ZX Diesel pump swap: CAV to Bosch - How?

beezer · Post by **beezer** » 19 Nov 2004, 03:18

I would probably use the old belt too but wouldn't suggest somebody else do so just in case!
A tenner could be several hundred if it went wrong.

tomsheppard · Post by **tomsheppard** » 19 Nov 2004, 14:05

No! NO! Change the belt. It will have taken a "set" on the old pulley profile. The new one will be microscopically different and therefore less likely to last the full life. Don't chance it!

beezer · Post by **beezer** » 19 Nov 2004, 14:07

It will also have stretched so tensioning would be hit and miss.

NiSk · Post by **NiSk** » 19 Nov 2004, 14:16

It really amazes me how short sighted car owners/garages can be: the XM 2.5TD I recently bought had a new recon head fitted in feb this year due to a cooling hose blow out (I've got the receipt - from an authorized Citroën workshop; 28,000 SEK = about £2,200, which is slightly more than I paid for the whole car!) the IDIOT's didn't change the cambelt since it had only gone half the recommended distance (!!) makes you want to cry (especially when you find out what a B**tad of a job it is on that engine!)
//NiSk

Dave Burns · Post by **Dave Burns** » 20 Nov 2004, 00:45

The belt also runs over three other precision hobbed pulleys and of differing diameters, just a tad more than a microscopic differance there, so the belt can't take a set on any single pulley, a change of pulley wont make any difference.
The belt may have stretched but you wont have the gear to measure the minute difference if any, what would happen to your timing if any apprechiable elongation occured, these things aren't elastic bands but hardy pieces of kit and much tougher than they are credited for, they have to be to stay on there for fifty odd thousand miles.
Why would tensioning such a part used belt be hit and miss, the belt would need to have the same characteristics as an elastic band for such a statement to be true.
Any road up Alan, its your engine and your money so if the talk here has worried you enough stick a new one on, just be aware that many problems in this area arise from poor fitting like getting the timing a tooth or two out, and tensioning the belt when there is still slack in the front run of it, the tensioner wont take that out.
If the old belt is re-used just make sure it runs in the same direction as it does at the moment.
Dave

beezer · Post by **beezer** » 20 Nov 2004, 01:06

You seem to know enough about belts but they do stretch. Check the tension of one when newly fitted and one that has done 20 thousand miles or so. If you re-tension that belt to the specified tension you are actually overtensioning it. Ford are very particular about this. So you would have to calculate how much stretch is in the belt and adjust the tension accordingly. Hence the hit and miss aspect. Yes, Gates and other quality belts will no doubt stand up to extra tensioning but for a 'tenner' is it worth the risk? I would use the belt again but not for the specified mileage and I have enough equipment to change a belt easily. I certainly wouldn't be advising anyone else to do the same.

drabux · Post by **drabux** » 20 Nov 2004, 01:51

Interesting discussion!
On balance I feel that I will probably stick with the old belt. Why? Because I am hoping to get away with replacing the pump without having to fully remove the belt. I understand that you have to take off the crank pully to replace the belt, which can be a pig of a job.
Alan

beezer · Post by **beezer** » 20 Nov 2004, 02:03

On some engines you can remove the plastic guard around the crank pulley and get the belt in without removing the pulley.

Dave Burns · Post by **Dave Burns** » 21 Nov 2004, 05:08

So what your'e saying then beezer is that to re-use a belt you must perform some wierd calculation and put the belt back on slack, hehe yeah I'm sure.
The reason for tensioning a belt be it new or used is to take the slack out, maintaining proper belt/pulley mesh preventing skipping/ratcheting of the belt over the pulleys, especially the small 21 tooth crankshaft drive pulley, a certain amount of extra tension is part of the specified tension and is there to account for decay of the original value due to wear.
Too little tension and the belt may skip and the pulleys can kick back and forth as they rotate, too much tension and excess heat will be generated and this is a timing belts real enemy, the cooler the belt runs the longer it lasts.
Now I can't believe that anyone (including the boffins at Ford) would say that applying the specified tension, on two or more seperate occasions would cause the belt to be overtensioned or have extra tension, thats nonsense, (some manufacturers even specify a re-tensioning interval of the same belt) and the thing that makes it nonsense is the sheer fact that the tension applied to the belt from accelerational forces makes any pre-tension absolutely pale into insignificance, yet those forces are acting on it throughout its entire life, they don't get any less so why should a re-tensioned part used belt have any less tension applied.
This belt is driving the injection pump, camshaft(s), water pump, vacuum pump and in some cases the power steering pump.
Consider for a moment even starting one of these engines with a timing belt doing all that work, the starter turns the engine at a very slow rate (about 150 rpm) and then wallop, it fires and its almost instantly doing 900 odd rpm, what sort of tug do you think that puts on such a belt, and the m.o.t test, floor the throttle and rev it from 900 to 5000 rpm in half a second, yet the belt takes all that it in its stride right up untill it needs changeing at 50,000 miles.
Dave
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by beezer

