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MG MGF Technical - cam belt

Quick question - what are the consequences of not replacing the cam-belt at the right time?

Can anyone spare a quick pearl of wisdom?
Dave

Cam belt snaps. Pistons touch valves. Valves bend. Valve train knackered. Engine buggered!!!
Bob Millar

Bob, quick consise and correct ;-)
Will Munns

Nb. Does NOT apply to all MG engines ..... ;-)

Years ago, had a belt snap at 6,000 rpm on my MG Montego Turbo just as I was about to change up to third gear..... DOH!... Lost all power and coasted to a standstill. Before I identified the problem, tried several times to restart car ... nothing doing. Looked under car to discover snapped cam belt trailing along the road .... No harm done!

Colleague at work had same thing happen to his early GTi. He too tried several times to restart .... no harm done. AA man said he was very lucky..... So was I apparently but the design of the O series turbo probably means you'll get away with it sometimes ..... Dont know about the EFI version of the O series though.
John McFeely

John, this very much depends on the design of the head and pistons. Some engines, such as those found in the Mazda MX-5 for example are 'non-interference' designs - which means that even if the cam belt snaps, the valves will never contact the pistons (presumably there are valve 'pockets' let into the piston's top surface?

K-series engines do not benefit from this - hence the disasterous consequences of cam belt failure... :o(
Rob Bell

Had this happen on my previously owned renault 19 16 valve, it bent all the valves, destroyed all the tensioners and i was stood idling at traffic lights at the time, god knows what would have happened if i had been on the motorway.
Wayne D'Andilly

Hi,

<Some engines, such as those found in the Mazda MX-5 for example are 'non-interference' designs>

I don't know how the Mazda manages this, I would have thought that if there was clearance between piston and valves at max piston height and max valve depression then the resulting compression ratio would be about 4.5 to 1.

The K pistons are recessed a little for the valves, but this is for normal running clearance, there isn't much room in there at max compression.

My daughter had a cambelt snap in her small Fiat, and this too survived without any noticable damage. Funny really, who survives and who doesn't.

Regards, Kes.

Kes

No idea either - just one of those facts I remember reading somewhere. Might even have been the motoring section of the Telegraph?
Rob Bell

Many motorcycle engines - which have very high revs - have a shaped piston crown that allows for 'overlap' of timings to avoid engine damage, these 'dents' dont seem to effect compression ratios either - so is it possible that some of todays high performance car engines are doing the same?

Ted
Ted Newman

Few motorcycle engines have belt drive to their camshafts - significant exception is Ducati. Most use chain drive to the cams, some even have gear trains (the non-VTEC VFR for example). Bike engines also have high compression - but valve contact is not common.
I would guess that car manufacturers could do the same - perhaps it's because the K series is pushed way beyond its original design ?
Steve

The only car engine that I'm aware of that still uses chain drive for the cams is fitted to Saabs.

Just to repeat it once more... Don't forget about the cam belt tensioner - it was this that killed my F.

Ralph
Ralph

This thread was discussed between 30/01/2004 and 02/02/2004

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