Snakeoil wrote:A few miscellaneous, thinking out loud thoughts on this subject. These are rhetorical in nature. Not expecting anyone to have a hard answer for any of them.
How hard would it be to convert the engine for a totally pressurized oil system so that plain big end bearings could be used? Lot of work but has been done with Dnepr crankshaft, same big end bearings.
With that done, what would it take to fit a late model BMW airhead crank? $$$$$$$$
With a stout crank, could VW jugs and pistons be used? I'm thinking VW because airhead parts are getting expensive, even when bought as used parts. Been done on Ural 650s
VW heads are out because I'm pretty sure one head bolts to 2 jugs. So, BMW heads would be the next choice. Look up Half VW. Great Plains even made a head for half VWs
Hmmmm...Maybe BMW jugs, pistons, rods are the way to go. Solves fit-up problems. Just need to get the jugs mounted to the crankcase and have the pushrods line up with the valve train and sealed. Been done. BMW R 65 heads and barrels fit with only the need to move the bottom two studs and make a spacer plate and machine the crankcase mouth. Lack of oiling causes problems though.
Cam would be a custom grind. I wonder if there is enough meat on a stock cam to regrind. Might require metal spray. Might require a custom cam made from scratch. Easy Peasy to get a custom cam, just costs $$$$$
We need somebody that owns a CNC machine shop and owns a Ural. Would be great to have someone whittle out some custom jugs and heads. A custom, on-piece forged crank would be a great starting point.
Last point is Terry Crawford was involved with a Bonneville record bike based on a 650 engine. I'm curious what mods were made to the internals. Everyone with experience on these engines always points to the weak crank. So, I would think, if all those concerns are justified, something was done to improve crank integrity. Welded, pinned, duct tape?? Don't know what Terry did but TIG welding pins is common on Japanese 4 speedway engines.
All this seems to take me full circle back to the question of whether it is better to simply retrofit a BMW airhead into the Ural? Replaceing all the internals with BMW major parts is really turning a Ural engine into a BMW engine. There's a Polish Dnepr with a R850 engine fitted. No idea what was done, but it would have been extensive.
I like cateyetech's approach. All the usual "can't be done" people come out. But that's been said throughout human history, and it's generally been done. Reminds me of this song, just replace (Safety) Dance with Ural Upgrade.