mk1-forum.net

Hi all.

I continue with the restoration of my 1962 Morris Mini Super 850, and it is time to overhaul the engine.

As I need to rebore the block, wich is the maximum size of pistons I can use?
Can I use 1098 pistons with the original conrods? I have found the followings:

Dimensions:
65.596mm diameter
CH 30.5 -5.3mm
Total height: 58mm
Pin: 15.875 x 55mm

I am also need a new head, and I am thinking in a 12G202 skimmed to reach 9.5:1 com ratio. 12G295 are too expensive, and my head is really bad (rusted, two broken studs in the thermostat housing, etc...)

Thank you for your advice.

Ricardo

The 1098 standard size pistons would be the same as a 998, which is 64.58mm. If the pistons are 65.596mm they are +040 oversize.

Going from the 848 bore of 62.94 (which is the same as the 948 engine) to the std 64.58 is an increase of 1.64 mm or +064 thou. There are +060 pistons for 850 engines out there, so that might be possible?
http://minisport.com.au/pistonsa-rings-liner-sets

If you went from std bore (62.94) to 65.6mm that's +105 oversize which might not work? I'm no expert though.

If you were able to bore the block that far, the CR would be well down, even with 998 pistons which are taller than 1100 ones, you'd be about 0.156" short of where you'd like to be.

I haven't done a lot with the 850 blocks, however the early 850 blocks if I recall don't usually have a lot of meat in them to bore with and you could be pushing your luck beyond 0.080" over stock.

I've been round this problem and eventually decided to just put an 1100 in my car. Everything needed machining anyway or replacing (I had the thin tail wet primary crank and I'd planned a cam upgrade so would have been looking at fitting and line boring of new cam bearings too) and given the cost of 850 pistons, their general poor design and quality, it made sense to 'archive' my old engine and build something a bit better.

Current spec is a +10 1100 with NOC BMC High Compression pistons (think they're the MG1100 spec one) Cooper 997 spec cam and a 295. I've yet to obtain a set of twins and an appropriate inlet and exhaust manifold and I've decided that this development will have to wait until after the car's built. We'll see how it runs with a single HS2 and the peashooter exhaust...

Good luck.

Have a look way down at page 3 on this 850 Speedwell project to see what can be done: http://www.mk1-forum.net/viewtopic.php? ... 0&start=20 aint impossible!

Maybe a little off topic, I have a UK Moke with an 850 of course. I rebuilt another Moke 850 last year and was able to locate O/S pistons NOS. All four pistons were slightly different sizes. Not a problem as they left .005 in the bores to be honed to correct finish for each piston. Cost me a little more but extra attention to the details is worth it. Before finding the pistons I looked long and hard at having the counter weights on a 998 crank machined to get the crank in an 850 block. The engine I did build could have used a single HS4 but owner wanted it to appear stock. I spent quite a bit of time working a single HS2 and the intake. Tried everything Vizard suggests to get a HS2 to flow with an HS4. Steve (CTR)