Austin Healey 3000 bores in a 100/6 block.

Over the years I have been asked many times if there is any good reason why a 100/6 (3.125” bore) should not be overbored to 3000 specifications (3.281”). Back, before I knew better, I believed that there was no good reason not to do so and in fact we successfully rebuilt more than one 100/6 engine to standard 3000 specs.

We never realized how lucky we were.

In the late ‘90s we had a customer bring in a 100/6 that was having problems as a result of coolant leaking into the oil. The leakage only seemed to occur when the engine was warm and the cooling system was at operating pressure. After a long and involved period of analysis we discovered that the water was leaking through a tiny crack in the block about half way down one of the bores.

Our local race engine shop had recently acquired a sonic cylinder wall thick tester and using that piece of new technology we determined that the walls of that block were perilously thin in many areas as a result of being bored to 3000 specifications and the only solution was to rebuild the engine with a 3000 block.

After I read in Geoff Healey’s book “The Story of the Big Healeys” that the engine block had to be redesigned to produce the 3000 I put two and two together and decided not to “overbore” any more 100/6 blocks.

Unfortunately this problem was not common knowledge and just before I sold my business we purchased a “race ready” 3000 for a long time customer never imagining that the engine had been produced from a 100/6 block.

The engine ran for about 20 minutes before it “grenaded”.

The following photos illustrate the result and should serve as testimony as to the folly of “overboring” a 100/6 block.

An Indication That All Was Not Well

Look Closely…Something Is Missing

It Takes A Lot of Force To Do This

You Can See How Thin The Cylinder Walls Were

The Remnants of a “Race Ready” Engine.


About Michael

Who is this guy? Born in New Zealand some time back. Went to Maori Hill Primary School then Kaikorai Valley High before joining the RNZAF and an Airman Cadet in 1968. Graduated 1972 with an NZCE in Aeronautical Engineering. Then embarked on the typical Kiwis "Big Trip Overseas". Got to see quite a few places, and spent a while in the U.K. "home" as it was refered to by many New Zealanders in those days, before travelling on to New York and then to Canada by bus!! This trip is presently on hold (has been for the last 34 years). Met my dear wife Judy not long after arriving in Ontario and we have been happily married since 1976. After travelling around New Zealand and the pacific in 1979 I started Precision Sportscar andfor the next 23 years grew the business and helped raise 2 boys Drew and Robin.
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