pdm
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| posted on 24/2/11 at 01:41 PM |
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cam belt change
I just posted on a MX5 forum about a belt change but figured I'd get good answers here.
Please bear with me here as I trying to get to grips.....it's the theory I'm not clear on....
To my mind with a cambelt change, you start at the crank and go past the idler and onto the exhaust cam.
At this point there surely must be a set number of belt teeth between 6 o'clock on crank pulley and 12 o'clock on the cam ? And surely
this dictates the tension in the cam belt so far ?
then carrying on going around the inlet cam - 19 teeth between top I and top E mark (is correct) surely sets tension - by which I mean IF you could
stretch a belt so engine was in time but there were only 18 teeth between top I and top E THEN the belt would be too tight.
Finally slip belt over tensioner....
Then move crank clockwise round 1 5/6, undo tensioner, it moves on spring, you tighten it.
Now on mine 1 5/6 turns, undo and retighten has set the tension but why ?
As far as I can see all that will set is the tension from inlet cam to crank. Does this tensioning affect the top deflection ? If your engine is in
time but your tensioner really pulls in, how come your crank and/or inlet cam doesn't move whilst your exhaust cam stays still ? Can you take a
timed engine out of time with the tensioner ?
Mike says the other runs crank to idler, idler to exhaust cam, inlet to tensioned and tightened tensioner and tensioner to crank should be as tight as
banjo strings. Surely up the crank to exhaust sides depends on if you pulled hard enough when putting belt on.
The reason I ask is that I expected my crank/idler/exhaust to be very very tight, however, they do have a small amount of deflection. I don't
think I have but I'm wondering whether it's possible to put more belt teeth in than should be there in crank/idler/exhaust run and
therefore have a loose flappy belt and the slack is accounted for on the "other" side by the tensioner or the belts just jumps off on
first start.... Or would it be SO loose that you'd "just know".
thanks
Paul
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DRC INDY 7
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| posted on 24/2/11 at 05:51 PM |
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Some times you have to be cruel to be kind............... I would suggest you have this job done by some one that can do this job because you do not
seem to have a clue
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Puddle Dodgers Club
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pdm
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| posted on 24/2/11 at 06:02 PM |
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never will either unless I have a go - how did your first cambelt change go or looking at your sig were you trained in it
looking to learn here that's all....
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DRC INDY 7
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| posted on 24/2/11 at 06:17 PM |
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Im not knocking you for learning you just need somebody that knows what they are doing to show you because if you get it wrong its big money as you
are aware
it is easier to show how its done than try to explain
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avagolen
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| posted on 24/2/11 at 06:30 PM |
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Hi Paul,
I do not have experience on your engine, but the usual setup is that the tight side is from the Crank to the Cam - regardless wether it is inlet or
exhaust.
The tensioner is just to control the 'extra' length.
When the cams are timed correctly and the tight side is tight, all timing marks should be very close.
The rotation of the crank when tightening the tensioner bolt is to settle all bits in the belt run - make sure you rotate it in the correct direction
- very important.
Len.
The Answer for everything, but never the last word....
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ed1801
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| posted on 24/2/11 at 06:40 PM |
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Probably the best engine to learn on - non interference! (as long as it is one of the old ones...)
I wouldn't worry about the top deflection, there was always a bit on my mazda - the slack is taken up when the engine is dragging the cams along
at several hundred rpm.
Once it is all setup,and the engine is running, it will be really tight from crank to exhaust to inlet because of the drag and then the tensioner will
keep the required tightness on the bit that has no 'pull' to keep it tight.
My head had been skimmed twice on the mazda so the marks never lined up properly. I put the belt on in the 4 different correct looking configurations
one day and it seemed to make no difference at all to the butt dyno.
PS What is critical is making sure the crank pulley bolt doesn't ever come loose - that chewed up the nose of the crank on my car, use a new one
and loctite.
[Edited on 24/2/11 by ed1801]
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pdm
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| posted on 24/2/11 at 06:40 PM |
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quote:
Originally posted by DRC INDY 7
Im not knocking you for learning you just need somebody that knows what they are doing to show you because if you get it wrong its big money as you
are aware
it is easier to show how its done than try to explain
Ok fair enough - long day so I'm probably too quick to bite.
