From: Dave Baker
Subject: Re: Broken Rings
Date: 06 Apr 2000 13:26:08 GMT
>> Pulling apart a 308 V8 the other day, the top ring of 5 of the 8
>> pistons were broken. One entire bank, and one on the other bank. Now
>> why would they break, I'm told it's unheard of on new engines, so why
>> do old one's break 'em.
Most likely cause is that the engine was high mileage and had usually been
driven gently. A wear ridge develops at the top of the bore in a specific place
dependent on rpm. The higher the engine revs, the higher the piston rises due
to rod stretch, thermal expansion, crank flex etc. At 3000 rpm the stretch will
only be a few thou but on a high rpm race engine you can need 30 thou or more
piston to head clearance to avoid contact.
So it only needs one high rpm thrash on an old engine that has trundled about
all its life to whack the top rings into the bore ridge and break them.
Using the full rev range every now and then throughout an engine's life is
actually a good thing to help even out the wear at the top of the bore. Drive
at low rpm for too long and that's as high as you'll ever be able to run the
thing without risking a broken ring. Most engines that have been driven
normally i.e. a good thrash every now and then, have a sort of graduated wear
ridge spaced out over 15 to 20 thou. An engine that has never been past a
certain rpm can end up with a very defined, sharp edged ridge at exactly the
same height in each bore.
Dave Baker at Puma Race Engines (London - England) - specialist cylinder head
work, flow development and engine blueprinting. Web page at