XS 850 Re Model

Eugene, thats looking good. The black with red speed blocks is one of my favourite colour combos. You also have nice brakes, and it looks like a Yoshi pipe and I think I can almost make out that you have some Mikuni RS carbs.
Also like the single face dual dial speedo /tach unit.
Let us know how it goes. and thanks for the encouragement.

Back to my current problem.
As you can see in the photo the pin had been well and truly rogered. So after reading on the interweb a thread that you may be familiar with Eugene my plan is to drill a hole 180 opposite the old one and cut another slot in the advance unit.
Crank pin.jpg

First I dug up a XS 750 crank and got the pin out, luckily it was still proud of the surface. Long nose pliers skidded off so I used some heat (about 1 minute of mapp gas) and magnets and it slid out easily.
Pin out.jpg

With a small dremel grinder I cleaned up the area of the snapped pin and blued it so I can see the remains of the old pin .
I have now started to make a drill guide for the new hole, it has a slot to locate over the old hole and will have a hole to drill through 180 degrees opposite.
Jig.jpg

The crank material is reasonably hard so I will get some carbide drills to give me the best chance of succeeding.
Then I need to cut a slot 180 degrees opposite the existing one in the ATU. I will epoxy the old slot so I can't install it in the wrong position.
Advabce.jpg
Then need to check the timing marks and move the TDC pointer if needed.
Before all this started I had finished thw number plate mount and stripped the bike down to a frame and motor ready for cleanup .
Bare.jpgBare 2.jpg


I will then do a leak down test, and check that the 750 inlet cam will fit. I found some info on the 750 cams that makes me hopeful but want to try it before I strip the motor.
I also need to make a puller to get the driveshaft out of the swing arm. This bike just keeps giving.
 
Spark erosion is often a good, if slow solution to removing hard difficult to access parts. Sometimes it's the only viable solution. Not something you can do at home. Just a thought passing through my head.
 
She’s keeping you busy.
I looked at that option of the 180 swop. If the dremmel grinding hadn’t worked that’s where I was heading. My engineering skills aren’t in your league so the easiest solutions are usually my go to options.
Drive shaft shouldn’t get stuck in swing arm. Is it off the rear drive?
 
Just looked closer at that timing pin 😳 someone’s certainly had a right old go at it and buggered it up good and proper. It’s hard to even see the old pin.
 
The drill guide worked and the dowel is now 180 degrees opposite.
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I was able to drill the crank without buying carbide drills however recutting the slot in the advance unit was more difficult.
The material was very hard so had to grind the slot but it all worked. Just need to recheck the timing pointer is still correct but will do this when the motor is painted and reassembled.
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I have done a leakdown test and as far as I can tell its good with results well into the green sector and little variation between cylinders.
I may not go ahead with the 896 plan.
Had time to set the 750 inlet cam up in the mill and remark the timing dot 180 degrees from the inlet mark timing mark. Interestingly the 750 cam sprocket has 3 dowel holes spaced 1 tooth apart whereas the 850 cam has one dowel hole.

I also made an adaptor so I can put my degree wheel on the alternator rotor , not sure if it will be needed but I will then be able to check the timing marks are accurate and i feel better about turning the crank with the alternator securing bolt.

After dinner started to watch this again for info.

Found another small job to do- make a crankcase breather filter holder. No a biggy will mount it off a crankcase bolt.

Need to get some zip lock bags and plastic boxes to hold the motor internals during the motor strip.
 
Just mounted mine direct. Kept it simples.
 

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The motor is now apart, it was quite a job. Found a few unpleasant surprises nothing that money and time cant fix though.
I bought some plastic storage boxes and some wire cages that fit inside and a ton of zip lock bags. All the internals are now mostly stacked away.

The 850 is good to work on as you can take the top end off with the the motor in the frame which is good because the motor is a porker.

When taking off the sump the strainer was clogged so at least that works.
Also in the sump found the head of a sheared bolt and could not find where it came from.
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The rods are pretty shot almost all of the copper has worn off the inside of the little ends, new ones are available but very expensive. The bore is on its first oversize . Still ruminating on the 896 kit.
Also found the starter bendix gear was worn (et tu 850).

When taking the transmission out the manual reads something like remove the bolt and take of the gear to withdraw the shaft. I could not see a nut to undo and the manual is in places a bit vague. I found the rest of the bolt sheared off flush in the shaft.
I don't think the gear could become jammed in use but if it did it would lock the back wheel the thought of it sent a shiver through me.
20250621_154525.jpg

I have another starter gear and transmission shaft if my plan to get the sheared bolt out fails so apart from time no real cost except the rods which will more than make up for it.

