for this motor yet, initially as a spare, but I may go 120 on it. All depends on what I find when I get it. The guy says he bought it for a project that he never started, so he has no real history on the motor itself.
I plan to bring our XR's kick-starter and do a compression test before I buy it. Since it's not in a bike and there's no history, I plan on getting it for cheap or not at all. I've bought boat-anchor motors before and I don't want another one.

Smacaroni, on 18 July 2012 - 04:01 AM, said:

Serial number is only located by the kicker.  Serial number progression only indicates year.

http://www.cmsnl.com...ist/SERIAL.html

These engines utilize plain bearings for the cam, bad news if it was laid over on the left and let idle while "Johnny" licked his wounds.  Heads are not cheap, if you see anything sketchy walk away.

Cam chain, crank and cam chain sprocket wear, tensioner wear is very common, damage left side cases are common due to piss poor drive chain guide.

Early versions (Pre 1985)
had a shorter kick shaft than later
had valve seals on both valves.

1985 - 1992 virtually unchanged,
1992 and up had CDI ignition, early versions can be updated to CDI but it's not perfect.

Edited by theraymondguy, 18 July 2012 - 03:01 PM.

theraymondguy, on 18 July 2012 - 02:58 PM, said:

I read about someone who bought a motor with a short kick-shaft. Now I know to avoid the early ones. Dumb question, I'm only seeing up to 84 on that link you posted.
So if 81 is 50, 82 is 51, 83 is 52 and 84 is 53, you're saying the method is first two numbers after the dash plus 31 for the model year? And that sequence carries the whole way through for three decades?

Ok, so I need to avoid anything prior to -54 because the kick-shaft is short and would require splitting the cases to fix.
-64  or higherwould be "worth" more (to me) because it should have better cooling than the earlier ones and
-74 and higher is better yet, because according to SolCalXR, they have a better main shaft and valve seal.

Since I can't start it, let alone ride it, I'm going to have to take an educated guess on condition, besides of course doing a compression test.

Thanks!

Serial turned out to be -63 and it was on the left side of the motor near the shifter. That's a '94 by my math. Not ideal, but not horrible.
Condition was not so great, but it made 145 PSI compression with me holding it down with one foot while I operated the kick-lever by hand.
Came with what appears to be all the electrical bits, but somebody cut the wires the stator plate, flywheel, cracked magneto cover, a carb without a slide and some miscellanious crap that I think is from the CA emissions "stuff".
I bought it with a '89 (supposedly) YZ80 motor that looks like it has everything to run (carb, pipe, wiring) and supposedly needs re-ringed for $110.
As long as I don't go through monumental efforts to make either one run, I can make my money back if the parts are junk. <- I need to remember that, the last time I bought a motor like this, I spent almost as much as the bike was worth and it still doesn't run.