1. Many of the piston kits out there bump the compression. This moves the power lower down in the power band and also increases the engines running temperature. One of the great things about Thumper Racing's XR300 kit is that is uses the stock 10.2:1 compression ratio.
   
2. Increasing the displacement will increase airflow which in turn leans the jetting. The previous owner should have re-jetted for the piston kit. I assume it is a Wiseco Piston, if so then thumper racing can give you jetting specs. (sorry, but I don't have the number with me- they have ads in the back of dirt rider). Check the plug to see if it looks really white (lean); Although, if I were you I would pull the carb apart to see what jetting it has in it...
   
3. The '86 XR600 did not come with an oil cooler. It uses a dry sump motor and the oil is carried in the backbone of the frame. There is extra space between the tank and the frame to allow air to pass and cool the backbone and the oil inside it. Some idiots from dirt rider, (Tom Webb et al) were putting XR250R tanks on the XR600 to make the bike slimmer; The XR250R tank fits very tight to the backbone of the frame obstructing airflow; Dumb, very dumb, just goes to show ya that ya can't believe everything that ya read...
   
4. The motor does run hot in stock form and will run hotter with the piston kit installed due to the increase in compression. You should use a synthetic oil and change it religiously every couple of rides. I would suggest using Honda HP-4 or Spectro 20W50 unless you live in a cold climate then use 10W40. Mobil-1 works fine as well and can be obtained for around $3.50 a quart in most places (it seems to be on sale around here quite often). You will hear claims that Mobil-1 will make your clutch slip -> this is absolute B.S. I have run Mobil-1 and so have several friends of mine in XR250s, XR600s and also an '83 XR350R, which ran exceeding hot and also initially had weak clutch springs just like the XR400R has now).
   
5. Unless you are racing all-out or live in a very hot climate, you should be just fine without adding fins to the cylinder. I am currently having fins added to my XR300R, but only because I am running it flat out for eight hours at a time in ISDEs.
   
6. Jag makes an oil cooler for the XR600. You might also be able to get an XR400R oil cooler to fit with a little bit of work. I added one on to my XR300R, but had to machine off the top fittings to get it to work. Mine was able to mount to the stock xr250r oil cooler mounts but only because the steering head had been extended downward about 30 mm to mount on a set of KYB upside down forks.
   

Jeremy,

I was just reading the section on your web page under "Web Advice" and
the "Internet Q: XR600 running hot." What caught my eye was Jeff
Dunham's response. In particular was item 3 in his "list".

He states that the XR engine is a dry sump motor and carries oil in the
backbone of the frame. He then goes on to call Tom Webb an idiot for
mounting an XR250 fuel tank to the 600, because according to him the
"tunnel" between the fuel tank and backbone allows air to circulate
around the frame and helps cool the oil. Jeremy, Mr. Dunham is the idiot
here!

Look at any XR600 frame. The oil tank is situated behind the steering
head in front of the fuel tank, and in the front down tube of the frame.
NOT the backbone of the frame! The oil tank extends to about one inch
behind the front of the stock fuel tank. There is an oil fitting on the
bottom of the tank near the front of the frame, this has to be the
lowest point on that part of the tank in order for oil to circulate. If
oil was indeed carried under the fuel tank in the backbone of the frame,
it could do no more than sit there, as the frame slopes downward to the
upper rear shock mount.

Another point that makes his statement even more ludicrous is the fact
that the XR tank mounts on two rubber "bumpers", one on the front, one
on the rear of the tank. These rubber pads are mounted securely to the
frame and the tank sits securely on them. Plus there is a fabric
"cushion" that wraps around the frame between them that acts as a
vibration damper for the fuel tank. Again, if oil was carried in the
backbone as he suggests, then this vibration damper would act like a
blanket holding heat in and negating any cooling effect that what little
air could pass under the fuel tank would have.

As for the mounting of a 250 tank on the 600, it is my understanding
that the tank actually sits further back on the frame, which would
expose more of the oil reservoir tank, therefore using his theory, it
should help cool the oil, not cause it to heat excessively.

Many more knowledgeable people than the Dirt Rider crew have used 250
tanks on 600's successfully, including Geoff Ballard of XR's Only in
Australia. In fact he makes a rather trick aluminum tank for the 600
that absolutely hugs the backbone of the frame. So far, I've heard of
zero problems with reliability after mounting either stock Honda 250
tank, or Geoff's cool looking piece.

Jeremy, please add this to your site, as I feel that the real facts
concerning this issue need to brought to light.

Thanks!
Bruce McCrary
Lexington,NC
'94 XR600R