In 1968 on labor day weekend I bought my first mini. I removed the 850 power unit and began work on it right away. Reading Clive Tricky and Tripple Cs I came up with an engine spec.

850 block line bored for three cam bearings

Bored .060 O/S

zero decked

flywheel lighened

complete assembly balanced with Cooper S balancer.

Con rods lightened and balanced shot peened

big ends reconditioned all bolts upgraded

510 Cooper S camshaft with duplex timing kit

Isky lifters push rods springs and caps

295 cyl head surfaced .125 with O/S intake and exhaust valves.

small amount of clean up in exhaust ports.

Twin 1 1/4 SUs on S intake

Cooper three into one exhaust

small Cherry bomb with 4 inch chrome tip

Cooper S non vacuum dizzy

std rebuilt gear box with remote diff housing

4.10 CWP

The only gauges I could afford were a fuel pressure oil pressure and tach

Areas I'd improve now would be in carburation exhaust and valve train.

I'd give thought to a vacuum dizzy built and curved for this unit. Some minor improvements to the carbs and the air cleaners small bore roller tip 1.3 rockers. Ultra light flywheel and back plate with green or blue P plate Cooper S ratios in remote tranny converted to majic wand and a 3.9 CWP

Steve (CTR)

Thanks for the input. I have yet to pull the 850 block apart. The PO had removed the cyl head for a look and put it right back. I'd like to get it running on the test stand and see what I have first. It would be interesting to do a couple of dyno pulls before and after if I build this thing. Running the numbers have a soup to nuts 850 costing more than a built 1275. I considered a complete crank assembly swap either 998 or 1100. I even have a 970 crank on hand. In the end if it's not an 850 basic bore and stroke it won't be the same. Steve (CTR)

I'm still thinking at some point the factory installed all CBs in the block. I did have the block line bored and the additional CBs installed last time. I was using 8,000 as a red line and felt the bearings were required. Can't wait to check this 67 block.

Steve, back in the day (the `60s), I used to set 850 blocks up with the bearings on a 1.750" bar in my old lathe, and used a knife tool in the bar to face the sides of the bearing bosses and caps to take a 1098 crank and 1098 thrust washers.
Then I bored it and fitted a std 1098 crank rods and pistons in. With the stock 850 head and cam the torque was tremendous, no more rowing it along with the gearlever. It would just about climb a tree in 4th gear.
I never had cam problems with the 850's single (front) cam bearing, but back then we only used an 18-58 or 20-60 cam grind for sporty road motors, and watched the valve spring pressure. 

If you were going racing though, you had the block line bored for the other 2 bearing shells.

Chris

Steve what i would do is fit a 295 head aim for standard compression. Bit of flywheel lightening, a good exhaust and a fuel injection kit.

I think here's the post referenced from TMF:

//www.theminiforum.co.uk/forums/topic/258340-tuning-an-850cc-engine-where-to-get-machining-done/

Thank you Mal, went to the turbo mini forum and spent some time looking around there. The UK forums are even funnier than ours. Steve (CTR)

BTW I met a guy once who claimed he was having a 970 crank machined to fit an 850. I think the strokes are the same. I spent some time reading Vizard yesterday but little on 850s.

Steve TMF is TheMiniforum.co.uk i believe.

Guess I should Just Install my 1275 GT with 10.2 CR SW5-07 mildly ported head water /oil pumps and timing. Steve (CTR)

There , I fixed it for you.

Interesting to see what can come of an 850.  Test away - there are plenty of other blocks around if one is boogerd from too much cutting!

That's going to be a hunk of metal. .165 give or take. I don't know much about deck thickness on these blocks. I can see the D crown pistons helping. I'd like a little better set up. I'm sure I can come up with something. The new 850 pistons from down under will do all I want to do.

Guess I should sell my 1275 GT with 10.2 CR SW5-07 mildly ported head water /oil pumps and timing. Steve (CTR)

If I remember correctly, the small bore rods are all the same length.  However, you should use the later press pin ones from an 1100/998.  If you use 998 pistons you must deck the block.  The wrist pin location on the 850 piston is different than the 998, necessitating the decking of the block.

This has been a plan of mine for a few years now. Thanks to folks here on the board, I collected a few sets of used std Cooper D-top pistons to use in the build so I don't have to skim/deck more than reasonable to get compression halfway decent.

I already bored part of one block using the kwik-way boring bar I bought off of craigslist to see if the block would take opening up. No problems there, apparrently.

I opened up an 850 that someone gave me about 2 months ago and I found that it was already bored for +.060 850 pistons. less than 10-thou more and the cooper pistons will fit in (top of piston fit in but skirts didn't).

I keep distracting myself with other projects, though.

This all stems from wanting to build a 3-engined mini for LeMons.

Steve once you go 850 you never come back!!

Not sure I know TMF I'll look for it. Thanks It seems all but the rare 970 5.875 rods all mini rods are 5.750. That and the compression distance of the pistons makes a piston transplant unlikely. Steve (CTR)

I've just finished reading a thread on TMF where a chap is about to embark on a virtually identical build.  Might be worth looking at it.

With the purchase of a 67 UK Moke I'm now the proud owner of a 1967 850 power unit. While rust has eaten the floors out of my Moke the engine and tranny look very nice. I'm sure someone has been into this one. All alloy bits have no paint and the engine block and head are painted.

My plan is to rebuild the old girl to a very high std. Kevin was good enough to put me in touch with new pistons. I also was able to buy a complete 1100 off this board. There is my source for a 12 G 295 cyl head for the project. I have contacted a few UK engine builders just to chat the project up. All have suggested it has been 20 / 30 or more years since they have done anything with an 850. Checking my records it appears to have been 44 years since I built one.

I recall talking with George Sykes while building that one. We threw around the idea of using 998 pistons and picking a set of rods that would match. At that time we could buy Power Max pistons for the 850 so we never went down that road. If you have played with the 850 or have heard any wives tales please share with me.

I know early 850s have only one cam bearing in the block. It seems at some point in production they went to three bearings for the cam shaft. Can anyone place a date on that? Thanks Steve (CTR)