You seem to know enough about belts but they do stretch. Check the tension of one when newly fitted and one that has done 20 thousand miles or so. If you re-tension that belt to the specified tension you are actually overtensioning it. Ford are very particular about this. So you would have to calculate how much stretch is in the belt and adjust the tension accordingly. Hence the hit and miss aspect. Yes, Gates and other quality belts will no doubt stand up to extra tensioning but for a 'tenner' is it worth the risk? I would use the belt again but not for the specified mileage and I have enough equipment to change a belt easily. I certainly wouldn't be advising anyone else to do the same.
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beezer · Post by **beezer** » 21 Nov 2004, 06:07

Ford say never to use a belt once it has been off full stop. I would happily use the pump belt on a Ford again because if it goes it is not going to kill the valve gear. But why do Ford say never to use a belt again once it is has been off? Why does a belt eventually give up the ghost? Because its real enemy is heat generated from too much tension? That is a contradiction of your own argument. The belt also becomes compacted over time and retensioning would force it to run at more acute angles which would increase the chances of it breaking. More belts are broken by fatigue and oil contamination than being fitted too tight when new. That they slacken slightly - and I say slightly - as they become tired is probably not a bad thing. If you then put full strain on them you are increasing the likelihood of breakage. The acceleration forces would be increasing the likelihood too if the belt is under more stress to begin with. If you forgive me using an analogy here: A string on a musical instrument is constantly under tension and does a fine job. As the string wears it loses tension. The musician retensions (tunes back up to pitch) to compensate. Does a fine job again but one day it will twang and break. This is almost invariably when being tuned or played hard. In other words fatigue played its part. Another thing to increase the likelihood of that string breaking is to slacken it off completely and then bring back up to pitch. This is applicable to steel, gut and nylon. Consider also that tension meters include the thickness of the belt in calculating tension (mine certainly does and is a professional piece of equipment). And a thinner belt will give an inaccurate reading. The MOT test instructs the tester to take the engine to maximum speed in 'less than one second' and 'quickly and continuously but not violently'. I certainly did not advocate calculating how much less tension to put on an old belt. This can not be reasonably calculated. Neither did I say put it on slack. I said it was not a good idea to use an old belt. I am not alone in thinking this. Drabux did not say how many miles the belt had done anyway, only that it was 'newish'. So if he does as he says and keeps the 'newish' old belt and then it breaks will you come back and say 'it was not the condition of the belt'? As I said, is it worth the risk for the sake of less than 20 quid? If you want to re-use a cambelt for the sake of a few extra miles for your money then fine but I will not be going to you for any work done on the timing gear!

drabux · Post by **drabux** » 21 Nov 2004, 14:06

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by beezer

Drabux did not say how many miles the belt had done anyway, only that it was 'newish'. So if he does as he says and keeps the 'newish' old belt and then it breaks will you come back and say 'it was not the condition of the belt'? As I said, is it worth the risk for the sake of less than 20 quid? If you want to re-use a cambelt for the sake of a few extra miles for your money then fine but I will not be going to you for any work done on the timing gear!
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I did! In my third post I said 6k on the belt. The belt should then have around at least 44k left in it. That doesn't sound like I'm trying to squeeze a few extra miles for my money does it?
Alan

