Clay Test -- checking clearance between Valves &Pistons

[billwill]

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.


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Original topics:
elan-archive-f16/piston-valve-clearance-t10646.html
elan-archive-f16/valve-piston-clearances-t11532.html


Fred Talmadge:

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


Rob_LaMoreaux:

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

types26/36

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

"Roger Sieling":

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


"Brian Goodison":
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



Elan45

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



ardee_selby:
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!




(From elan-f15/one-man-his-twink-from-1985-t23399.html)

Richard

[Sea Ranch]

Awesome compilation, Bill. Thanks so much for putting this together. Will "subscribe" so I can reference it again when I do a rebuild.

Randy

[billwill]

Yesterday, Sunday, I had a go myself at doing a clay test.
This is how I did it.

The bottom half of the engine is still in the car, the head is brand new.

The block and pistons are prepared and blobs of plasticine modelling clay pressed into the cutouts in the pistons. the chain tension adjuster its little spring, carefully removed, guide rods screwed into two head bolt holes.

Block with Clay on Pistons

The timing chain was fished up from the water-pump boss and a handy piece of wire attached, so that I would not need to go fishing at the full depth of the head.

The bottom surfaces of the valves had engine oil squirted on them, so that they will not stick to the clay.

New head, with valve surfaces oiled and chain tensioner in place.

Pistons and cams are set at top dead centre.

At age 71 I'm getting old & feeble 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.

Anyway then I hoisted the head clear, then I removed the bolts and screwed and the guide rods back in.

Head hanging

The wire for the chain was passed up through the head chain area then the head swung over and onto the guide rods and carefully lowered a little, checking the position of the chain tensioner. To get it past the chain guide near the dipstick it was necessary to remove one of the guide rods for a moment, lower it a little more, but not yet seated down, then put the guide rod back.

Head on guide pins

Finally all was clear and the head was lowered down on the head gasket, two head bolts put in loosely so that the guide rods could be removed. Then I put in 4 more head bolts and gently tightened them. Then fished the chain out, hung it over the cam shaft bosses, removed the piece of wire, checked the tensioner yet again. I didn't remove the hoist rope as I would be lifting off again shortly.

Head on, Sprockets on

Then I put the tensioner spring and adjuster back in, loosened.

Chain tensioner adjuster back in.

Then I fitted the cam sprockets and chain, starting with the exhaust sprocket and verifying that I had a clear run of chain from the crankshaft sprocket at the bottom to the cam sprocket, and that the straight run up from the crankshaft is reasonably tight when the sprocket is on the cam and the timing mark is level with the centre of the top of the head. This ensures that I am not one-link out of place. Taking extreme care that I do not drop a washer or bolt down into the sump I inserted the sprocket-securing bolt and did it up loosely. (While doing this I find it best to put the spring washer and big spacer-washer on the bolt, then hold the bolt head in a socket spanner with just an extension piece in the spanner, no ratchet or tommy bar, so that I can support the washers with my other hand.)

Then I did the same with the inlet sprocket. Then the tensioner adjuster was screwed in to tighten the chain until I had just half-inch up & down movement in the chain at the top between the sprocket wheels. You will see from the pic that I have those nice fully adjustable sprocket wheels in my engine. That was one of the mods, Rob Morley did for me back in about 1998.

Checked that the timing marks on the sprocket wheels are both aligned correctly, if not I might have had the inlet sprocket one-tooth out of place.

Double-check the cam timing marks.

I used 6 head bolts, not all of them for this test, but at this point I torqued them to 60 ft-lbs. All was then ready to turn the engine by hand for two whole revs (at least) using a socket spanner on the main pulley nut of the crankshaft. When looking at the front of the engine, I rotated it clockwise, that's the normal direction of rotation of the engine.

Six head-bolts torqued to 60 ft-pds, ready to turn

{can't put enough photos in one posting, continued in the next message, please don't post any comments until my second posting is in place}.}

[billwill]

After I had turned the engine two revs, by hand I did two more for luck, leaving it at top-dead centre.

Then I reversed all the procedures above to free the head and hoisted it gently with two very loose head bolts to ensure the valves cleared the head & gasket, before I removed those last two and hoisted it clear.

Assembly reversed and head lifted clear.

Then the four head bolts & rubber mat balancing trick was used so that I could remove the hoist rope and lift the head off completely. The pistons were put at half-mast for this by turning the main crankshaft pulley.

The four head bolts & rubber mat were used again to enable the hoist ropes to be removed

It was fairly obvious immediately that the exhaust valves had made little or no impression on the clay, so clearly I could have put a thicker blob on the exhaust positions, but the inlet valves had indented the clay in the inlet positions.

Using a razor knife, thin slices were cut from the clay and then the thinnest point measured with a digital (vernier) callipers (which I forgot to include in any photo).

