With my brand new SAS Cylinder Head it seems a wise precaution that I check the clearance between the Pistons and the Valves. This is because valves and pistons can occupy the same space, but not at the same time. Towards the end of the Exhaust stroke, the piston is chasing the exhaust valve up the bore as the exhaust valve closes and on the intake stroke the inlet valve is chasing the piston down the bore. The pistons have small sections carved out of the top to get adequate clearance. If they intersect due to: dimensions, cold or hot, or due to weak exhaust springs or wobble due to too much piston to bore clearance, something will bend or break.
We’ve had recent broad discussion on this forum about a Clay Test, which is the idea of doing a temporary build with squeezable modelling clay on the pistons then rotate the engine by hand, then take the head off and measure the thickness of the clay. I thought that I would try find out best procedures on the Internet,but frankly the blurbs out there were poor, then I fond that we have had quite good descriptions on here in this website, but most of them back in the Archive messages. Because those messages are really fiddly to read, I’ve taken the liberty of collating the best messages that I could find and here they are below. I’ve done a tiny amount of editing to make them a more coherent sequence, with no change of opinions or technical description. 
Original topics:
[elan-archive-f16/piston-valve-clearance-t10646.html](http://www.lotuselan.net/forums/elan-archive-f16/piston-valve-clearance-t10646.html)
[elan-archive-f16/valve-piston-clearances-t11532.html](http://www.lotuselan.net/forums/elan-archive-f16/valve-piston-clearances-t11532.html)
[b]Fred Talmadge:[/b]
I want to check the piston to valve clearance, and I got a few of dumb
questions. First is the clay a special type or will any modeling clay work?
Do you compensate for compressing the head gasket or is it not enough to
worry about? Finally what is the recommended clearance for the twin cam?
My engine is all standard.
Fred T. '65 S2
[b]Rob_LaMoreaux:[/b]
This is easy I learn it in school! Well actually it was in a community
college class called Vintage engine rebuilding.
Put a layer of clay on the top of the pistons where the valves would meet.
About 0.100" will do. Now put the head gasket on and the head and then
torque to spec. The head gaskets have a special coating that cures form the
heat of the engine to seal so as long as you don't start the ening the head
gasket can be reused.
Put the timing chain on and make sure the cams are set to the right timing.
Rotate the engine a couple of full cycles.
Now remove the head and with a razor cut through the clay where the marks of
the valves are and remove the clay from have the valve area. This will give
you a profile of the distance from the valve to the piston.
Measure this distance to find the clearance.
The clearance should be greater than 0.060" with an absolute minimum of
0.040". This clearance pretty much applies to any engine with longer valves
needed a slightly larger clearance to compensate for more growth in length
at temperature.
The other method involves using light springs that you can compress by hand
and then with the engine assembled you rotate the engine until the valve is
at full lift and with a dial indicator on the valve tip you push down until
the valve hits the piston to measure the distance.
The clay method is cheaper and quicker especially if you want to do all the
cylinders just to be sure.
Rob LaMoreaux
Ann Arbor, MI USA
[b]types26/36[/b]
When I have done this I have used plasticine (modeling clay?) I use
an old head gasket but I doubt it makes a great deal of differance
unless you are working to very fine tolerances.
All the books I have read say a MINIUM of 60thou (1.5mms) clearance
and turn the engine two full turns.
It helps to lightly grease the valves so the clay does not stick to
the valves.
I have done this process quite a few times, basically you do a "dummy
build" on the engine,pistons,crank,head,cams,t/chain etc. but first
you put a layer of clay over the valve pockets, stick the clay to the
piston the thickness is not critical....3-4mm is fine it can be just
a blob of clay.Lightly grease the valve heads, fit and torque the
head (old gasket) valve timing must be right. Turn over the engine at
least twice, remove head,,slice the compressed clay and measure its
thickness = valve clearance. Sometimes the clay will have stuck to
the valves (thats why the valves were greased to prevent this)Its a
bit time consuming particually if you have to do it again and unless
you are fitting non standard pistons/cams not nessesary.
