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Mr_John111

Cambelt Snapped

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I am going to do mine every two years on the advice of my local specialist.

At the end of the day negligence is not the reason for this failiure as John changed the belt last summer, it sounds like extremely bad luck.

Any update yet John?

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Important to remember that a variable was introduced to John's belt - it was rubbing on a hose. That may or may not have been a contributory factor.

I frequently do belts-only jobs at the owner's request. It's neither difficult nor expensive. Very often it's probably belt and braces, probably unnecessary but it gives peace of mind, especially if the car's history is sketchy. Of course belt technology has moved on, but adherence to the Lotus service schedule is a sensible thing to do, especially if you want to sell it at top dollar.

I've removed a few diabolical belts that by rights should have trashed their engines, so much better to err on the side of caution!

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No update yet

Weather at the weekend was not conducive to having a look.

Sparky is right, I am kicking myself for not playing safe and renewing the belt just giving it a visual inspection and thinking it was ok.

Will let you all know if I can find a cause and what the damage is

John

Sparky said - "Belt and braces"

If you are supposed to use braces too that's where I went wrong :)

Edited by Mr_John111

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Looking at the broken ends of the belt, did the break go straight across the belt or was it at 45 deg. If its straight across its been crimped and the cords have been damaged, if its at 45 deg then it has seen a lot of load. An 18 month old belt should be OK for normal running so I would advise that you look for a mechanical problem before putting another belt on. If the crank pulley (drives the belt) is in good condition then the increase can only come through the pulleys driven by the belt eg oil pump pulley or the cam pulleys. I have seen a lot of belt failures, but rarely seen one break without it being extremely old and work hardened or a mechanical fault. I have even run belts on dyno engines and the teeth have sheared off because the belt has been aged so much, but they have continued to run quite happily. Strange sight to watch.

Cheers

Ralph

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Update

I believe I have found the cause.

The bolt on the end of the camshaft (nearest the turbo, not the cam wheel ) had worked loose this explains the harmonic vibration/rattling that I had diagnosed as the clutch release bearing.

Engine out soon

John

Edited by Mr_John111

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That'll be the setscrew that holds the thrust washer? I wouldn't have thought that would cause a belt to snap... Before you take the engine out, it might be an idea to check the valve clearances....turn the crankshaft so that the pistons are in the middle of their stroke, without going past TDC (this stops damaging the valves, if you've got away with it so far!) Then you can rotate the cams and check the valve clearances; if they are all OK, I would fit a new belt to the engine and then do a compression check...if that's OK then you've been a lucky boy!! It worked for me...my only really puzzling thing when my belt snapped was that the engine started, ran for a few seconds, then stopped and wouldn't start again. Much messing around with timing and accidentally disengaging the distributor and suchlike..finally, it turned out that - whilst working crouched in the space behind the engine - I had kicked the inertia fuel pump cutout switch. The teltale red thingy was fully down...which was why I didn't suspect it....but I'd dislodged the connecting plug underneath it; that wasn't obvious either, but I ought to have realised the fuel pump wasn't running....! It was early days, then...only had the car for a couple of weeks.....

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Moved the pistons to half way but the inlet cam has moved forward about 1/2 an inch and won't turn (and I don't want to force it ) so I am taking the engine out as everything is easier then.

John

Edited by Mr_John111

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Definitely the best thing to do....! The cam moving forward may well have got the cam lobes off the cam followers and jammed it. Now THAT could snap a cambelt.....! Finger's crossed for you....

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The 912HC and 910 both run 104 degree maximum opening point (mop) cam pulleys, also known as 'green dot'. The hc uses 104 and 107 cams, the 910 two 107 cams.

You will need three htd cam pulleys, and the crank pulley. Lotusbits advertises them used for sale on their site (so check with Mike to see if they have any), or also check with Garry Kemp to see what he may have (I bought a couple of green dot htd pulleys from him a few months ago).

I believe most of the htd engines use a manual cam tensioner, the 912hc did, where your 910 probably has an auto tensioner. I am changing over to the manual tensioner on my 907 rebuild with htd pulleys, and many 912hc parts (head, carbs, intake, crank) to wake the old 907 up a bit.

