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650 motor with new 750 heads

Ural 650 upgrade 650 motor with 750 heads

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#1 Dahmer

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Posted 24 November 2016 - 03:10 PM

This is the upgrade I'm doing currently with one of my engines.
As the way it is - quality for aftermarket 650 spares has greatly declined, valves especially, and the factory only produces 750 stuff.
So, after seeing this done in Russia,  I decided to do the same. The link is in Russian, but there's pictures.

http://skuns99.blogs...5/02/650-3.html

Since Mr.Crawford was curious how this is done, and I'm sure some other people are as well, here it is.

What you need:
2 new 750 heads, l&r plus rocker  covers
2 new 750 head gaskets
set of 4 new roller bearing rockers
set of 750 pushroads (optional)
and, an access to some machine shop equipment, as well as experience (or someone with it) in simple machining operations.

As you know, the main difference is that in a 650 motor the cylinder is bolted to the crankcase  with four  m10  short studs and head goes on four

m8x1 studs that are threaded into the top of the cylinder.
On the new 750 (and Dnepr as well) the M10 studs go all the way through the cylinder and head.  So, first thing is that you'll need to drill holes all the way through the ribs. You'll need either a vertical milling/drilling machine, or a good drill press, or even better - a jig bore.
1.Cylinder modification
   Cylinder has to be fastened upside down to the milling table with couple pieces of 2x4 underneath, and you can use original mounting  bolt holes in the bottom flange as guides (yes, it will match the new head's pattern!). Drill all the way through, until wood chip starts to come out. I used just regular HSS 7/16 jobber bit.
Then, you have to cut down top of the cylinder on the lathe so that the new head gasket fits on much like on the new 750 cylinders.  Russian dude that wrote above mentioned article also did cut the cylinders down even more to increase compression ratio, since combustion chamber on  a 750 is larger to fit domed pistons. And used 79mm pistons (mine are 78.75 now, so good enough). This is really optional, and I'm not going to go that far, ratio will drop to about 7:1, but I really don't care. I'm not racing it. Plus you can use regular gas, e85, old gas scavenged from scrap cars or any horse ###### that burns. :)
Also, make sure the surface where push rod tubes go in seals tight.
2. Long M10 pins, or studs, or whatever you want to call them.
There are two ways to go about it - either get stock 750 studs and cut them down and re-thread, or make them yourself on the lathe. Since 10mm stock on this side of a ditch is not realistic to find, I used 7/16 machine steel stock machined to, 9.95mm diameter on threded ends, and a bit thinner in the middle. Length - just eyeball it , make sure you got enough thread to go into the crank, long enough piece to go through the cylinder and head, and enough thread on the other end to hold your rockers.
Thread is m10x1.5, very common. Use dies to cut it, or just cut it on the lathe - which ever you feel more confident with.
  3. Rockers and pushrods.
Since the spacing between the new m10 studs is larger than between old m8 studs on 650 heads, the old rocker assemblies will not fit.  Of course, if you got access to lathe, hardening facilities and a cylindrical grinder, then from a proper size drill rod stock  you can make yourself longer rocker pins , drill out holes in mounting blocks, and make some spacers to position rockers where they have to be, but it's a lot easier to just use the new ones off the 750. And they're better anyway.  
Pushrods can be used from 750 model, but they have to be shortened, and also the ones from Dnepr/MT will do.
4. Head modifications.
As you'll notice, the oil hole on 650 cylinder and 750 head don't match, so in the head you'll have to machine a slot in the head's surface to connect them, and cut a corresponding opening for it in the head gasket. I just machined the slot in the head on the mill using simple two-flute 1/4 hss mill end. And drilled two holes into the gasket and chopped out a slot with a small chisel. Marking it propertly before you do the cutting is important, so take your time. I used paper pressed together between cylinder and the head to get it right.
And of course, you'll need 750 rocker covers and gaskets. and either use 750 exhaust elbows, or just weld on flanges on your old ones.
Carbs - up to you. I use Pekar k65's  - i had to file out holes in the mounting flange a bit for wider spaced mounting studs.. But if you got export CV's  - sure, why not.

Please note, that this modification is not to increase performance, but just to improve reliability, since quality replacement parts for 650's are long gone. Last season I had to change valves twice in the new aftermarket heads, as they burned off after about 2k. And after seeing completely technically illiterate dudes riding new 750's with no problems, I decided to do this upgrade.

#2 Dahmer

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Posted 24 November 2016 - 11:22 PM

Sorry guys, I mentioned the thread on studs as M10x 1,75  - it's actually M10x1.5..Don't want to give false info there.... Just the thing is, prior to this post I had to thread about 50 holes to M12x1.75, so 1.75 the pitch just got in my head. It's not CNC , you know...
Already corrected the original text.

#3 Dahmer

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Posted 19 October 2017 - 04:12 PM

Since this upgrade it has been almost a whole season (where I am), and over 9000km. Works great. I posted a video of how everything works. New parts - all came from e-bay.
https://youtu.be/EwV58jMLaPc
Please note that using new heads decreases your compression ratio a bit, but I really don't notice much difference. Again, this was done to increase reliability, not engine performance.
Other improvements were Herzog gears and 10:35 final drive. The 35t gear of the final drive came a bit too thick for teeth to engage correctly,so I had to bring it down from the back side about .030" on the surface grinder.  Now it cruises about 60 mph at about 3000rpm. Not bad, considering the fourth gear is still not direct 1:1 ratio like on most 4-speeds , but more like a 4th  on a 5-speed. Originally 4th came something like 1.4:1 , but Herzog seems a bit less than that.  With the old gears it hit 60mph aat about 3200. Getting the gearbox out on this model is a bit of a pain - you have to disconnect all engine mounts and move the engine forward. But on later models from M-67 and up the gearbox can be just unbolted and taken out of the frame to the left side.
I did replace original ignition coil with a pair of 6v coils I took off one of my Jawa bikes( when I converted to 12v CDi), wired the pair for the twin spark and they work much better than what was there originally. Dual-fire coil of appropriate voltage from a Harley will work as well.
Anyway, the whole thing works great, even though some were skeptical if it will work at all. If you got an old 650 model that has no compression and grinds gears - that's the way to go. New heads plus Herzog and it will be just as dependable as the new 750. And yes, this idea will work for Dnepr as well, and new gears too, if you change the whole gearbox. Also forged pistons are a must, with Hastings rings.
Another possible upgrade is replacing bronze bushing at the rear end of the camshaft  with a bearing (like on the 750 model), but that requires removal and complete dissassembly of the motor. I haven't gone that far yet.




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