The next little while will be spent cleaning and prep for painting the motor and plating stuff also going to build a platen for the head to fit on the flow bench.
A little bit of welding to finish on the frame and that can also be blasted and painted.
 
Qucik update.
New rods in cupboard found one in NZ , and 2 on eBay. Also found another 2 on ebay.
Bearings abit bit of a nightmare. Got a full set of 750 crank bearings but the 750 has one odd shell that has flanges so managed to find a pair of 850 shells the right size.

I have 2 rods that need the green size and 1 that needs the blue size.
After a lot of searching and checking of parts numbers I think I have all the bearings I need. Need to get some plastigauge now.
We are so blessed with aftermarket parts for the 650.

As part of this project I need to change the motor on my compressor to a petrol one. Going to use the one off the paddock starter as that is no longer needed. Running a blasting machine is too much for the electric motor and it is tripping the thermal overload. The plan is to fit a bigger muffler for the motor and a muffler for the compressor intakes as well.

Welding is done on the frame and it looks factory which is daming it with faint praise ( I am not a welder).

Today I spent the my time cleaning up and replating the case bolts and thinking about how to tackle the job of fixing the flogged out rear engine mounts.
 
Note a lot to show but I have been busy.

The electric to petrol compressor conversion is almost done, just need to modify the belt guard and fit the second muffler to the engine.
I also made a quieter air filter (hopefully) for the compressor , fitted two more wheels and a steering handle thingy. Then I need to adjust the bull whip throttle control which works of the unloader circuit.
comp.jpg

A did a labour swap with a friend who will paint my motor in exchange for cross drilling some Harley discs for a Hard tail he is building.
I will never offer to drill Harley discs again. I have cross drilled many Yamaha 6mm discs and they are easy, the Harley discs were so hard I bought good quality 6mm cobalt drills and still had to sharpen them after about 3 holes, very slow going.

In between jobs I showed a friend how to rephase his XS 650 crank, pressed it apart and rephased it , gave him quick how to on trueing and off he went home .
He did a great job of trueing the crank and did it pretty quickly as well seeing as it was his first one.

Most of the motor is ready for painting, the barrels are being bored for the new pistons and 850 head is in the blasting cabinet.
The internal parts are bagged and put into plastic storage boxes .
850 in boxs.jpg20251004_171808.jpg


The new platten for the flow bench is done I put an O ring in the top to make sealing easier, also need to make an adaptor to depress the valves .
platten.jpg
The ports on the 750 and 850 head come from the factory a lot tidier than the XS650 heads but that would not be hard.
The 850 also has a bath tub combustion chamber and the 750 has a hemi.
 
Thank you, the design for the flow bench and a spreadsheet to do the calculations was generously given to me by a tuner from Invercargill. I also know very little and the flow bench has certainly taught me some lessons on what not to do.
 
As far as the XS650 heads I have only been practising on orphan heads with no matching rocker box.
One attempt with oversized valves and multi angle cuts had mixed results.
The exhaust flowed very well, the inlet was a bust.
I have been told by a local tuner that he found 2 valves heads don't like the multi angle cut, but have yet to experiment with this as there is always seems to be something more urgent or interesting to do.
Of late I have been watching Salter racing you tube clips on porting and had a couple of light bulb moments.
the plan is to be conservative , test then step it up with the 750 head then apply that to the 850 head.
The two heads don't look that bad to me which is probably a combo of ignorance and looking at stock 650 ports.
 
This is a bit out of order but progresss.
Mounted the lower engine case in the mill and overbored the flogged out holes then turned some solid stock to fit. Slugs in situ before welding.
slugs.jpg


I was wondering how to fix the less flogged but also loose frame bosses. The frame is too big to fit in the mill while contemplating this my Brother in Law appeared like a fairy godmother and gave me a set of adjustable reamers. He said he hasn't used them in years and they were all carefully sprayed with soft seal to stop them rusting.

The plan was to take the bosses out to 14mm and use glycodur bushes to take them back down to 12mm.
It worked ok.
Reemer.jpgbushed boss.jpg
Then using the front case mounts and a quickly made punch guide I was able to use the frame to mark the slugs for drilling . So back to the mill and another little reem and the job is done.
case repair.jpg
This is the good looking side the other hole is not even close to concentric.

