Hi,

I was wondering what are the best pistons for a small bore boosted application? Size wise i was thinking 998 + 0.60". Does anyone have any ideas? I assume you cant get Hypatecs in small bore sizes?

Thanks, Rob

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Mini - The Wizardry on Wheels!

You can get Hypatecs, but they were out of stock a while back.
Try Ken Nelson- www.miniauto.com.au/home.htm

<edit> I think kazjim is running ACLs in his, with 15+psi boost.

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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.

Yep...

had the head off this weekend for a look-see and its all good.. crowns are still perfect and the bores are showing NO scuffing or ridging after 15000 boosted miles....

Keep the bore size as small as possible, keeps more rigidity in the bore and prevents as much "bore flex" ... also allows for another rebuild later !

Mine is only 0.020" over ......(73HP@Wheels - over 100@FW...) gotta be doing somthing right.....

J

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Thanks for the help. I'll have a look around.

I was thinking whether i should do a 998 or an 850? any idea on the average cylinder wall thinknesses on either blocks? I have heard people say that 850's are among the best to boost because they have such a small bore and stroke they are less prone to detonation?

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Mini - The Wizardry on Wheels!

I wouldn't use an 850 block because:
1. Main bearing widths are 1/8" wider, as 850 crankwebs are thinner (won't fit a 998 or 1098 crank in there without machining block).
2. No centre or rear bearings for the camshaft, it ran straight in the iron block.
3. Block is no thicker around the bores than a 998-1098 one is.
4. Thrust washers are smaller than 998, I think (hey, it's been 30 years since I did one.. )

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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.

Doc, what do you think of a 998 with a 850 crank in it (894cc)? According to Vizard this is do-able and makes avery nice short stroke small bore motor. I know you have to deck the top of the block a lot.
I would have thought the 850 cranks would be quite strong because of the journal overlap too?
Just throwing a few idea's around.

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Mini - The Wizardry on Wheels!

If you do it, make sure it's a late 850 crank with the thicker tail. Early ones were only 1-3/8" diameter.. I broke one, so did many others back then.
IMO unless you are trying to fit it into a 900cc? class for racing, it's a waste of time. For a road Mini, more cc works better than more rpm.
Can't beat cubic inches..

As for the 850's extra journal overlap, that's offset by the fact that the 850 crank webs are thinner and weaker.

If you really want to build a strong short stroker that'll breathe and rev, I'd PM minicranks and get one of his Sth African made cranks.
They are pretty much a 1071S crank, with similar stroke to an 850, but have 2" mains and 1.75" big ends and will go into any 1275 block. Then you have an oversquare motor.
You could even destroke it more if you really wanted to.. but then you would probably need 970 S rods, or specials.

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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.