Mr Avis lyrically performs his duet of spanner and Ural....

Pissed ‘n’ Broke

Some thoughts on Pistons, brakes and also gearboxes arising from a Ural M66 rebuild.

The missus and I are getting married in Dublin in a month, and I persuaded her that using my M66 with a borrowed sidecar (thanks Tony) would be a Good Idea. Following our last, ill-fated and very smoky trip to Donegal, she suggested I might put some effort into sorting the bike out first…on which I retreated to the shed for a couple of months of officially-sanctioned bike fiddling. The following are some thoughts on the outcomes:

Pistons: my bike had std. CX500 pistons in it. It ran, but didn’t develop too much power and burnt a LOT of oil. Having splashed out loads-a-money on a pair of Ural hi-comp std pistons, I made a comparison: CX500 std bore pistons in my bores (which are not stepped) will allow a 9 thou feeler up the side, as a tight sliding fit. These pistons are a little scored…..but I guess that’s where the burning oil was coming from. I put new valve guides in anyway, just in case. (For comparison, the new Ural hi-comp std pistons allow a 3.5 thou feeler tight sliding fit, and will clamp a 4 thou feeler - so under the 0.1mm clearance spec. in the manual.) If I was buying new CX500 pistons I’d get 1st oversize(but see (3) below). CX500 gudgeon pins are 21.00 +-0.02mm - measured with a vernier, so not too accurate. I guess I was lucky in the pins which came with my new pistons (colour-coded red) - same fit as Honda ones, with gentle pressure and some resistance but turnable between thumb and forefinger. No small-end noise before or after. The biggy is crown height. The CX pistons have an odd ‘lump’ on the crown, since (I think) they’re for a 4-valve head. They stop being cylindrical a full 6mm below the point at which the Ural ones do likewise (measured from the pin) - meaning less compression than even flat-top Ural pistons. I thought about taking 3mm off the cyl. Base flange, and 3mm off the head-end of the barrel - but am a little worried about the flange breaking off or the head-bolts pulling out. If I ever have a go at this I’ll write a follow-up article…

Brakes: I’m using ‘pastry-cutter’-hub wheels. They were both very oval, and the ABS effect got a bit wearing. A friend of mine works at a well-known brake and clutch manufacturer, and got me some shoes re-lined (getting old Japanese shoes for £5 per pair to the door from Jim Allen M/Cs in Scotland made me loath to pay about twice that each for new Ural ones!) - so while the wheels were off I thought I’d skim them - but I only have a small lathe................missus.

So - I put the wheel up parallel to and behind the back of the bed, with a spindle at 90° to the bed on a load of packing (odd bits of metal) and clamped to the bed using a pair of MZ wheel spindles and some home-made ‘clamps’ and v-blocks. It was pretty rigid…I then partially deflated the tyre and ran the wheel by friction between the tyre and a face-plate. All I then had to do was get the tool into the drum…which meant squeezing a bit of 2" sq. bar into the toolpost and g-clamping a tool to the end of it. It chattered a bit…but the result was not too bad if I ran the lathe flat out and used plenty of lubricant. I also tightened the wheel bearings as far as they’d go. On the worse wheel I took off about 0.5-1mm dia. off the drum. Both work really well - no periodic ‘lunging’ any more with the brakes on - although I half expect the reduced thickness drum to overheat and go oval again. So far, so good (anyone in the North-West wanting to see the arrangement is welcome to get in touch).

Gearbox: If you’re turned off by part numbers and exploded diagrams, stop reading now… I’d bought a high-ratio Polish 4th gear through the club, and wanted to put it in - someone had cut off the air-cleaner lug off my gearbox casing, and I had a replacement into which I wanted to put all my internals incl. the new 4th. I found out a few things in the process I thought might be useful to anyone else silly enough to find this interesting:

Getting the old 4th off the clutch shaft is hard. I broke my 2-leg puller - so I extended my vice jaws with some bits of heavy angle clamped in, slid the old 4th over them (still on its shaft) and hit the end of the clutch shaft as hard as I could with a big, soft hammer (I still have bits of exploded MZ crank in my hand from a teenage accident with a not-soft hammer in similar circumstances…) Eventually, gear, bearing and spacer left the shaft (what’s the little paper washer for?)

New Ural (reverse) gearbox parts are not always interchangeable with old (non-reverse) gearboxes. I wanted to replace a worn 1st / 2nd, and the toothed sliding ring with which the selector fork selects them. Here come the part numbers…old numbers start 620***** or 720*****, and modified parts start 8.103-***** or 8.101-*****.

The dogs on all gears are more substantial. They will still work with the old sliding rings (6204221 - 1st/2nd and 3rd/4th use the same part no.). However, if you want to change the sliding rings too (since the internal dogs on these will also be knackered) you have problems:

New sliding rings have more substantial internal dogs. This means they wont go on the old version of the small gear which they slide over, and which keys them onto the mainshaft. So - for every selector-slider you want to replace, you’ll also need a new 8.103-04219 which takes the larger dogs.

The new 1st/2nd selector-slider - with a toothed outside which I presume is something to do with a reverse gear - has a narrower slot in it than the old part and will not accept shift-fork 6204306. Lots of bad experiences with MZ selector forks made me loath to take anything off the thickness of the fork (reducing hardening) with an abrasive belt - luckily the new 3rd/4th selector-slider will work as a 1st/2nd, as long as you obey point 2 above and get the small ‘gear’ too.

All this means that if you were to want a new 1st/2nd selector fork (8.103-0403), you would also need to replace the selector-slider (point 3) and small ‘gear’ (point 2) above. 3rd/4th shift fork keeps its old part number and is, I guess, unchanged.

Saving the worst for last…the old first gear 6204209 has a hardened hole though the middle and runs direct over the mainshaft splines. The new one (8.103-04209) has a plain (brass?) bush in the middle, and runs on a flat plain region on the new mainshaft 8.103-04236. So - if you want a new first gear and have no lathe, you’ll need a new mainshaft too. If you have a lathe, you can do as I did…push the plain bush out and turn about 0.5mm off the ID of the resulting hole, to 26.0mm…taking about 2 hours with inadequate tooling and a very hard gear!

After all this, the resulting ‘box works OK but whines a bit maybe ‘cos the new 4th is a straight-cut spur gear, not angled like the old one maybe ‘cos I should have been more careful with shimming the new shaft into the box. ( No...........it's because it's a Ural..( Chris)

I think it’ll get me to Dublin...........