I have spent a great deal of time on the flow bench since the last update.

The plan is to use stock valves and cam.

First up was testing the 750 head in stock form, it has the same cam lift and valve sizes as the 850. The throats stock are around 85% of the valve sealing diameter on both the 750 and 850 and that’s where I believe it should be.

After a quick cleanup of the ports just removing the tops off the casting lumps another test showed minimal change as expected.

I then opted to do a pocket port making the transition from the throat to the bowl smoother. All up about 5 mins work with small burr, then re test.

The gains were so great I did doubted my testing and repeated the tests a few hours later with near enough the same result. Great stuff however any further work I did reduced flow so I now had a plan and thought it would be easier to make the 850 head flow well in no time at all.


The 850 head is a later design, and stock it flows as well as the 750 head I had played with. For those of you with 750’s a 850 had will not fit 750 barrel but the good news is its very easy to get some decent gains on the 750 head.

I did a cleanup as before and retested with no real gains, so blended the bowls and got a small increase in flow. Nothing like the 750 head.

I then had an inlet valve back cut and again got a small gain at low lifts but very small, then looking at the head it looks like the valve is shrouded. I checked this with a ball bearing the right size on a bit of welding rod and decided to de shroud the inlet valve and retest. I was going to loose compression slightly but the compression jump from 850 to 896 would compensate for that.

After retesting again there were very small gains at low lift and full lift but nothing in the middle where I wanted it. I decided to stop there rather than loose what gains that had been made.
I believe a multi angle valve job would help but that is not in the budget.

So after all that work I can say that at least it is not any worse than when I started ,slightly better than stock and as a novice will take that as a win and a lesson.
Flow test.jpg


To make resurfacing the head easier to set up I removed the cam cover studs and as usual all but one easily came out, the problem one stripped the threads on the stud. Put a nut on and hit it with the welder and the stud came out with its hands up.

The head has been skimmed now to clean up a few imperfections in the gasket surface, my mill and flycutter are not rigid enough for a head that size. 0.2mm was taken off roughly 8 thousandths.
Head.jpg

I also bought a valve refacing machine it works but the wiring could do with a revamp, this will speed up experiments with back cuts on valves and I will use it to do the 850 valves which are not in a good state. Just about every Garage had one of these when I was young but now they are a bit of a rarity.
cutter.jpg
I will reface the valves and lap them in the head. Then clean up the cases and head a few more times to remove blast media etc then mask the gasket surfaces for painting.

Next up after finishing the cases is removal of the broken bolt from the gear shaft. My plan is to use the lathe centre drill then reverse grind a bit and wind it out . Like most things on this motor it wont go like that but that should be the last of the remedial work fingers crossed.
 
This is a bit out of order but progresss.
Mounted the lower engine case in the mill and overbored the flogged out holes then turned some solid stock to fit. Slugs in situ before welding.
View attachment 361114


I was wondering how to fix the less flogged but also loose frame bosses. The frame is too big to fit in the mill while contemplating this my Brother in Law appeared like a fairy godmother and gave me a set of adjustable reamers. He said he hasn't used them in years and they were all carefully sprayed with soft seal to stop them rusting.

The plan was to take the bosses out to 14mm and use glycodur bushes to take them back down to 12mm.
It worked ok.
View attachment 361115View attachment 361117
Then using the front case mounts and a quickly made punch guide I was able to use the frame to mark the slugs for drilling . So back to the mill and another little reem and the job is done.
View attachment 361116
This is the good looking side the other hole is not even close to concentric.

I have spent a great deal of time on the flow bench since the last update.

The plan is to use stock valves and cam.

First up was testing the 750 head in stock form, it has the same cam lift and valve sizes as the 850. The throats stock are around 85% of the valve sealing diameter on both the 750 and 850 and that’s where I believe it should be.

After a quick cleanup of the ports just removing the tops off the casting lumps another test showed minimal change as expected.

I then opted to do a pocket port making the transition from the throat to the bowl smoother. All up about 5 mins work with small burr, then re test.

The gains were so great I did doubted my testing and repeated the tests a few hours later with near enough the same result. Great stuff however any further work I did reduced flow so I now had a plan and thought it would be easier to make the 850 head flow well in no time at all.


The 850 head is a later design, and stock it flows as well as the 750 head I had played with. For those of you with 750’s a 850 had will not fit 750 barrel but the good news is its very easy to get some decent gains on the 750 head.