Brian
64Elan/72Sprint/J.P.S.Europa
[b]"Roger Sieling":[/b]
What this process tells you is the relationship of the opening and closing valves in relation to the moving piston. The cam timing must be exactly right where you want it to be, stock or otherwise, but the clay thickness willvary if you change the cam timing. You turn over the complete engine. At this point, you might also leave the rings off the pistons too, unless the bottom end is already to go.
One of the solutions to too close or even no clearance is to change the camtiming, in fact, I have built pushrod motors, or for that matter, any engine with a single camshaft and part of our determination of final cam timing, when things are gettin' close is to advance or retard the cam to even outthe clay thickness on intake and exhaust. On a twin cam, you need only vary one or the other cam to do so, but unless you've done something very special w/ the cams or milled a bunch off the head, most street TC engines should be OK.
The idea of using lighter valve springs is to not wear the cam or tappets out turning over the engine by hand as well as it is very hard to do, compressing full strength valve springs. Often, you just leave one of the pair off, then when you do final assembly, you install both spings in the pair. I usually use the inner since it is the weaker of the two.
Roger
[b]"Brian Goodison":[/b]
suggestions, hints or details of the procedure.
Take the clay and press it into the piston cutouts, about 5mm thick.
Grease the valves heads (this is to stop the clay sticking to the
valve)
Do a dummy build on the engine with an old head gasket.
Turn the engine over two full turns.
Remove the head, take a knife and cut the clay in half and measure
the thickness of the clay, this is = to the clearance.
All the books I have read recommend a minium clearance of 60 thou (1.5mm)
Brian.
64Elan/72Sprint/J.P.S.Europa/Birkin T.C.Seven
[b]Elan45[/b]
When "claying" the engine to check valve clearance. Do NOT just rotate the cams with pistons at TDC. They will touch and you may damage the valves or pistons.
The intake valves will open just before the piston reaches TDC, but will only be open just a little bit. It will not be fully open until the piston is halfway down down the bore.
After firing and power stroke, the exhaust valve will begin opening just before BDC, be fully open as the piston is about half way up the bore. It will still be open a little bit as the piston approaches TDC.
So, to clay the engine, you must assemble the engine with the old bearings and no rings. Don't bother to torque things or stretch the rod bolts. You want things snug, but not tight-tight. Use the old headgasket as someone else suggested.
If you are not going to degree in the cams, make sure you at least have the intake and exhaust sprockets on the right cams and set to factory marks. The sprockets are marked "INT" and "EX". You will need to remove the head to check the clay blobs, so you might want to use only 4 head bolts at first for this checking process. If you leave the cams in the head, be very careful that you do not sit the head down on the bench with the valves extended. I like to work on the head with it sitting on the front timing chest.
Always turn the engine to TDC before you remove the head!!!!!
This way, the cams should be on their marks and the crank is also on its mark.
Be very careful! Don't force anything! Good luck.
Roger
[b]ardee_selby:[/b]
And if the valve timing is being done!
This little "snippet" has stuck with me since I read it all those years ago. An example where the guy had checked clearances and THEN timed (degreed) them!
[img]http://www.lotuselan.net/forums/download/file.php?id=17362&t=1[/img]
(From elan-f15/one-man-his-twink-from-1985-t23399.html)
Richard
and can no linger hold a 19kg twink head at arms length while trying to jiggle it onto the guide rods, so I take the weight on my engine hoist, which is fastened to my garage ceiling. To avoid damaging the valves, some of which protrude a little at top dead centre I first put the 4 outer head bolts in the block and put a soft rubber mat over them and then lift the head onto them and it balances there while I fix the hoist ropes. This is because, due to the shape of my garage, I can’t by hand move the car out from under the hoist, attach the head, lift it and push the car back under it.