These 4 photos show the cross sections of pistons 1 through 4 in sequence.

Thin slices of the clay were cut out an measured. Piston 1

Piston 2

Piston 3

Piston 4

And finally the extracted samples and my notes of the measurements.

It seems I have plenty of clearance, typically over 140 thou.

The samples and the measurements

Cyl 1
Inlet > 136 thou, >177 thou Edge to cutout clearance 118 thou
Exhaust. Didn't seem to touch the clay at all so was at least 141 thou


Cyl 2
Inlet : 250 edge 104
exhaust . Didn't touch clay so > 250 thou

Cyl 3
Inlet 170 edge 150
exhaust Didn't touch clay so > 240 thou

Cyl 4
Inlet > 180 edge 147 thou
exhaust Barely touched the clay so > 240 thou.

[billwill]

Ouch I think this writeup was more effort than the actual test.

[CBUEB1771]

I have never cared for the clay method. I have always done a test assembly with valves installed on only one cylinder and with light springs. The light springs allow you to push the tappet and valve away from the cam lobe and toward the piston with a screw driver or similar tool. With a dial indicator on each of the inlet and exhaust tappets you can rotate the crankshaft and every few degrees in the TDC region stop and measure just how much clearance there is from valve-to-piston or valve-to-valve by easing the tappet away from the cam lobe and going until you feel the valve contact something.

[billwill]

The dial method won't really tell you the clearance between the edge of the valve and the edge of the piston cut out, will it? Only that there is some clearance while cold.

~~~~~~~~~~~~~~~

On subsequent thought, I've decided it was not really a good idea to torque 6 head bolts to 60 ft-lbs. The head is designed to be clamped down by 10 bolts, so the head gasket might very faintly possibly cause some distortion of the head near those missing 4 bolts.

Rob Morley suggests that it is not necessary to use high torque on the head bolts for a clay test, 'finger pressure' will be sufficient.

[CBUEB1771]

The dial method won't really tell you the clearance between the edge of the valve and the edge of the piston cut out, will it? Only that there is some clearance while cold.

Fair enough, the dial indicator method will tell you if there is interference when everything is cold, but nothing about how much clearance there is to accommodate thermal expansion. Next time I'll think about adding a strip of something of suitable thickness to the valve pocket in the piston and painting die maker's blue onto it.

[foggy]

all around the valve is important. Things really thrash when at full chat. The clay will tell you better about the area around the plane of the valve face, but to read the important space from the face of the valve to the face of the piston:

Use a nylon wire tie with a .05 or so thickness and run it into the space between the piston and valve through the spark plug hole. Turn the motor by hand, only, and you will feel the resistance when the wire tie will catch. You need a bit more for the exhausts. Big .05 at min. the intakes can be .04. This closest proximity occurs not necessarily at TDC. Use the offset pins when you use a serious cam. careful turning one way or the other, as a bit of slack in the chain will give a different timing in reverse. Best to do this on a stand. You should be using a dial indicator and degree wheel. You may have to give up a bit of cam timing or cut some more clearance.. depends on priority. Often times, I will time the cams according to what clearance is available, giving up a bit on the timing.

My usual is Alfa's and they have the vernier type adjust giving 1.5? incremental changes. Not sure what the offset pins are available in, as I have not been doing Twinc's .

If you need this described in more detail, especially the dial indicators and TDC and all that, please ask. I will check this in the near future.

Steve

[elj221c]

My usual is Alfa's and they have the vernier type adjust giving 1.5? incremental changes. Not sure what the offset pins are available in, as I have not been doing Twinc's .

Usually 1 to 9 degrees at 1 degree steps.

[SJ Lambert]

Rohan, do you prefer the plasticine or dial indicators and inner springs method?

[SJ Lambert]

Al Bartz, engine man extraordinaire, was a dial indicator / no springs, then do it again with springs man with his famed Bartz Engine Developments Chevys (et al) - as this extract from Phil Henny's book relates Don Nowell's reminiscence. Don was Al's first employee.

[SJ Lambert]

Don preferred this method so as to know what the valve & piston are doing at all points of crankshaft rotation.

[rgh0]

Rohan, do you prefer the plasticine or dial indicators and inner springs method?

I use the plasticine method. Yes it only gives the minimum clearance and you don't know exactly where it occurs around TDC but its enough to know if your clearances are OK and its a lot quicker and easier. The dial indicator method may suit a pushrod engine but a pain to do on the twin cam as you need to ensure you don't get the valves clashing.

The information it gives that the dial indicator method does not give is the side clearances between the valve and the valve cut out in the piston so you can check that the valve cut out is large enough in diameter and positioned correctly .

cheers
Rohan

[SJ Lambert]

Any disadvantage in doing a clearance check without rings on the pistons?

(Pistons are gonna get sent off for a coating before final assembly).