Yamaha 660 water cooled to air cooled conversion

[Lawton883]

I asked advice a while ago about fitting an air cooled engine to a MZ Skorpion frame and was given advice from Steve H that an XT 600 top half could be fitted to a 660 Yamaha bottom half. I now have enough parts to begin to build my air cooled Skorpion special and have a very good 660 bottom half. Has anyone any advice about what top half to obtain and any problems that I might have with the conversion. if anyone has an XT600 head and barrel for sale I would be very interested. Thanks in advance and thanks to Steve H for the original tip. Alan

[Steve H]

In theory, as long as you have a cylinder head and barrel from the same bike it should be ok. The front and rear studs on the later XT600E heads/barrels are further apart so you cannot mix and match. I would try and get a later XT600 head and barrel simply because it will be closer to the 660 in design.
I believe Andy230 had an engine built this way. A guy called Bill Jurgenson http://home.arcor.de/w.jurgenson/Tuning.html may be best placed to help, he built and ran one of these engines in a Skorpion

[Lawton883]

Thanks Steve very much appreciated. Alan

[Steve Lake]

As you all may know, I have the project bike Andy230 started, and in fact both Andy and I have run it on track days, sadly the engine lunched itself at cadwell... looks like a tab from a (reused... a timely warning NOT to re-use tab washers I think!) tabwasher found a home between the teeth of the balance shaft... LOTS of bits, mainly teeth lying about. When it was running it was a stonking motor, nearly 70bhp on the dyno...at the back wheel... in fact it burnt out the motor of the dyno whilst trying to swing it over... rather high compression....
anyway Andy230 can tell you exactly what he did to the motor... which is currently in bits in my garage... and will probably go to a bike tuner in Denver to repair and reassemble...

then.... well it won't go back in the tigcraft chassis it came from.. as I've had that back to Dave Pearce who has welded up some cracks (oil bearing frame).. and replace some brackets that were removed to accommodate the mz660 bottom end... so I shall be fitting a nicely tuned srx600 twinshock motor into 'tiggy'... then the repaired 660/600 mill will go into a monshock srx chassis (should be a direct swop, and chain line should be spot on)

good luck with the project.. be good to see/hear it once its viable

[andy230]

Hello,

Good project!

I did put together an aircooled 4v top end on the 660 cases.

It works. Quite well!

Look at the early posts at

Www.team-thumper.blogspot.co.uk


This talks about what top end I used and some of the potential pitfalls.  You will need to blank the internal air cooled oil ways and make up an external oil line like the 660. Also a blanking plate which is tapped to feed the cam (internal on the air cooled motor).

Steve, do you have the build log? That diary has all my notes. It is literally a little black book,  I am sure I gave it to you?

Sorry, I have no more info than the blog...

And I haven't been here fora while,  sorry for the slow reply.

Cheers

Andy

Ps. As. The bottom end had just been build, I didn't split the cases. Should have! That tab washer would have been replaced and saved steve a whole lot of trouble!

[Steve Lake]

yes Andy, got the book somewhere... prob under all the bu%%ered bits in the garage.. will ferret it out at the weekend

[andy230]

Good man steve!

Of use that I can think of:

Xt engine code (3aj?)
Valve sizes? I think for the race motor, we did not use the biggest valves possible. Different xt heads have different valves. Better flow? Not sure now...
Cam clearances
Collet info?

May not be a whole lot of use as its probably rather specific to that motor, but there will be some stuff there which is not documented in the blog.

Cheers chaps,

A