Sunday, 16 December 2007

Decisions, Decisions

Its been a time of head scratching and fact finding of late.

I've finally solved the problem of finding out what studs I need for the crankcase to bolt the aircooled barrel onto the watercooled cases. The XT600 used a few, of varying lengths which complicated matters.

And I'm working on a way of getting rid of this big rotor on the end of the crankshaft.

Can't imagine its doing much to help the motor pick up speed....
And up top, I've stripped the valves out. Both the inlets were a bit hammered, and one was "mushroomed" such that it couldn't slide out through the guide. The offending article:

I had to very gently hacksaw the top off of it, and dress it so that I didn't wreck the guide. I found out that the guides have been shortened to allow the cam clearance.

The latter is a Megacycle Stage 4 item, and looks good. It has nearly 0.500" lift! However, this makes it hard to find springs. So I've opted for RD items with Ti tops, as recommended by Megacycle. The ones I took out had flats on the inside of the outer springs, and the outside of the inners, possibly suggesting that they'd been closing up...
The ports look good:

However, there appears to be a bit of a "step" where the inlet manifold bolts to the head:

You can just see it here on the right hand side, the little white ridge inside the inlet tract. I think I'll have to smooth that out....
I'll be getting an order off to Kedo soon for new valves, seals, some gaskets, a box of plugs, a cam chain.... and those studs which caused so much puzzlement!
And then in the New Year I can get it built, and take it up to Boyd asap to see how its going to go into the chassis!

Sunday, 2 December 2007

Small steps for an anorak...

I went out to see my old mate (and Team Thumper supporter) Barry Johnson at the weekend. He plied me with beer on friday night, and we talked a load of old rubbish! Cheers Baz!

However, come Saturday, after a fry up, I had a look at a couple of XT600 motors he has. I wanted to measure the length of the studs in his crankcases.

The 34K motor (Jap import) has 4 studs of 31mm (case surface to top-of-thread)

The 3AJ motor (ie. "my" barrel) has 2 left hand studs of 41mm, and the RHS ("inside" the barrel) are 31mm

My 660 cases have 3x 31mm and 1x 36mm stud!

All barrel flanges are in the region of 19.5-20.5 mm deep.

The fixings for the RHS of my motor use a 12mm deep M10 nut (14mm head) and 2.2mm washer. Total depth = 20mm barrel depth + 12mm nut + 2.2 mm washer = 34-35mm

So on the RHS, I'll need studs of at least 36mm. This corresponds to the one stud which looks ok on that side.

However, on the LHS (where the the nuts go down inside the barrel, its recommended to use Yamaha "thick" washers / spacers if possible. (I'll have to ask Martin Sweet why, I didn't question it at the time!) These are 5mm thick, so the total lenth of available stud necessary is 37-38mm

So I reckon I can use 2x 36mm (visible above case) studs on the RHS and 2x 41mm (visible above case) on the LHS, with the thick washers.

I've also just checked the squish and valve-pison clearances. I think I was worrying about nothing, there an easy 2mm on the exhaust valves, and the inlet valves didn't even touch the plasticine.

On the minus side, I think some of the rockerbox and camchain tensioner threads are getting a bit tired. Will have to be careful, but may end up helicoiling them...

I'm going to have to measure the valve srpings tho. I thought on removal that the inner and outer springs were touchiing/rubbing/ or with very little clearance. Now is the time to investigate!

However, its a fairly simple fix- I just need to spend some money!! Then we can start to think about actually building it.

(And sorry for the lack of photos, I left the camera at work...)