960 Why oh why an interference engine!

[wooble]

It's an almost necessary evil of modern engine design, the market demands that any engine has a reasonable level of performance, is easy to drive and gets reasonable fuel economy, therefore a high compression ratio and 4 valves per cylinder with a high lift are the norm. When meeting these criteria, it's hard to design an engine where the valves and piston don't share some space.

[foggyjames]

Exactly ^^^.

What's a 720, by the way?

cheers

James

[CliffHeaton]

Seems to be a massive truck, you certainly wouldn't want to interfere with that. I got the numbers wrong, it was a long time ago... should be 740. I'm sure that the valves worked within the head chamber

[StanC]

Originally Posted by CliffHeaton

For the sake of a few millmetres valve to piston clearance my 960 is now a scrapper after one of the pulley bearings snapped off. I took the head off today and sure enough the valves are scuppered Does anyone know why a manufacturer builds destruction into a perfectly good engine?

Cliff - sorry to hear about your misfortune. But, incidentally, what mileage/age were the old pulley/timing belt assembly that failed?

Knowing how critical they are to the survival of an engine, I've always tended to replace my timing belt and pulleys at about half the mileage/age recommended by Volvo, which for my 1996 960 is 80,000miles/8 years. I therefore do them at more like 40,000miles/4years. But I still sometimes wonder if even this is not often enough, which is why I'd be interested to know how worn/old your failed pulley was.

[loki_the_glt]

Originally Posted by foggyjames

Exactly ^^^.

What's a 720, by the way?

cheers

James

It's a Boeing aeroplane - http://www.google.co.uk/imgres?q=boe...,r:5,s:0,i:123

Other images of this aircraft type are available.

[smithy]

Originally Posted by CliffHeaton

For the sake of a few millmetres valve to piston clearance my 960 is now a scrapper after one of the pulley bearings snapped off. I took the head off today and sure enough the valves are scuppered Does anyone know why a manufacturer builds destruction into a perfectly good engine?

What year is your car mate if it is an early 1992 to 1993 can I have the front anti roll bar off it . It has to be the rear wheel solid axle type that uses the 24 mm front bar

[CliffHeaton]

Hi Stan, 230,000. but was running great. I had to get the AA out a few months ago when a heater pipe came off and I had no tools with me. The AA guy was impressed with how sweet the engine was. The belt was 20k old and I was going to replace it this year. The belt was fine, I didnt replace any other parts when I did the timing belt about 3 years ago, but everything seemed ok. It was an idler pulley that snapped from it's mounting plate, that did the damage.
Smithy it is 96

[StanC]

Originally Posted by CliffHeaton

Hi Stan, 230,000. but was running great. I had to get the AA out a few months ago when a heater pipe came off and I had no tools with me. The AA guy was impressed with how sweet the engine was. The belt was 20k old and I was going to replace it this year. The belt was fine, I didnt replace any other parts when I did the timing belt about 3 years ago, but everything seemed ok. It was an idler pulley that snapped from it's mounting plate, that did the damage.
Smithy it is 96

Thanks, Cliff - yes, I had my first 960 3Ltr 24v in 1991 and the first few times I got the timing belt changed that was all I did - the belt alone and nothing else. But then I started getting strongly advised to always replace the tensioner and pulleys at each belt change, because these wear out almost as quickly as the belts do. Some folk also recommend getting the water pump replaced at every belt/pulley change, but I tend not to go that far - I hope that water pump bearings aren't that prone to failure!

As I said in my earlier thread, I'm quite suspicious about just how long it is safe to leave the timing belt/pulleys before changing them. Volvo say 80k/8years for my 1996 model 960, but there seem to be so many that I hear about that fail before then that I tend to halve it for peace of mind. If anyone has any other ideas on this I would be interested to hear.

Stan.

[foggyjames]

I'm suspicious that they're OK for the recommended interval in the majority of cases, and the "premature" failures are likely to be caused by the idlers being at least one belt-change old. I found when I did the swap on my V70R that the "kit" for the whole lot was not as much as I thought it might be. £80 rings a bell, while the belt alone was £25. £80 sounds like excellent insurance to me.

cheers

James

[Paddy Wagon]

Hi Cliff, sorry to hear about your misfortune. To answer your origional question, it's a deliberate policy by some manufactures, it's what they call "planned obsolecence". There is no justification for not building a "safe" engine. The easiest and least costly way to do that is to use "dished" or "recessed" pistons. It's quite annoying, particularly when Volvo's B200 series can break belts without any danger of internal damage, the same applies to Ford's "Pinto" engine. Is there any visible damage to the corresponding piston and cylander, such as gouges in the cylander wall or piston, and apart from the valve's is there any damage to the combustion chamber, have aother look at it, you might be lucky and simply need to fit new valve's. I've fixed countless engines just by changing the valve's, this is due to the fact that the valve stem will usually bend before any other damage occurs. Have any of the valve's actually broken or are they still in one piece because if they are still in one piece then that eliminates any possible cylander damage and could mean no serious piston damage apart from a slight indentation on the piston face which is not going to be a major problem. You might get lucky and be able to save the engine without having to spend big money on it.