I did a cleanup as before and retested with no real gains, so blended the bowls and got a small increase in flow. Nothing like the 750 head.

I then had an inlet valve back cut and again got a small gain at low lifts but very small, then looking at the head it looks like the valve is shrouded. I checked this with a ball bearing the right size on a bit of welding rod and decided to de shroud the inlet valve and retest. I was going to loose compression slightly but the compression jump from 850 to 896 would compensate for that.

After retesting again there were very small gains at low lift and full lift but nothing in the middle where I wanted it. I decided to stop there rather than loose what gains that had been made.
I believe a multi angle valve job would help but that is not in the budget.

So after all that work I can say that at least it is not any worse than when I started ,slightly better than stock and as a novice will take that as a win and a lesson.
View attachment 361118


To make resurfacing the head easier to set up I removed the cam cover studs and as usual all but one easily came out, the problem one stripped the threads on the stud. Put a nut on and hit it with the welder and the stud came out with its hands up.

The head has been skimmed now to clean up a few imperfections in the gasket surface, my mill and flycutter are not rigid enough for a head that size. 0.2mm was taken off roughly 8 thousandths.
View attachment 361119

I also bought a valve refacing machine it works but the wiring could do with a revamp, this will speed up experiments with back cuts on valves and I will use it to do the 850 valves which are not in a good state. Just about every Garage had one of these when I was young but now they are a bit of a rarity.
View attachment 361120
I will reface the valves and lap them in the head. Then clean up the cases and hea a few more times to remove blast media etc then mask the gasket surfaces for painting.

Next up after finishing the cases is removal of the broken bolt from the gear shaft. My plan is to use the lathe centre drill then reverse grind a bit and wind it out . Like most things on this motor it wont go like that but that should be the last of the remedial work fingers crossed.
I have had pretty good success with using Torx bits as screw extractors. Just drill a slightly undersize hole and tap the Torx bit in. Sort of a poor man's Ridgid/ Ridge Tools extractor. 12 point XZN bits may also work, possibly better.
And I'm sure you are aware of regular left hand drill bits (which also can provide some entertainment in the hands of the unaware 😉)
 
Pressing on, I could not resist trying one more thing to improve low lift inlet flow on the 850 head.

The back of the inlet valve is not smooth so after removing the ridge and polishing I retested and there was a small gain at low lift, for the amount of effort involved it was worthwhile so have decided to do this to the inlet valves.

Before painting I gave the cases another clean and removed the drilling cap from the oil pump to clean it and found that from the factory the main feed from the filter to the bearings had a sprag still stuck to it. It was not a major problem s the motor had lasted many years but certainly was not helping so I removed it and replaced the bung.
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The snapped off bolt in the gear shaft came out but it fought all the way. My idea of using the lathe in reverse did not work. When you think about it the lathe everything is fixed but for the bolt to reverse out something must move. This is not a problem with a hand drill. Any how the bolt is out with no damage to the threads. On the 750 parts motor I have the shaft is different.
20251120_093930.jpg


The 850 valves have all been refaced and lapped I then spent an eternity masking the motor ready for painting.

Either way (bare or painted) to get the motor looking even halfway decent was going to take a lot of time.

The motor is now painted and I have started assembling it. The valve clearances were all out as to be expected but by swapping shims around only 2 new ones are needed. Waiting now for shims, seals and a cam chain.
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While checking the crank I noticed that some of the chamfers on the oilways were not complete so I touched these up with a die grinder and then blew out the oil ways with compressed air. The NOS crank bearings checked out ok with plasti gauge. However I have never used it before and will get my work checked by my mate who is a mechanic.
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Ran into a head scratcher with the con rod bearings. Crank bearings ,conrods and big end bearings are hard to get, and I had to source them from different ebay sellers.

Two sets of big end bearings are fine, but one set came in the correct packaging but the colour although very faded did not look like the other of the same colour. They all check ok with plastigauge. I will also get this checked before I begin the search for another bearing but I am doubting my self now.


I paint stripped and sand blasted the frame and swing arm. I don’t have a cabinet big enough to take the frame so did it in the back yard. It now has etch prime on it ready for paint.
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Still playing around with the instrument panel and on a whim bought some carbon fibre vinyl wrap. It was not to hard to apply apart form a bubble right in the middle . I think it looks cheesy so its not staying. Wrapping is on the table still just not that pattern.
aaaa.jpg
The work continues...
 
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