Stupid car Q of the week.....loosening sprocket to tension cambelt?

[visor_down]

Hi

Just reading up on changing cambelts (I am not planning to do it but just interested), does anyone know why on some engines you should loosen the camshaft sprockets to tension the cambelt and yet on others you don't?
I can't find an exact answer to it despite searching google, the general consensus seems to be that they should be loosened to allow correct tensioning of the belt yet on other they lock the sprockets together and no mention of loosening them at all, so how are these correctly tensioned between the sprockets?

I am sure the answer is obvious and i am missing something!

 

[normspanners]

It normally is to adjust the cam timing slightly to allow for wear of the belt or should I say the difference between the belts, but it is so small an amount (just a few thou) if it can be done without, IMO you are better leaving the cam locked to the sprocket as the difference is negligible.

 

[visor_down]

Great cheers that explains it. Understand now why it is possible to do without loosening them

 

[waddycall]

If you loosen the camshaft sprocket you take away the risk of getting the valve timing a tooth out on the sprocket as the sprocket is free to rotate independently of the camshaft when you tension the belt. It makes sure the valve timing isn't affected by tensioning the belt.

 

[ben8vgte]

Kind of depends on how the cam drive is fixed to the cam, if they are pinned then loosening them wont make any difference. But if it's on a taper then it can be adjusted.

 

[doubleboost]

Some engines have no key in the bottom pulley
1.125 some Renaults

 

[CKJDJK]

On some engines the timing is set on a part of the camshaft not the sprocket which is not keyed to the camshaft. Can also be true of the injection pump. Once the camshaft, pump and crank are locked in place with the pulleys free to turn on the unkeyed shaft the belt is fitted and then the pulley securing bolts tightened. Any error in the belt length is removed and all timing is correct. Especially if the head has been skimmed or any work on the cam itself. Some belts when replaced can be a half tooth out so this method can be more accurate.

 

[the snooper]

only ever loosen the sprocket if the camshaft has been locked in place otherwise disaster could strike when you try starting the engine

 

[Robotstar5]

only ever loosen the sprocket if the camshaft has been locked in place otherwise disaster could strike when you try starting the engine

Which is why you should wind the engine over by hand a couple of times, you should feel any problems.

 

[ibrooks]

Thing is that there is far less force involved when turning an engine by hand than when spinning it on a starter or when it fires up so turning by hand may well be fine but as soon as you turn they key something moves.

We've just done a Vauxhall Corsa with a 1.2 that had skipped a tooth on the chain - yes chain. Unbelievable how weedy the chain is. The chain is tensioned by oil pressure and a lack of proper changes had resulted in it not tensioning and there was enough whip in the chain to allow it to jump a tooth (small chain equals shallow teeth on sprockets). The camshaft sprockets aren't keyed to the cams so the cams need to be locked with a tool and the sprockets are allowed to spin freely whilst the chain is initially tensioned. Once the chain is in the right place the camshaft bolts are torqued back onto the cams.

Apart from the wimpy chain I like the idea but it does mean you start needing special tools for each engine.

A mate always used to ditch the woodruff keys from Cosworth YB's and just use friction to hold the sprockets onto the cams rather than using verniers. The reason was that anything but the pin type verniers have a habit of slipping more than the standard sprockets torqued onto the cams. Nowadays proper Quaife verniers aren't much more expensive than the Kent or Pipercross ones so he tends to use those. His kit has won plenty of rallies so I'd suggest he knew what he was doing.

Iain

 

[dunk]

Thing is that there is far less force involved when turning an engine by hand than when spinning it on a starter or when it fires up so turning by hand may well be fine but as soon as you turn they key something moves.

We've just done a Vauxhall Corsa with a 1.2 that had skipped a tooth on the chain - yes chain. Unbelievable how weedy the chain is. The chain is tensioned by oil pressure and a lack of proper changes had resulted in it not tensioning and there was enough whip in the chain to allow it to jump a tooth (small chain equals shallow teeth on sprockets). The camshaft sprockets aren't keyed to the cams so the cams need to be locked with a tool and the sprockets are allowed to spin freely whilst the chain is initially tensioned. Once the chain is in the right place the camshaft bolts are torqued back onto the cams.

Apart from the wimpy chain I like the idea but it does mean you start needing special tools for each engine.

