OT: timing belts

[Tony Duell]

> 
> Gordon JC Pearce wrote:
> 
> > Well, that's just down to stupidity.
> 
> That's called an interference engine.

I suspect it's difficult to make a Diesel which isn't -- the compression 
ratio is pretty high in such engines (I am used to changing the timing 
belt on the Citroen BX Diesel engine (XUD engine)).

> 
> Do these engines sit transversely in a front wheel drive (fwd) car? 

Old Citroens actually had longtitudinal engines, sometimes with the
transmission _in front_ of the engine (!) (Citroen made fwd cars long
before they were common), but I think those all used timing chaims, and
weren't overhead cam (I've never seen a non-overhead-cam engine with a
timing belt). The ones I am used to (and the previous cars with timing
belts I've worked on) have transverse engines. The only problem due to
that is the limited space for getting the belt between the timing-gear end
of the engine and the inner wing panel. 


> Most cars here are fwd, and have the engines positioned transversely.  A 
> lot of times you will have to disconnect at least one of the engine 
> mounts and lift the engine up.  Also, since it's bound to be an overhead 
> cam engine, the accessorys will have to come off.

OK, the procedure for the BX diesel, as far as I remember it is : 

Jack up the front of the car, remove the road wheel on the timing-gear 
side. Take off the plastic cover on the inner wing panel (press-in studs).

Take off the air filter and intake box (this is not essential, but it 
makes life a lot easier!).

Loosen the alternator mounts, unscrrw the belt tensioner (this is the 
only car I've worked on with a tension screw for the alternator belt, 
rather than having to pull out the alternator to tension the belt and 
then tightenting the moutings without letting go!). Remove the alternator 
belkt

Engage 5th gear, handbrake on (this works on the front wheels..), undo 
the bolt on the crakshaft pulley. This is tight, and goes in with locking 
compound, so it takes a bit of moving, still.... Remove the crackshaft 
pulley. Put the bolt hack in, so you can use it to turn the engine.

Mremove the top parts of the belt cover (slot in), and the lower part (2 
bolts).

Enggage neutral. Turn the engine with the pulley bolt until the timing 
marks all line up.

Put a 'timing pin' (a Torx key of the right size works) in the hole 
behind the starter motor, so that it goes into the flywheel.

Put 1 8mm bolt in the hole in the cam pulley, 2 in the holes in the 
injector pump pullet to lock those at the timing point.

Put a jack under the timing case end of the sump. Remove the top part of 
the engine mouting on that side (the belt runs _through_ the mounting 
block!).

Loosen the tensioner fising bolts. Pull back the tensioner (there's a 
square hole in the plate to take a 3/8" square drive), tighten one of the 
bolts to hold it. 

Take off the timing belt, fit the new one (remember everthing iomportant 
is locked at the timing point, so there's no chance of it moving). Check 
there's no slack on the front side of the belt)

Release the tensioner bolts, let the tensioner do its job, retighten. 

Remove the locking bolts and timing pin. Tuen the engine through 2 
complete revolutions by hand. Release and retighten the tensioner. Check 
the bolts and pin will go in again.

Put the engine mounting back on, rtemove the jack, put the other bits back.

> 
> > As to why we use timing belts, well...  The problem with timing chain 
> > designs is that the chains wear and go slack.
> 
> I don't know too much about chains used with overhead cams, but on push 
> rod engines this doesn't happen until many miles are on the vehicle.

Having read many workshop manuals, it seems that engines with 
chain-driven ocerhead camshafts can be a pain to pull the head from. You 
either have to dismantle the complete timing case (which often involves 
dropping the sump, and which may not be possible with the engine in the 
car), or puse a special tool to hold the cam sprocket after removing it, 
and hope it doesn't slip (or you'll have to remove the timing case to get 
it all back. The belt, being essentially outside the engine, is a lot 
easier to deal with.


-tony