Suggestions for replacement 998 Cooper Pistons

[rolesyboy]

My standard 998 Cooper Pistons and rings are a bit tired. What can I replace them with? Something Plus 20 but what?? Only seem to be able to find flat top stuff which I guess means skimmed head? Any advise welcomed. Cheers. Mark

[Rusty old S]

Mark, flat tops ,then get volume of the bore plus the head when the engine is built
in CCs then work out the compression ratio ideally 9.75 to 1 .Your head has a good chance of been skimmed in the past , someone on here will know the measurement of a stock head. The combustion chamber CCs in a 295 are much bigger than a 202 casting that's why D top piston for the 998 cooper were used to fill the space to gain compression , normal 998 dished pistons with a small cc head (202)will give the same CR but you have small valves

[rolesyboy]

Thanks Rusty. Unchartered territory this 998 business.
Anybody tell me the measurements for a stock 295 head and what the maximum I can skim from it.
Cheers

[wantafaster1]

I'm going to be building one for the wife's mini, what I wanted to investigate was decking the block and taking a bit off a flat top to leave the d shape. I've not got any old pistons to measure though.

[CooperTune]

I have built several 998 s this past year. I have also done Spridget 948 s using 998 flat top pistons press fit with correct 998 rods. I have used a 202, 295 and 940 cyl. heads. None were 998 Cooper units. The H/C flat tops will make 10 to 1 with the correct volume chambers. You get compression much faster reducing piston to deck. The smaller chamber small bore cyl. heads help get compression up but have poor chamber shape with valve shrouding. The 295 casting can be surfaced from .080 to .125 depending on location oil passage feeding rocker shaft. This can and should be measured along with head thickness. Depending on amount surfaced and cam a 940 big bore head requires valve pockets cut in the cyl. bores to clear exhaust valves. The ports and chambers on 295 and 940 cyl. heads are nicer than the balance of small bores. There is one more small bore head 205/206 which is nice but harder to find. I liked using a 940 head as it allows drilling and installing the 10th and 11th head fastener ( like a Cooper S ) and the chamber starts 4 or 5 CC s smaller than a untouched 295. With the fuel issues we are seeing I'm leaning toward 9.5 CR s unless customer wants more.

As for making D pistons from flat tops and decking the block I don't think there is enough material for all that. The 948 with H C flat tops zero deck and .010 off cyl. head worked out at 10.45 to 1. Since the head had been surfaced before I don't know the total removed. It's a Lotus 7 show car, only gets out for shows and meets. Runs well on Hi test and likes a gallon of 110 added to a tank. With a Kent 266 cam, twin 1 1/4 SU s, a nice exhaust and good dizzy she is a scream. Steve (CTR)

[mk1]

The "D" is 2cc.

Easiest way to sort this is to get 24 thou skimmed off the head & use flat tops.

Job done.

M

[Daz1968]

Above sounds easiest option, but depends how much of a rush you are in as original d tops turn up for sale quite often. I managed to get a standard and +0.20" set with not too much effort

[850man]

I use +100 Hypertec flat tops, 0 deck height and then skim the head to suit whatever comp you require.

[BLT]

Here you go, someone selling some on eBay -

http://www.ebay.co.uk/itm/Classic-Mini- ... SwnNBXXYLo

[Spider]

http://www.ebay.com.au/itm/Classic-Mini ... SwHaBWjXIK