What parts are needed to fit an oil cooler to the 3TJ ?
Can bits off another (more common) bike be fitted if the 400 parts are rare/expensive ?
cheers

Off the top of my head you will need the oil cooler sandwich plate / mount, the sp radiator & pipework.
If you're not using it for racing its probably not worth doing.

When 3TJ's are used for racing they sometimes fit the zxr400 radiator because of it's bigger cooling capacity, but it's really worth the hassle for the road.

I think the 4DX has one too

Why would a road bike be unlikely to need one?

Because it doesn't get hot enough for long enough.

After the bikes warm up, and you ride away you'll see the temp gauge go come up to the red. You'll be thinking it's going to boil but next time you look the thermostat has opened and the temp comes down. You get used to it and it's not a problem.

But if Im thrashing the bike on the road (high speed motorway or country roads) then I'll be regularly accelerating at high revs and holding the throttle open for quite long periods. Surely a thermostatic controlled oil cooler would be a good idea?
Ive never been on track so pls excuse the daft questions..

If you thrash the bike hard enough to need an oil cooler on the road you will get banned anyway!
You won't be able to hold it on the red line through the gears time after time like on the track.
The UK SP bikes were really only bought in for racing anyway I think that's why they fitted the oil coolers, even in summer it should be fine as it is.

I reckon the coolers on the 400 engine were overkill anyhow.
A race bike will have had the rad fan removed to reduce weight and in semi std trim the engine rarely overheats.
It is only once very radical tuning has been performed that the ZXR radiator seems to be fitted; Manx GP bikes and such, where extended time under race conditions is encountered.
I would suppose that one of the first things a racer would remove is the cooler assy to aid weight loss.
I have coolers fitted as std on a 2TK and a very early 3HE. The later 600s dispensed with the cooler as unneccesary.
I believe that on the earlier series engines; to remove the oil cooler baseplate at the filter requires a bit of crankcase machining in order for the filter housing to seal.This may be relevent to later engines with a cartridge filter if the cooler is to be retro fitted.

If you think about it its not when you have the throttle wide open that the bike heats up because your getting a lot of air over the rad. Its stop start heavy traffic where you use low gears and smaller amounts of air over the rad that's the problem. Country roads and motorways you'll be fine.

Actually the later 600 foxeye and TCat had an oil warmer at the back of the filter