998 pistons into an 850?

[petrolhead]

This is all hypothetical at the moment, just mulling things over.

I have some +0.030 998 plat top pistons and would like to fit them to my 850 block, this would the equivlant of a +0.090 overbore for the 850

Why I hear you ask, well why not. Would make an interesting high reving engine

Anyway, has anyone done this?

[Tim]

Its very common to fit 998 pistons into 850 blocks over here, especially with 850 pistons 2 - 3 times the price of 998 ones.

I haven't done it but I would expect that 30 thou ones would be OK. you need to deck the block because the pin height is different and the 998 pistons don't come to the top of the bore.

With the right head and camshaft I guess you should end up with a little 916cc version of the 1071. The one problem with your interesting high revving engine is that the 850 cranks don't have a great reputation for longevity at high revs...

Tim

[petrolhead]

I haven't done it but I would expect that 30 thou ones would be OK. you need to deck the block because the pin height is different and the 998 pistons don't come to the top of the bore.
Tim

Do you know by how much approx

[Tim]

I've been told that 4mm needs to be decked off the block, but hopefully someone who has done it can confirm?

To do it properly you'd really need to do a dry build and measure it, but thats an extra step when you probably want to get all the machining done at once.

Tim

[Jono]

Peter,

You used to drive a Caterham R500 and now you're looking to tune a Mini 850 - what's going on

[petrolhead]

I've been told that 4mm needs to be decked off the block, but hopefully someone who has done it can confirm?

To do it properly you'd really need to do a dry build and measure it, but thats an extra step when you probably want to get all the machining done at once.

Tim

Yes, it was our intention to dry build to check total combustion area. Looking to achieve about 10.5:1

I have done some quick calculations based on the following:

The pistons are flat top +.030 so that would give me approx. 914cc, which is 228.5cc/cylinder
The head has 23.1cc
Gasket 4cc
Top ring to top of piston 1cc
Piston to deck, well this is the bit I don't know but lets say its level with the top of the block (I know from your post its not)
So that's a total of 28.1cc

I think that would give about 9.5:1

Hi Jono

This is a project for Neilson

[CooperTune]

I'm currently rebuilding several 850s from early UK Mokes. I have 3 1275s and two 1071s and a 997 to build this fall and winter. Finding .030 pistons (NOS) for the 997 was a stroke of luck. I'm thinking finding three sets of NOS 850 pistons is not going to happen. I have access to a fully equiped auto machine shop. Even so wacking .160 off the top of three blocks will take half a day. There are pistons out of OZ but they are not cheap and and not a performance type piston. I have an idea and if it works out I'll share it with the board.

In way of introduction I'm Steve Gibbs and I have a little 5500 Sq Ft classic mini only shop on the east coast of the US. My collection includes a 64 1071 Cooper S, 67 UK Moke, 85 1000E (looking a lot like a MK3 S), and a pair of 60 850s. All but the 1071 are LHD, being in the US I'm fine with that. Steve (CTR)
PS CTR stands for CooperTuneRacing which was my US vintage race team in the 90s.

[Chalkie]

One word from me

Turbo.

[CooperTune]

You might be missing the point. Of all the mini models the Moke seems to be the big winner in value. They ( the UK ones as far as I know ) were all 850s. A proper restoration would include the matching number power unit. Bits for 850s are thin on the ground. Not so much about performance as appearance. Anyone could slam a 1275 Metro unit in place. I have never been a one word kind of guy. Steve (CTR)

I have been ask where I'm located. My shop is in Hampton Va 23661 (757 880 9283)
This is the south east corner of VA. on the water.