As I under stand it;


So what engines are better suited to being bored out to a "square" bore?
I'm looking at the small bore 998 and 1098 engines, ideally I'd like the 1098 as we all know cc's win.

I'm pretty sure the 1275 is over square as the cyl diameter is greater than the stroke. I also don't want to use a 1275 because they're harder to get/more expensive and I already have a few of each small bores. So what are the possibilities with the small bore engines?

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Respect mine and I'll respect yours.

All A series are undersquare except the 970S and 1071S.

A 1275 is 70.6mm bore and 81.33mm stroke.

Bore a 998 or 1098 out to +.100" is about all you can do really. Shortening the stroke on a road car (if it was possible) is a waste of time, it just reduces displacement and torque.

An overbored 998 or 1098 can be persuaded to give nearly 100HP at the crank with a lot of work. If you want heaps more mumbo, bolt a turbo on.

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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.

How about a certain someones square 850 with 50hp almost Formula Junior, and thats just the test motor, the real motor will have a 544 cam and twin 1.5 SU's

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P76 alloy V8

3.5inches x 3.5inches



sorry, couldn`t help it

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

i think you answered your own question.

seeing as how an 1100 can be made to rev to 7000, on a road car the squareness really doesnt matter as the capacity is the main factor in the power from the engine.

For drivability in a road car ie. torque, as the Yanks say "there ain't no replacement for displacement"...

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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.

What is the stroke of 998 and 1098 in mm?
and what is the diameter of a 100" over bore in the same engine? also what is the diameter of 30 60 and 80 thou over bores etc?

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Respect mine and I'll respect yours.

998 stroke = 76.2mm
1098 stroke = 83.7mm

bore of both is 64.6mm

.030" (an odd size now) is 65.35mm
+.060" = 66.1mm
+.080" = 66.6mm
+.100" = 67.1mm
+.120" = 67.6mm (pistons for 998 only, I think)
+.134" = 68mm (Hillman Imp pistons, suit 1098 stroke only)

These sizes are close, but not exact to 2 decimal places....

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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 made this little table once:

Bohrung = bore, Hub = stroke

Red: Std displacement
Green: Std overbores
Blue: Other common bore/stroke combinations
Grey: Other (theoreticly) possible combinations

Stroked 1071 crank could give a square engine?

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No speakers, no bodykits, no six-inch spacers, no spak filler, no bullshit

The only 1071 crank you could stroke is the Sth African big journal one... as a 1071 Cooper S crank only has 1.625" crank pin journals.

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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.

So the closest to a square bore is the 998 bored out to 120" but I'm assuming 120" is a fair bit of stress on the block. Plus I guess that would require staggered boring correct? And I'm assuming the imp positions are hard to near impossible to get these days?

Seeing as the small bores are a bit far off square (out of curiosity) what about a 1275...can they be squared, what are their dimensions in mm?

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Respect mine and I'll respect yours.

You can probably straight bore a 998 to +.120" but the 68mm ones need a bit of offset, I've done 3 over the years.
GOOD Imp pistons are hard to get (from UK) and expensive too.
Matt Read says there's a 68mm Suzuki? one that works.

A 1275 is a long way off square but less so than a 1098.
All are 70.6mm bore and 81.33mm stroke.

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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.

998cc +100" can be bored straight down no worries.....never done a +120"....yet, i think they do have to be offset.

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http://www.youtube.com/watch?v=nDMkVq1jRGU

when i was talking a bloke in england that used to race them he told me.

He used to race in the 850cc class, he got a 1275block and had it bored out, and then made a custom super short crank (2inch stroke from memory). he said it would he would rev it 10700rpm
That would be nearly square or could be past square

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That would be well and truly over square. Stock 970 Cooper S stroke was around 61mm I think.
While 10,000+ rpm sounds impressive, any A series doing that would would have had a very short life...
Ask Graham in aus, he used to race Super Se7ens over there.

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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.