A mate always used to ditch the woodruff keys from Cosworth YB's and just use friction to hold the sprockets onto the cams rather than using verniers. The reason was that anything but the pin type verniers have a habit of slipping more than the standard sprockets torqued onto the cams. Nowadays proper Quaife verniers aren't much more expensive than the Kent or Pipercross ones so he tends to use those. His kit has won plenty of rallies so I'd suggest he knew what he was doing.

Iain

which is the whole reason for turning the engine over a few times by hand, ( ratchet and socket on cranknut) so if there is a problem with your timing being out you will feel it and save yourself a lot of work, if something moves after when turned by key you`ve cocked up by leaving something lose, or, got belt tension wrong

 

[ibrooks]

The Corsa would spin fine by hand when we first got to it but there had been mild valve/piston contact.

Spinning the engine by hand and not feeling any resistance is not the be-all and end-all that the post comes across as. You must use the locking tools to make sure that everything is aligned before either slacking or tightening the sprockets.

 

[the snooper]

The Corsa would spin fine by hand when we first got to it but there had been mild valve/piston contact.

Spinning the engine by hand and not feeling any resistance is not the be-all and end-all that the post comes across as. You must use the locking tools to make sure that everything is aligned before either slacking or tightening the sprockets.

I would say its a must to turn the crank 2 full turns by hand which should bring everything back into line and re check everything does line up, the only time you will fail is if you don't re check
the corsa tensioners also have a spring in them which do become weak which will also allow a worn chain to slip

 

[ibrooks]

But the tensioner wasn't weak enough and the chain doesn't whip enough at hand turning speed to skip another tooth. The exhaust cam had skipped one tooth and fortunately stopped the engine (actually prevented it from starting with a "splutter"). It would turn as much as you wanted by hand in that condition with no contact and looking at it visually all appeared fine with what little markings there are. It's only when you actually stick the locking tool in that you realise that those slots on the back of the cams that looked horizontal aren't quite level.

Fortunately with a strip-down, skim and valve re-lap it all went back together (with a new timing chain kit) and works fine again. Lucky lady.

 

[dunk]

Seems you were fairly lucky,your clue to the wear was the horizontals that wer`nt. If this had of been a cambelted engine the cambelt would be changed as a matter of coarse along with the tensioner and guide roller and possibly water pump and done right it would not skip a tooth. A lot of horror stories are associated with broken engines because the cambelt broke which is why people shy`d away from cars with these engine`s, but the truth is its not the cambelt that`s the problem its that some car owners who for some unknown reason will not get these belts changed at manufacture`s recommended times or mileage, usually because of the cost involved.
Now because an engine has a chain to drive the cam/cams, this important item gets overlooked as they are more robust than a belt, but that said chains do wear as do tensioners and instead of breaking they jump tooth, but the chain still looks ok, the end result thou can be the same for a belted engine, so when a strip down of a engine is done its non productive not to change it regardless if it looks ok.
With your chain there must of been more play than acceptable for the old chain to jump, dont forget with chains they need plenty supply of oil when running

 

[ibrooks]

Chains weren't prone to this sort of thing originally - in most cases even without the tensioner they wouldn't have enough free play to skip a tooth. They'll sound bloo...min awful and run like a bag of the proverbial prompting investigation before they actually kill something. Multi-cam engines though need a longer chain so the free play is greater and on this thing the chain was such a weedy thing that it wouldn't need much free play to allow the chain to skip a tooth honestly my bike has a beefier chain.

I liked the good old Essex V6 - so long as you replaced the cam gear with an all steel one. Direct drive with a cog on the crank and another on the cam. The Cologne was similar until the Cosworth version - 5 cams! (well pretty much).

I also remember the kits you could get for the old 2286 Land-Rover lumps where you replaced the timing cover and converted them to a geared drive (Zeus seems to ring a bell as a trade name).

 

[dunk]

That brought a smile to my face When you mentioned the fords v6..i remember when working at a Ford dealer as an apprentice when the mk3 zodiac came out and had these v6`s, the spare was at the front of the engine at an angle which made the distance front bumper to bulkhead huge, me not being huge in height made it amusing for some other mechanics when they saw i was streched out along the engine trying to reach items, more often than not thou out came the spare wheel so i could sit in the well to work, oddly now thou they dont seem as big as they did in them days