Further work on the YZ

Firstly decided to clean up the frame prior to painting (using the Hammerite solution). First problem of the day was a cracked frame to the rear of the main top tube. One of the side stays had cracked at the base of the tube as it connects to the welded end. Needs fixing, obviously !

The frame tis cracked

Starting stripping the engine and first problem is that the drain plug had seen some action from the molegrips. Managed to hammer an 11mm socket onto it to undo it, but thats going to need to be replaced.

Off with the head and barrel and though the bike was running okay, with plenty of power (I didn’t check the compression), the rings are worn and there is clear evidence of them blowing down each side of the piston. No scoring on the piston, though the gudgeon pin holes are worn to an oval shape (?) Piston was at 70.25mm, therefore

Clutch side first and no problems with the getting the clutch bolt off (with an airgun !). The primary drive nut is a 26mm special, but managed to get it off with the long bar and a thin sided hex 27mm socket, saving another special trip to the tool shop.

The clutch basket has plenty of notching from wear, though the plates aren’t original and in pretty good condition.

Clutch basket wear

Removing the flywheel / rotor should have been straightforward, but like a few other things on the bike, it needs a specific M18 x 1.5mm RH thread puller. Alas the ones listed in most kits are solid and like the puller for the MX250 it needs to be hollow for a depth of around 22mm. Therefore drilled my puller with a 12mm bit down to 20mm and this did the trick, which amazed me. Off for a celebratory cup of tea.

Drilling the puller

Onward, and time to split the cases, still thinking about why the engine kept slipping out of neutral. No problems with the puller and again like the MX250 there are two handy M6 threads so the puller can sit on the primary drive end of the crank. Cases came apart easily, though they’d been put together with what looked like contact adhesive.

As it was apart, I’d got some new main bearings (as OEM, Koyo) and crank seals both of which were provided by Keith at Motolink. Heating the cases, popping out the old bearings with a couple of swipes of the hammer on a large socket and then more heat to the cases and the bearings just dropped in; easy. As seems to be the norm, as couple of shims were found on the bench under the engine. Did some checks on the parts diagrams online at North West Vintage Cycle Parts and managed to get it wrong a couple of times. Some input from Gavin and there still seemed to be too much movement in the clutch shaft, but I think is taking up by an external shim underneath the basket on the outside.

The reason behind the gearbox dropping out of neutral was also becoming clear; for the 1979 and 80 models, the neutral selector bolt and the drain bolt were combined (they were always separate on early and later bikes) and as mentioned above the one with this bike is knackered (internally and externally).

For torque settings:
|Cylinder Head bolts|25|
|Cylinder bolts|33|
|Clutch Nut|75|
|Drive Sprocket|75|
|Flywheel / rotor bolt|35|

Nothing listed in the Clymer manual for the primary drive bolt, which is interesting…well not really. I went to 75Nm for this also.

Cases split

Further inspection of the piston and ring showed it to be a 1st oversize Wiseco in old money (that 20 thou oversize, rather than metric), so again on the phone to Keith and a 2nd oversize piston kit with little end bearing acquired. Also, had a look at the Wossner options, which list a piston for the MX and YZ250’s; cheaper that Wiseco, available and seem pretty good.

Hammerite sprayed frame

Sprayed up the frame and got the compressor working a lot better than I’d done in the past, in particular managed to lay the paint quite thickly without the excessive overspray. Much happier with the result than from when I did the MX250 a couple of months before.

Time to order some bits, get the rear wheel sorted (a bit of a special saga all of its own) and move on to completing the bike. All great fun !

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