Lotus Elan

pistons and valves contact

PostPost by: checkrail » Wed Sep 20, 2023 4:16 pm

Hi all, just replaced the head after renewing the gasket to hopefully cure oil leak on the timeing case. I stood the head on end to check the valves which was a stupid thing to do because the tappets came tumbling out.
The cams I had already removed. It taken ages to sort all the shims again which I have done with the cams back in the head, and done one cam at a time with the timeing chain disconnected.
The engine is on TDC and the cams on no. 4 wth the lobes pointing inwards
If I turn the cams using the sprocket bolts with the chain disconnected they will jam as one of the lobes presses down on the tappet, I have tried altering the shims but no difference
If the pistons and valves are contacting I do not understand how this has happened, I have not forced anything.
It looks like the head has got to come off again, I dread to think what I'll find,

Hope that I'll hear from some one where I've gone wrong thank you John
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PostPost by: Europa88 » Wed Sep 20, 2023 4:31 pm

If you are turning the cams and they are not linked by the chain, they may well be out of timing and you could have valve contact.
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PostPost by: Andy8421 » Wed Sep 20, 2023 4:40 pm

As Europa88 mentions, the valves and the pistons occupy the same space, just not at the same time.

Before the cams or crank is moved, the cams need to be connected to the timing chain and timed correctly, then as the piston move up the bore, the valves will move out of the way.
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PostPost by: nmauduit » Wed Sep 20, 2023 5:15 pm

checkrail wrote:The engine is on TDC and the cams on no. 4 wth the lobes pointing inwards
If I turn the cams using the sprocket bolts with the chain disconnected they will jam as one of the lobes presses down on the tappet,


if the timing chain is disconnected the engine is not on TDC, it is just partly disassembled.

I would at least consult the shop manual to get a feel for what to do on an engine I'm not familiar with before embarking a serious maintenance operation : now you should as well control that a valve has not been bent depending on the force of the valve/piston contact you have applied... no time saved here.
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PostPost by: Europa88 » Wed Sep 20, 2023 5:35 pm

As nmauduit says...You may be lucky and no damage done if you haven't forced the cams to turn. But you are right the head will have to come off to check for damage!
Last edited by Europa88 on Wed Sep 20, 2023 5:37 pm, edited 1 time in total.
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PostPost by: berni29 » Wed Sep 20, 2023 5:37 pm

Hi

If all you want to do is check the rotation of the cams then use a screwdriver down a plughole and a socket on the camshaft to set the pistons at midway. Then you can rotate the cams safely as much as you like.

If you have been gentle and stopped when you felt contact you should be fine.

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PostPost by: Europa88 » Wed Sep 20, 2023 5:42 pm

I'm just wondering that if its a large valve engine and the cams are rotated independently, that the valves could touch when both fully open?
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PostPost by: promotor » Wed Sep 20, 2023 6:31 pm

Europa88 wrote:I'm just wondering that if its a large valve engine and the cams are rotated independently, that the valves could touch when both fully open?


This is exactly what happens, even with standard size valves.
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PostPost by: checkrail » Wed Sep 20, 2023 6:32 pm

thanks chaps for all the replies
I forgot to mention that I connected everything up as it should be and all I could get with a long reach spanner on the crank bolt was about half a turn before it locked,
It is a big valve engine

John
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PostPost by: Europa88 » Wed Sep 20, 2023 8:45 pm

checkrail wrote:thanks chaps for all the replies
I forgot to mention that I connected everything up as it should be and all I could get with a long reach spanner on the crank bolt was about half a turn before it locked,
It is a big valve engine

John

If it was timed correctly there shouldn’t be an issue. Did you account for all the shims that fell out?
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PostPost by: alan.barker » Thu Sep 21, 2023 4:23 am

Sorry but i suggest you contact someone who knows Twinks to sort it out.
Good luck
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PostPost by: 512BB » Thu Sep 21, 2023 7:23 am

A very simple method to determine where the pistons are whilst setting up the cam timing. All wooden dowels are the same length.

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PostPost by: checkrail » Thu Sep 21, 2023 6:32 pm

thanks Leslie for the photo, it's not the first time that I've removed the head. Last time I set it with the timeing marks level on the sprockets, the marks on the pulley/ timing case correct, the cam lobes on no.4 pointing inwards and a rod down the plug hole confirming TDC.
I've done the same this time, removed the cams and then the head. Of course I had the tappets fall out but can't understand why i've got this piston/valve problem
Senior moment but why should no.1 piston be moved from TDC?

Thanks for your patience John
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PostPost by: Europa88 » Thu Sep 21, 2023 8:31 pm

Just be aware that TDC should be on the compression stroke.
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PostPost by: Europa88 » Thu Sep 21, 2023 8:37 pm

Sounds to me that something has fallen into the timing chest and jammed! You don’t know for sure that it’s valve/ piston contact or valve/valve contact.If there is nothing jammed then at this stage I would definitely have the head off!
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