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pdm
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| posted on 24/2/11 at 06:41 PM |
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quote:
Originally posted by avagolen
Hi Paul,
I do not have experience on your engine, but the usual setup is that the tight side is from the Crank to the Cam - regardless wether it is inlet or
exhaust.
The tensioner is just to control the 'extra' length.
When the cams are timed correctly and the tight side is tight, all timing marks should be very close.
The rotation of the crank when tightening the tensioner bolt is to settle all bits in the belt run - make sure you rotate it in the correct direction
- very important.
Len.
Thanks Len - my marks are spot on and the "tight" is ok. I'm going to recheck deflection tonight and then put covers back on I
think.
Thanks again.
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pdm
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| posted on 24/2/11 at 06:44 PM |
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quote:
Originally posted by ed1801
Probably the best engine to learn on - non interference! (as long as it is one of the old ones...)
I wouldn't worry about the top deflection, there was always a bit on my mazda - the slack is taken up when the engine is dragging the cams along
at several hundred rpm.
Once it is all setup,and the engine is running, it will be really tight from crank to exhaust to inlet because of the drag and then the tensioner will
keep the required tightness on the bit that has no 'pull' to keep it tight.
My head had been skimmed twice on the mazda so the marks never lined up properly. I put the belt on in the 4 different correct looking configurations
one day and it seemed to make no difference at all to the butt dyno.
Again thanks for help - it is an early one which is non-interference which is one reason why I'd persevered. I figured worse case I break down
on test run and have to do it all again.
Can you imagine my noob posts if I was doing it on an interference engine 
My marks are definitely good - I've turned it loads to check and then again after tensioning.
I guess I'll measure deflection again tonight and then put it back together as said.
Thanks again.
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adithorp
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| posted on 24/2/11 at 06:50 PM |
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quote:
Originally posted by DRC INDY 7
Im not knocking you for learning you just need somebody that knows what they are doing to show you because if you get it wrong its big money as you
are aware
it is easier to show how its done than try to explain
As someone who could borrow DRCs signiture (but add a much earlier date) I read your original post and couldn't make head or tail of it.
It's so confused that I'd agree with the above about getting someone who knows what they're doing to show you. His first reply was a
bit blunter than I'd have put it though 
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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DRC INDY 7
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| posted on 24/2/11 at 07:21 PM |
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quote:
Originally posted by adithorp
quote:
Originally posted by DRC INDY 7
Im not knocking you for learning you just need somebody that knows what they are doing to show you because if you get it wrong its big money as you
are aware
it is easier to show how its done than try to explain
As someone who could borrow DRCs signiture (but add a much earlier date) I read your original post and couldn't make head or tail of it.
It's so confused that I'd agree with the above about getting someone who knows what they're doing to show you. His first reply was a
bit blunter than I'd have put it though
Yea i can be a bit blunt straight to the point
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Puddle Dodgers Club
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britishtrident
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| posted on 24/2/11 at 07:48 PM |
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I think you will find this is an interference engine --- turn it over gently by hand first and get somebody who knows what they are doing to look it
over before you reconnect the battery and turn it on the starter.
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McLannahan
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| posted on 24/2/11 at 08:00 PM |
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quote:
Originally posted by britishtrident
I think you will find this is an interference engine --- turn it over gently by hand first and get somebody who knows what they are doing to look it
over before you reconnect the battery and turn it on the starter.
I disagree. I'm in the "non-interferance" camp.
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imp paul
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| posted on 24/2/11 at 08:23 PM |
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ok chaps auto data says the engine is a freewheeling engine in witch the possibility of valve to piston damage in the event of a timing belt failure
may be minimal or very unlikely. all the best paul 
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MikeRJ
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| posted on 24/2/11 at 10:39 PM |
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The Gates web site suggests the early 1.6 is an interference engine, and the
later 1.8 is not.
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britishtrident
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| posted on 25/2/11 at 08:39 AM |
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All petrol twin cam engines with a hemi or pent roof combustion chamber are interference.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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