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I am looking to rebuild the engine from my '67 (?) K-750M. I have never rebuilt one of these before, so I am looking for any tips/techniques/traps associated with this engine.

 

I've been reading a lot of bike rebuild stories from this and other sites, but I haven't found a thread dedicated to rebuilding these engines.

 

Any suggestions/advice?

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I have been doing a lot of searching around to try to find out what the piston/bore standards for this K750SV. I have the cylinders at a shop now being magnafluxed and cleaned up. I am looking at an overbore, but I am not sure what piston clearance standards are on these bikes.

 

Does this look right?

Piston Cylinder Bore

1: 78,13 78,2 (+0,03)

2: 78,43 78,5 (+0,03)

3: 78,68 ?

4: 78,93 79,0 (+0,03)

 

Does anyone have any advice on bore matching pistons? I am looking at buying, what look to be, EU quality pistons from old timer garage.

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I would ask Leszek what he recommends. I have heard that with the stock Soviet pot-metal pistons that the overbore should be as much as .06, because the pistons expand so much when they heat up, especially under load with sidecar. With newer alloys the clearance is less.

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Thanks. I just got a reply from Old Timer about their 78,13 pistons. They said the Pistons may be a bit bigger sometimes, maybe 78,15. Clearance should be 0,08- 0,09. That's for their pistons made in Almot Poland.

 

So the old manual says 0,07-0.10, Old Timer says 0,08-0,09. Seems close...

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  • 5 years later...

Hey everyone! I am back after a long delay in work on this bike. I am going back to catch up on what I had previously learned, but subsequently forgotten. Looks like this one was solved...got to get back out there and measure some clearances now.

 

Next question for the experts. What piston ring end gap are you all running in K750 motors?

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So the advice I recently got was to just was apx 0.3 mm for the end gap for a 79.00 mm cylinder. I will work on that one. But there is a new problem. I measured the cylinders to see how my machinist did in nailing the 0.08-0.10 clearance we had spoken about (note that machining work was done years back when I started this last time and I have subsequently moved about 4 times...).

 

I set my bore indicator to 79.00 and gave it a shot:

IMG 20191227 154026 542

 

 

IMG 20191227 153907 308

 

IMG 20191227 162340 478

It looks like it came up short.

 

I don't know what I'm doing, so if anyone has any advice on my technique let me know. But, I think I need to find a new machine shop.

 

Can anyone recommend a machine shop in the Seattle, WA area?

 

Thanks!

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For custom machining expertise I recommend Loder Instrument, in Redmond, WA.

They are a small traditional custom machine shop run by a Mechanical Engineer. CNC work also.

Times I have worked there (helping out the owner when one of their machinists was out) found us doing work on diverse things including antique tractor axles, and precision deep water sampling titanium devices, all in the same day. A high precision facility.

There are undoubtedly more automotive centric machine shops in town too, perhaps better suited to your needs. I am just not familiar with them.

 

While in Redmond you can always visit the URAL Motorcycle headquarters. too...

RussN

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Notwithstanding what racepres asked, and assuming your cylinders are honed and measurements are correct then note Ural Zentral sells 78.9 mm pistons that they claim 78.90 to 78.93 tolerance. This would seem an ok fit given your measurements and the required clearance.

 

All your measurements seem to be within a range of a hundredth of a millimeter from smallest diameter to largest diameter. Pretty good machining in my books and compliments to your machinist. You just need pistons to fit.

 

ND

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HI guys, thanks for all of your quick replies.

 

Y'all asked some very valid questions!

 

NorthernDuck: The pistons I ended up getting were the ALMOT 78.96 mm. Now, if I remember correctly, it is because the cylinders had scoring that required quite a bit of material to be removed.

 

IMG 20191229 091708 666

 

Racepres - I am not familiar with the differences between those...but the cylinders were bored and honed with a nice cross hatch and were supposed to be ready to install. This the first time I've done a total engine rebuild, so I may be missing something.

 

 

Russ Noe - thanks for the recommendation. That's not too far from here, I may check those guys out.

 

Like NorthernDuck said, they did a good job with the work, I think we may have just miscommunication on the desired piston clearance. I took a look at the uralzentrale/oldtimergarage options and the best is that 78.9. However, they can run up to 78.93 and with my min bore of 78.991 that only leave 0.061 which is tight

 

I think I'll search out a machining option first if I can to shave off that extra 0.06 or so and stick with the same pistons.

 

I really appreciate all of your inputs! Thanks guys.

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If they are in fact assembly ready... they should have been "washed" with very hot water, then wiped until perfecvtly clean with an oiled [paper] towel..

If ya wipe with a clean paper towel...the towel should come out perfectly clean and colorless...except some oil!!!

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