850cc Piston Query
Turbo Mini Forums

I helped a friend from my mini club pull apart his 1965mini 850cc engine as for rebuild.

We found a set of aftermarket pistons in it with just +30 on them. All looks good apart from one ring being snapped and worn out big end shells.

I have found the shells available on minispares website however no standard measurments are given in the haynes manual for big end journal diameter - so how do I know if its undersize with no base figure to compare my measurments to (no writing on the back of bearings either?)

I cannot find anywhere online selling mini850 pistons or ringsets standard or otherwise - Any ideas on suppliers?

Thanks Guys

Dan

pistons are a problem in 850's. There are a few sets still going around, but they arnt generally for sale anymore. Last time i checked DSN still had some for sale on their site, but this was a while back and i dont know the source of them. I wish minispares would release a 'mega' version of them.

An australian piston manufacturer JE/JP still make 850 pistons, but im unsure on the quality. Price isnt too bad, but they obviously have to be shipped over and you'll have vat to pay, price used to come in at around 140-150 + pnp +vat. I've heard some good comments about them, Mark Forster from MK1 performace rated them as good, but i havent ever heard anything othe than that.

There are other options in that you can bore to +62 thou and fit std size 998 pistons. An old thing to do in the late 60's and 70's was to fit +40/+60 998 pistona taking the cc's over 900, but this does mean a wedge of an overbore at +102 or +122thou. The deck also has to be substantially modified with around 150thou off the deck.

There are yet more options in that you can use 998 block in replacement of the 850 if you so wish, this would get around having to give the block a huge rebore/overbore, but larger thrusts washers have to be used as the 850 crank has a very wide centre main journal.

I've seen the future and tbh its Pie

IIRC, the '1071' chap (sorry cant remember his name) has his fingers in a few pies in the 850 front, maybe he knows more.

I've seen the future and tbh its Pie

http://www.jp.com.au/JPPistons.html

I've seen the future and tbh its Pie

The 998 piston idea sounds like a good plan. Would the standard a+ 998 con rods be suitable, or would a swap need to be made to the old 850 rods

you need rods to suit the pistons, im not sure if there were any press fit 850 rods? If you get the press fit hepolite pistons from minispares then the later rods from a+ engines would be fine. AFAIK, i might be wrong however, there is no such thing as a 998 A+ rod, i belive they were the same as the earlier press fit piston'd engines? Whatever they are they'd be ideal, i have some for my own 850 engine (also fitting 998 pistons, but will be using an a+ 998 block aswell).

Do bear in mind however that the skim of the deck is seriously big with the 998 pistons and might give you gasket problems.

I've seen the future and tbh its Pie

I'd watch Ebay. They do pop up occasionally..

Also, the 998 rods are both in press type gudgeon pin, and circlip. The old A ones uses circlips. As fas as I can remember, the only difference between a 998 rod and 850, is that the 850 uses a screw to fit the gudgeon pin. ANyway, 998 rods would be dirt cheap....