2 Strokes Down Under

Anyone have any knowledge about this? I have a set of RZ250 barrels which whilst otherwise are good, have wear in the powervalve bush bores, it's the inside bores that are not brilliant, outer ones usable I think. Not really bad, but centre bushes would be a pretty sloppy fit if installed. Presumably this wear has been caused by the bushes 'fretting' away at the aluminium, probably a lack of lubrication too.
I was wondering if anyone else has encountered this, and whether anyone has tried machining the PV bores oversize and fitting (say) bronze inserts to bring 'em back to the correct size for the bushes. There's not a lot of spare 'meat' in the cylinder castings so the wall thickness of the inserts would be pretty limited, (very approx 1.5mm). And there would be the issue of differential expansion, being so close to the exhaust port.
(I do realise it'd be a tricky machining job too).

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Obviously the other option would be to bore the cylinder PV bores out and have oversize bushes machined up.

Stan Stephens did inserts, but you could make bushes as an exact fit, or just silicone your existing ones in, its ugly but it works LOL

I'm very interested in this subject jools. I have 350 barrells that came to me with 1.2 mm ported of top of the exhaust port. the power valves where locked into the open postion, the removal of the exhaust port material just exposed the power valve bolt. raised exhaust port to the max was the outcome. I rebuilt the motor and unlocked the power valves conecting back to the power valve servo. New seals and orings on the original bushes.. bike ran great but within 4-5 thousand k's the power valve were spewing allot of oil around the bushes. Also 2 stroke smoke and exhaust leak around the bushes, especially when cold. eventually i decided to pull the top end to check the specs of the power valve assemble. The bushes had become worn. I don't have the exact clearance numbers but the bushes had worn 1 to 1.5mm over the new bush spec. Yamaha want close $300 for all 4 bushes. I'm with you, bore the bushes to except a sleeve that will bring them back to spec. My bearing supplier has an engineering work shop out back i would think they can bore then resleeve the bushes for allot less then the cost of new oem. Thing is what is the best material to use as a sleeve? bronze, Brass? stainless, alloy?
From what i understand Yamaha engineers realised that the valve didn't need to be a complete seal it could deliver the dessired effect without close tolerance. As we know the power valve only rotates some 45 degrees in it's cycle at low speed. i believe the liberal clearance allows carbon and oil build up in the systenm without seizure. I think the original power valve to bush clearance is somewhere near 1mm or close to it, i think over a few thousand k's that liberal clearnce becomes greater due to cylinder pressure fluttering the valve and wearing the valve bushes. when the clearence becomes to great there is exhaust leak and back pressure drop hampering performance. I do plan to take my worn bushes to see if they can be resleaved.
As another option i recently purchased a parts bike from a bloke that did some of his own mods. Biggest mod was he removed all the power valve bushes , turned down the power vale ends to 15mm to except a closed bearing instead of bushes. Ok this bike hasn't run for 10 years but the prior owner said the bike had a few thousand k's with the bearings on the power valves in place. they did turn beautifully without any slop. whe inspecting the bearings i found one of the inner bearings was somewhat seized. Maybe heat or carbon caused it or possible ten years of no use. i do plan to use these cylinders on an rz i have with the bearings installed. I will post my findings in regards to there suitability as an alternative.

I mentioned the raised exhaust port in my reply, wondering if the 1.2mm removal of the cylinder wall adds an extra 1.2mm gap above the powervalve in the closed postion, adding to valve flutter wearing out the bushes faster then if it was not altered???

Cocaine... good description!

For the 350 Yam didnt change the PVs when they went higher on the port later, but the billet PVs (from TSS/Legend/FFTP etc) did have and early or late option.

You can file the PVs to fit, or use a programmable ignition (I use Zeels) to have the PVs do anything you want, including setting th exact open and closed settings.

I think custom bushes is the way to go, a guy in the usa had some made to fit, and we have enough engineering skill here to have them done too, you could put proper bearings in them or bronze etc.

I know one engineer who might consider doing those jobs for people, but you would need to send the cyls and PVs to have them custom made, and due to the time involved its not going to mega cheap, but would be a good solution. Im not mentioning him by name here, he can do that if hes interested as he posts here, I dont want more stuff for others to slow his progress on his projects down really, he does too much for others as it is, hes good like that.

Stan Stephens might be the go for more advice on this. (My son and his son are mates, so I should exploit that relationship I guess). Oversize PV bushes sounds the simplest solution, but we'd need to source those trick rubber seals oversize to fit the oversize bushes.

Always worth exploiting that if you can, tho it looked pretty simple work really, bore out, heat, fit insert,cool, bore insert to size etc.

The outside seal is a simple O ring, easily found, and the PV bush could be kept if making a new bush as the shaft on the PV would remain the same, if we turn the shaft down we would need new seals tho.... and probably worth doing that if doing it properly, tho could reweld them and turn em back to stock... lots of options... including new billet PVs for the 350s of course

In fact if anyone is thinking of getting new PVs for their 350 Ive found that 1 set from the USA is the same cost as a bunch since $45 buys quite a big box and these are small and light, so if anyone else orders some I might be convinced to have a set if they are ordering, if say 4 of us did that post would be $11.25 each, A$16ish ea, PVs are $99 a set, A$140ish. OEMs can be had for a few hundy if youre lucky on ebay, the billets look nicer, but no real idea what they are made of, tho that can be said of OEMs which are monkey metal of course...

Jon, makes sense. Only worry I have with boring out and sleeving is differential expansion, -it's right near the (hot) zorst port, and when you look at the barrel casting you can imagine distortion there with heat. (I'm prob looking at the hole not the donut though). Simply boring out the wear and machining new oversize bushes may be the best option, the bushes are a pretty simple machining job, though they are anodised.
I might be interested in PVs, but I need to get (recently bought) barrels into my hands and inspect them first.

Yep, I agree, I was only stating what Stan did, I agree, I would bore it round and make a bigger bush too.

Leaded bronze would be the best material self lubricating. I would be inclinded to machine the PV end and shrink fit a bronze bush on that and not touch the cylinder. It could be scotch keyed in place. That is the least amount of machining and each bush could be machined to fit the size of the bore.

The problem is the bores are often worn oval now, I think you are going to have to touch the bore no matter what, unless you glue it all back in with silicone, works, but not ideal.

Have you measured the cylinder bores to check for ovality?

I'd have to save up but would be interested in the group-buy powervalves idea

Lozza I think Jon means the pv bore has gone oval in some cases.

I just bought a set of barrels that have been welded & re-machined,seems to be a nice job done but as Jools said it would be a bear of a job to get right in setup.
But if you guys doing a bulk buy/job lot Im guessing a jig of sorts (angle plate etc) could be made to machine multiple barrels & again as mentioned there is the issue of dissimilar metals.....an issue that a metalurgy specialist can answer?