Bob,
Generally speaking, BMC engines can stand quite large overbores. The old
fashioned casting process generally made for more material. You can increase
the capacity of many BMC engines by 10% just by overboring them. Sure - it
doesn't work on every single block - some you hit a water gallery - some get
porous - some you have to offset the bore centres - but it works on a good
majority.
For example - I had a sprite 1100cc block with 68mm Hillman Imp pistons for
many, many years - That took it from 1098cc to 1220cc - 130 thou overbored.
I also had a 948 with Hillman Imp pistons - thats nearly a 200 thou
overbore. The death of that old 1220cc engine was a broken circlip scoring
the bore - but I still drove it home from Melbourne to Sydney.......
Many sprite race engines run 74mm Omega pistons, which take a 1275cc engine
to a shade under 1400cc Thats a 130 thou overbore.
My old 100/4 was bored to nearly 3 litres - which was a pretty common mod
here in the 70's using Holden/ Chev pistons.
Many other Sydney 6 cylinder Healeys are running Volvo pistons - taking them
to almost 3.3 litres - which was a pretty common mod on road Healeys here in
the late 1980's - mid 1990's.
Conservatively, by comparision, my BJ8 has the Denis Welch Cosworth 85mm
pistons = +65 thou. (A std 3000 bore is 83.34mm = 3.281 in. 85mm = 3.346
in - ie plus 65 thou)
I suppose I'm trying to say 60 thou is actually a pretty conservative
overbore, and works on every BMC block I've ever seen, and doesn't affect
reliability/ cooling etc. The problems arise at much larger bore sizes -
offsetting them; porosity; compression etc. Most issues are more about the
'specific' block casting quality of a specific block - i.e. boring it
without going into a water jacket - than ongoing issues once the engine is
built and running.
Best regards
Chris
----- Original Message -----
From: "Bob Spidell" <bspidell@comcast.net>
To: "Healey List" <healeys@autox.team.net>
Sent: Tuesday, March 08, 2005 4:29 AM
Subject: Large Overbors (was: re: Piston Set)
> re:
>
> "Wall thickness at 0.060 and it was a little too thin for my comfort zone
> as I was a bit scared of overheating and of the cyls becoming barrel shape
> from being
> so thin. "
>
>
> I'd like to hear from anybody running 0.060 over pistons regarding any
> problems
> (I'm considering them myself). And, from the pros on the List, do you
> recommend
> for or against running 0.060 over.
>
> TIA,
>
> bs
> ********************************************
> Bob Spidell San Jose, CA bspidell@comcast.net
> '67 Austin-Healey 3000 '56 Austin-Healey 100M
> ********************************************
>
> Subject: RE: Piston Set
>
>
>> Carlos, I bought a set (for my 3000 engine) as you described from
>> British Parts Northwest in Salem, Oregon a few years back. Try them at
>> www.bpnorthwest.com. However, I opted to sleeve the engine back to std.
>> bore. I sent the 0.060 set back to them and they were happy to swap for
>> a std. bore set. Nice guys here.... I checked the cyl. Wall thickness
>> at 0.060 and it was a little too thin for my comfort zone as I was a bit
>> scared of overheating and of the cyls becoming barrel shape from being
>> so thin. Anyway, I am very happy with the sleeve job back to std. bore.
>> Lotsa Luck,
>> Jack
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