Lotus Elan

Head Thickness/Oversized Gasket

PostPost by: dlb123 » Wed Jul 12, 2017 3:14 pm

Hi chaps.

Just got the head back from the machine shop and it's looking a little on the thin side. I know that this topic has been covered before but I had a question regarding the prospect of an oversize gasket.

I've attached a couple photos to illustrate. Head thickness is now down to about 4.575" at its thinnest. As per photos, the intake valves are now sitting just proud of the machines surface. Do I need to shim (say with a +20thou gasket) to protect the valves. I've new pistons and the pockets look relatively deep. Obviously I'll build it up and turn it slowly before I commit to anything.

One more question and it's a daft one but when I picked up the head they had reassembled it with the cam sprockets in the wrong orientation. With piston one at tdc, and the cam sprocket timing marks aligned,should the cam lobes be pointing inwards or outwards for piston one .... Humour me please.

Thanks for the help as ever. Dave
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PostPost by: Elan45 » Thu Jul 13, 2017 3:02 am

I'm assuming you are going to move the sprockets to the correct locations. If you are simply using the marks on the sprockets to time the cams, you need to have the sprockets in correct orientation. The marks are about a single tooth different in location. Are you sure the cams are in their original locations or were they swapped over w/ the sprockets? Have the tappets been kept in their relative positions to the cams? I always keep the tappets in the same position relative to the cams.

W/o running off to look at my manual, you want the marks to be toward to center of the head, although it should also work if the marks are away from each other too, since the cams turn at half crankshaft speed.

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PostPost by: vincereynard » Thu Jul 13, 2017 2:40 pm

dlb123 wrote: With piston one at tdc, and the cam sprocket timing marks aligned,should the cam lobes be pointing inwards or outwards for piston one .... Humour me please.

Thanks for the help as ever. Dave


At TDC No1 compression. The lobes on No 4 should be pointing at each other. Inlet just opening / exhaust just shutting. (In overlap.) I.E No1 just about to start power stroke, (already fired). No 4 starting intake stroke.

Cam sprocket marks "roughly" pointing at each other. If the head has been skimmed both cams are likely to be retarded, therefore the marks may never exactly line up, if they ever did!
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PostPost by: dlb123 » Thu Jul 13, 2017 4:01 pm

Thanks for the responses. As you said Vince, is what I'm looking at. My confusion stemmed from the fact that two illustrations in the manual show different cam lobe orientations for the same sprocket position. I wonder if it's a dodgy drawing (photos attached)

It looks like an offset dowel has been used on the ex cam anyway. With the head skimmed as it is, I'll endeavour to time it up properly to make sure it's all kosher.

Any advances on an oversized gasket? Rohan if you're out there? Not sure what the correct course of action is.

Best. Dave
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PostPost by: cbguerrajr » Fri Jul 14, 2017 1:26 am

you could check valve to piston clearance with plastigage or putty, assembling everything temporarily with a previously used standard head gasket (already compressed) and turning the engine at least twice around.. I understand the minimum clearance should be about .080"
I recently did this, and in my case the clearance was about .100" and the head had been skimmed "a little", with Hepolite pistons. you may not need a thicker gasket.
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PostPost by: nmauduit » Fri Jul 14, 2017 9:21 am

dlb123 wrote:Any advances on an oversized gasket? Rohan if you're out there? Not sure what the correct course of action is.


Rohan will tell better himself, but in the meantime I believe he has a preference for composite gaskets for Lotus TC due to their ability to maintain seal under flex (over MLS type like Cometic - but which are available in a wide variety of thicknesses). Ajusa composite head gaskets typically are about 1mm compressed (they also do MLS), they also offer a 1.2mm compressed version (see their ref 10016300 and 10014600 for example), possibly other thicknesses on special order (I believe Burton do get some specific gasket custom made for their market for instance). Variation in head gasket thickness won't do a lot in terms of valve clearance, I would mainly consider that parameter for chamber volume adjustment (compression ratio).
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PostPost by: rgh0 » Fri Jul 14, 2017 9:34 am

nmauduit wrote:
dlb123 wrote:Any advances on an oversized gasket? Rohan if you're out there? Not sure what the correct course of action is.


Rohan will tell better himself, but in the meantime I believe he has a preference for composite gaskets for Lotus TC due to their ability to maintain seal under flex (over MLS type like Cometic - but which are available in a wide variety of thicknesses). Ajusa composite head gaskets typically are about 1mm compressed (they also do MLS), they also offer a 1.2mm compressed version (see their ref 10016300 and 10014600 for example), possibly other thicknesses on special order (I believe Burton do get some specific gasket custom made for their market for instance). Variation in head gasket thickness won't do a lot in terms of valve clearance, I would mainly consider that parameter for chamber volume adjustment (compression ratio).


Yes I agree - thanks saved me some typing :lol:

cheers
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PostPost by: nigelrbfurness » Mon Jul 17, 2017 8:59 am

Off topic but looking at your photo you might want to consider cutting the valve seats in a little deeper - I had a problem with a head skim recently that looked pretty similar to your image. A search around the web will tell you the optimum depth of valve/seat contact area though some Lotus TC engine reconditioners think the TC benefits from a very small contact area for the inlet valves, particularly on the big-valve head. I'm not sure why that is....

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PostPost by: john.p.clegg » Mon Jul 17, 2017 9:05 am

A nice radius on the inlet valve seat helps gas-flow , wheras a larger contact seat on the exhaust helps heat dissipation.

John :wink:
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PostPost by: dlb123 » Thu Jul 20, 2017 7:35 pm

Thanks for the responses guys.

When you say cutting the valve seats deeper, what would be the reason for this? We had to reseat the intake valves because the shims were down to almost minimum thickness (I think the original seats had been recut many times over).

The previous rebuild had addressed the exhaust but not the intake, so the variation in shim size between the two was significant.

Therefore the intention was to 'lower' the intake valves back down into the head against new seats.

What's the risk in this scenario? Have we gone wrong somewhere. The machinist is experienced and is happy with the work, but of course this is not gospel.

Best.

Dave
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