NiSk · Post by **NiSk** » 21 Nov 2004, 14:55

Citroën actually give instructions, in the official workshop manual, on how to tiewrap the timing belt to the various pulley so as not to upset the timing when changing the fuel pump. In the instructions I have it applies to the TD12 and the DK5 (2.1 and 2.5 litre diesels) but I assume the same applies to the 1.9. So Citroën seems to think its OK to reuse a belt.
//NiSk

oilyspanner · Post by **oilyspanner** » 21 Nov 2004, 16:16

To be brutally honest I would get all of the bits together and not do the job until I had to, I ran my previous GTI TD (Lucas pump) on vegoil for ages without trouble, getting up to 85% new oil, If the case is that the pump has had work then the seals may well have been replaced with synthetic ones. If not they can last a long time on vegoil, who knows you might get to do the swap with the sun on your back.
Stewart

drabux · Post by **drabux** » 21 Nov 2004, 16:17

Excellent tip acout tie wrapping!
Alan

Dave Burns · Post by **Dave Burns** » 21 Nov 2004, 16:26

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by beezer
"But why do Ford say never to use a belt again once it is has been off?"
Probably because they can generate much more revenue from spare parts who knows, fortunately I lost my appetite for Fords twenty odd years ago so what they say about them I couldn't care less.
"Why does a belt eventually give up the ghost? Because its real enemy is heat generated from too much tension? That is a contradiction of your own argument."
No beezer I don't contradict myself, you read but you dont see what you read, you type away but say little.
I said "too much tension and excess heat will be generated and this is a timing belts real enemy, the cooler the belt runs the longer it lasts."
Pay attention to the words "too much" it means what it says, it does not mean the specified tension, which on this engine is applied automatically by a spring loaded roller.
"The belt also becomes compacted over time and retensioning would force it to run at more acute angles which would increase the chances of it breaking."
Which part of it is compacting while the whole of it is stretching, and where are the angles changeing.
"More belts are broken by fatigue and oil contamination than being fitted too tight when new."
Agreed, thats why they have to be changed, if over tensioned (not specified tension) it wont go the distance either.
"That they slacken slightly - and I say slightly - as they become tired is probably not a bad thing. If you then put full strain on them you are increasing the likelihood of breakage."
If by full stress you mean specified tension then I don't agree and nor do Citroen apparently.
"The acceleration forces would be increasing the likelihood too if the belt is under more stress to begin with."
Agreed as more stress would have to mean excess tension, not the correct amount.
"If you forgive me using an analogy here: A string on a musical instrument is constantly under tension and does a fine job. As the string wears it loses tension. The musician retensions (tunes back up to pitch) to compensate. Does a fine job again but one day it will twang and break. This is almost invariably when being tuned or played hard. In other words fatigue played its part. Another thing to increase the likelihood of that string breaking is to slacken it off completely and then bring back up to pitch. This is applicable to steel, gut and nylon."
I hope the manufacturers of handbrake cables are looking in because your analogy certainly applies here[:D]
"Consider also that tension meters include the thickness of the belt in calculating tension (mine certainly does and is a professional piece of equipment). And a thinner belt will give an inaccurate reading."
Why does a proffesional tension meter only give an accurate reading on thick belts, surely applications where thin belts are required are just as critical.
"The MOT test instructs the tester to take the engine to maximum speed in 'less than one second' and 'quickly and continuously but not violently'."
What the tester is instructed to do and what he actually does are two very different things, don't forget that they are human beings too, I haven't had a diesel tested anywhere that they don't dust floor the throttle, but I suppose at your testing station its allways done by the book.
"I certainly did not advocate calculating how much less tension to put on an old belt. This can not be reasonably calculated. Neither did I say put it on slack."
Wether you did or not, it was what you implied.
"I said it was not a good idea to use an old belt. I am not alone in thinking this."
Nor do I think its a good idea to use an old belt when it really is old, but recently fitted and having covered only 6k miles (approx. 1/8 of its life) is not old in my book.
"Drabux did not say how many miles the belt had done anyway, only that it was 'newish'."
As said, you read but you don't seem to see the words.
"So if he does as he says and keeps the 'newish' old belt and then it breaks will you come back and say 'it was not the condition of the belt'?"
If its a good quality belt, with no contamination, properly fitted he wont have any problem, if he lets it go much past 50,000 miles he may experience belt failure, if it did break it would be one of those things, this does happen inside manufacturer specified intervals on otherwise good belts fitted correctly, I can think of such a manufacturer where 30,000 miles was specified for the life of a belt, but some didn't even reach that, probably a thin type of belt.
"As I said, is it worth the risk for the sake of less than 20 quid? If you want to re-use a cambelt for the sake of a few extra miles for your money then fine but I will not be going to you for any work done on the timing gear!"
I'm sure by now that Alan knows the usefull pro's and con's of his intended work and I wish him the best of luck with it.
Dave
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