1220 is an 1098 bored to 68mm and fitted with hillman imp 875 pistons(std size) and then decked a little.
i was thinking of the stroke/bore ratio
on the 1220 it should be 83.7\68 =1.23
the 998 is 1.17
the 1098 is 1.297
(imp pistons are shorter than 1098)
so the 1220 is a good comprimise
then again, standard 848 is 1.08,

correct me if im wrong here.

this is a good table, im not sure how you get some of these, i think they are impossable, one requires a 1098 crank and 73.5mm pistons... not so sure about that, 1275 block? i know the 1098 wont go that far out.

http://www.ime.org.uk/ime/b/bore.html

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speed costs....how fast do you want to go...

thats boring it out quite a bit right?
does it have to be offset bored?
this is all very interesting

i think it does have to be offset, theres not alot of meat between 2 and 3, a 1275 gasket is on the edge of the bore.
mine is offset bored, pocketed and decked, i think if anything goes wrong it will be fresh block time.

i hadent herd about these untill a couple of weeks ago, so this has been a good learning curve. very interesting.

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speed costs....how fast do you want to go...

Yes, you offset bore the block by moving cyls 1 & 4 apart, 2 & 3 together. One thing to watch is the oilway up from oil pump- you can break into it if unlucky. Then the oilway needs a sleeve.
<edit> plenty of meat between 2 & 3 on a smallbore block....
but very little in a 1275 gasket.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

i think mine has had the oil ways re drilled to suit the 12g940 head, i have heard they dont always line up.
its also been tapped for an 11 bolt head, will i have any problems with a 9 bolt?

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speed costs....how fast do you want to go...

ok so now i got thinking....head
so i have 2 standard heads(12g940) to choose from, i have one thats done about 1000km sinse a full recon to standard($900nz) (not by me, the guy i brought it off let his wife drine the mini into a pole after 300km)
i was thinking just take the lumps out of the ports a little, smooth it all off, not pollish, so it keeps the gas velocity up. standard valves are well big enough i should hope, not that i can go bigger anyway...
oh, the peaks have been reshaped slightly.
now, sinse the block has been decked, and the head has probably been slightly trimmed, what is this going to do to my valve lift? it will be running a plus (solid) rockers. not a plus head though, no big valves.

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speed costs....how fast do you want to go...

Decking head or block will not affect valve LIFT, but decking the head will bring the valve heads closer to the block to start with. So there is less room..
Valve lift with a stock cam is ~8.0mm, and you need around 1.5-2mm clearance in addition for safety, in case it gets over revved.
So, measure from valve head to head face, add head gasket & notch depth in block, & see what you've got.
If cam is not stock you need to check lift at the valve.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.