998-pistons in an 850

[Vegard]

I'm in a process of rebuilding an 850 engine, and as you might know 850 pistons are scarce.
So, people have successfully fitted 998 pistons, and I was thinking of doing such muself.
However, have anyone here any experience on how big 998 pistons which could be used? Would a +40 go?

[Striped shirt]

Hello.
Maybe I'm missing something, but if you're over boring an 850 block +40 to fit 998 pistons (and making it not 848cc), why don't you just fit a 998 engine block?

Sounds like a world of pain for no real gain.

[whistler]

Hello.
Maybe I'm missing something, but if you're over boring an 850 block +40 to fit 998 pistons (and making it not 848cc), why don't you just fit a 998 engine block?

Sounds like a world of pain for no real gain.

Probably because he doesn't have access to a 998 engine plus it sounds like he already has the pistons and a usable 850 block, crank and rods.
I might do the same as I have a set of +0.030" cooper D pistons.

[Vegard]

I have several 998 blocks as well, but this would be too easy. Also I’d like to use a period 62 block in the 62 car.
When people ask what drove past their 998 I could say it’s an 850

[MagicWandWoody]

I wondered the same - again for the reason of keeping the 1963 car original-looking.

Found this, and decided it was a non-starter: https://www.theminiforum.co.uk/forums/t ... 8-pistons/

My choices now are...
- Just rebuild the 848 as standard. Chances are I can't though - it's been standing since '74 and is properly seized.
- If it needs rebore, cam bearings, etc, then +060 it and experiment with a high-torque cam.
- If the 848 is stone dead, then chuck in a 1098... which looks a bit like an 848. If you squint. With Vaseline rubbed on your eyeballs. And then uprate the brakes.

BTW Somerford claim to have 850 pistons in STD, +20, +30, +40, +60

[Vegard]

Many have successfully done what I’m aiming for, so some have definitely managed to skim the block sufficiently to the correct amount. I’ve got several blocks, some dead ones too. I’ll have a go skimming them first.

I just build a std 850 with piper 255 cam, modified head, modified iron manifold opened up for a single h4. Gave 46hp on the rollers.

[Tim]

I just made some crude measurements on an 850 block, and I think 998 +040 pistons should fit.
If my numbers are right, 998 +0.040 pistons are approximately 65.6mm in diameter compared to a standard 848 of 62.9mm, that means the distance between cylinders will be reduced by 2.7mm. It looks like there should be enough material between cylinders 1&2 and 3&4 to do it without having to offset bore. It ends up around 923cc.

I'm contemplating something similar for my Traveller.

Tim

[irlmin]

I personally would go for a 1098 , looks 'right' and even in standard form has decent torque. easy to tune and some tuners have got some amazing results , one such tuner is Keith Calver ,have a look at Calver ST . with a decent build this engine will never be stressed .

['S'-type]

Vizard in HTMYM covered this. The 850 block can safely be over-bored up to 100 thou.
Back then A40 Devon pistons (std/+20/+40) could be used, having the same gudgeon pin and same pin to deck height. But skirt needed shortening and they weren't any good for high revs.
If using 998 Cooper pistons, the amount needed to be taken off the block is 1/8". The pushrods will need shortening by the same amount of course.
He also covers stroking it also with either a 998 or 1100 crank, but there are a lot of operations needed to make it work as well as the obvious need for fitting cam bearings in the 850 block anyway.
Given the economics and availability of parts for minis back then it may have made sense to do all this, but it's hell of a rave for little gain these days. As DV points out extra inches are the cheapest horses.
The mini seven 850s revved to 10k back in the 70s, so the 850 block is capable in the right configuration. Vizard was of the opinion that prior to 1964 1100 blocks used the same casting as 850 blocks. So as it came standard, 1100 blocks could be considered aldeady as an over-bored 850 block.
Vegard you must like a challenge.

[Exminiman]

Interesting topic, are you going to fit cam bearings ?

[Alex]

AC Dodd did it a few years back and posted updates on the dreaded book of face. the big difference was that he also used a 998 or maybe a 1098 crank by altering the thrust faces if I'm remembering it right.

[gs.davies]

+1 for just putting a 1098 in. My one goes pretty well for what it is.

[whistler]

I built a 1098 for my mk1 following Keith CalverST pathway. It produces 78bhp and 80 ft/lbs.
Basically +0.100" flat top pistons, BP255 cam, CalverST head, recurved 59D distributor, HS6 with BDL needle

[Vegard]

I built a 1098 for my mk1 following Keith CalverST pathway. It produces 78bhp and 80 ft/lbs.
Basically +0.100" flat top pistons, BP255 cam, CalverST head, recurved 59D distributor, HS6 with BDL needle

I’ve built the same. 1100 +20, piper 255, modified 202-head with 295 inlet valves.
Made 75hp and a massive flat torque curve.

Interesting topic, are you going to fit cam bearings ?

There’s no need with a piper 255 camshaft. I won’t need hard springs.

[Costafortune]

Vizard was of the opinion that prior to 1964 1100 blocks used the same casting as 850 blocks. So as it came standard, 1100 blocks could be considered already as an over-bored 850 block.

That's correct. It was when the 998 engine was introduced in late 1963 that the 998/1100 got their own block. Early ones have the '850' casting number ground off.

I'd bore the 850 block out and use 998 crank/rods/pistons, also doing away with the pinch bolt conrods used on the early 1100's.

I wouldn't bother too much with putting cam bearings in if you are using mild camshaft. Modern oil being what it is will cope fine.

Standard 998 pistons with an 850 crank is 894 cc.

[NZmember]

Talk to Simon Drew at Miniworks. He skimmed 60 thou off the 850 block and fitted +40 998 pistons, rods and crank etc

http://www.theminiworks.org/assets/imag ... wntown.pdf

Or Simon Naylor

http://mk1-forum.net/viewtopic.php?t=10615