Brian

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Update

Engine out now, hopefully the head will come off tomorrow and then I'll let you all know what the damage is

John

Edited by Mr_John111

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Guest allgoodpeter

Just read this and for what its worth - I have seen an example of a new cambelt going (on an Alfasud) that was due to a small stone picked up off the road surface that got inbetween the belt and the pulleys - the belts are incapable of stretching very far and just burst. That was on a race car - we found the stone in the engine bay.

So not only age is a factor but a bit of debris kicked up could do the deed - i know late HCs have undertrays but is there any protection to earlier belts from the elements?.

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John,

Hope that all is well and that pulling the engine was not too daunting of a task. Did Sue help ?

Will keep an eye on this thread for updates...tell Sue I said hello

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Hi Christopher. No I didn't help I had to look after our youngest but kept John going with supplies of coffee and and some chocolate (to keep his energy up). Bacon sandwiches on the menu for today. :)

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Head off now.

No obvious signs of Piston to valve contact?

All valves seated?

Am I being paranoid - how do I tell if a valve has been bent?

Pics to follow

John

headoff001.th.jpgheadoff002.th.jpgheadoff009.th.jpgheadoff007.th.jpgheadoff008.th.jpg

headoff005.th.jpgheadoff004.th.jpgheadoff003.th.jpgheadoff010.th.jpg

Edited by Mr_John111

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Put some fluid in each chamber of the upturned head, water isn't great for this (it's surface tension is too great), perhaps a dribble of petrol ideally with something which will easily be seen (paint thinners with a drop of red paint is good) then look in the exhaust and inlet ports for signs of the fluid going past the valves.

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Andy,

Presumably with the valve springs still attached to pull the valves closed?

John

p.s. before dismantling I checked the 'tappet' gaps and they were all within spec. 5-7 and 10-12 thou, (I had set them at 6 and 11) Apart from one which had opened up by 2 thou (13 thou so only one thou outside spec anyway?)

Edited by Mr_John111

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Presumably with the valve springs still attached to pull the valves closed?

Yes, just turn the head upside down & fill the combustion chambers with petrol or paraffin, if it leaks out the valves in that chamber need to come out. If you watch this episode of

you'll see Ed China doing it on a Triumph Spitfire. Once the valves are out place them on a flat surface & roll them around, that'll show of they're bent or not. If not just regrind the seats, refit them & thank the great god Chapman for your luck. :thumbsup:

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John, what do you use to check the. Belt tension? Just done my belt at the weekend using a kriket and twist test. Also got a laser tension tool but not convinced I am using it correctly

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Hi all,

A mix of thinned cellulose paint sat contentedly in all of the combustion chambers for 20 mins with no leakage so have to thank my lucky stars!

Wilf. I have ways used the Tun-it method aiming for 110Hz and that has served me well.

The belt ran fine for 6 months and it was the set screw falling out of the back of the cam which caused the catastrophic failure.

A thought. Has anyone ever contemplated a compression test on an engine out of the car?

I'm thinking of refitting the startermotor and attaching a battery and a switched feed to the solenoid

Is that completely daft?

John

Edited by Mr_John111

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The easier would be a leak-down test as you don't need to rotate the engine for that, beyond checking for leak at various points in the piston travel should you choose to.

With a compression test you'd need a very sturdy engine stand as the engine could throw a lot of weight around even just with the starter and yo'd need to allow enough room for the flywheel to turn.

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I'm thinking that without the carbs on I'll only have to ensure that the oil ways are all secure, turbo feed and drain connected, all coolant out anyway and with the plugs out it should just turn over. The gearbox is still attached so will need to ensure its in neutral

But must ensure high amp cable for feed and earth to the starter motor.

John

Edited by Mr_John111

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As long as it's secure I see no problem, it'll only be wizzing over at a couple of hundred RPM for a few secs but a leak down would make more sense if you can get the kit to do it.

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John,

I would think a good set of booster cables would be sufficient for what you are attempting to do. Rig yourself a simple starter relay switch, similar to what use to come with the ( you are going to laugh at me now ) the old fashion timing strobe lights. ( said like a person who has all ECU motor cars ).

Just have Sue help hold down the engine....( just kidding Sue !!! )

Glad to hear that all the valves appear to be fine...so far seems like you have dodged the bullet...

Edited by cjtpb13

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