piston-valve clearance

[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

[types26/36]

--- In ***@***.***, Fred Talmadge <ftalmadge@f...>
wrote:





Fred,
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.
Rgds Brian
64Elan/72Sprint/J.P.S.Europa

[richboyd]

I would like to try this "clay to test valve clearance" method also. But I
don't have a clue. So I will ask some simple questions.

Is the clay placed on the piston tops? Or, placed over the head and valve
surfaces? I assume the piston remains static at TDC (crank doesn't turn)
while one turns a single camshaft at a time. Does this sound correct? Only
one cam in place at any given time? Can you do two cylinders in one
"set-up," mounting the EX cam first, then the IN cam? Is there a "better"
procedure?

How thick of a clay layer should one use? More than the 0.060" than Brian
suggest (min allowable clearance), and less than the full chamber volume, I
would guess. But what is a reasonable thickness to start with? Or will this
become apparent as I start applying the clay?

Your advice is appreciated,
Rich Boyd


At 09:59 AM 7/12/2004, you wrote:

[Fred Talmadge]

Ron, unfortunately there is nothing in Wilkens or Vizard's book, but I did
find a couple of articles online by searching for "piston to valve
clearance" I printed the articles out at home and I can post them when I
get home. Basically you put the clay into the cylinder head rotate the
engine and you get an impression of the valves extended into the piston.
Measure the clay and you have the clearance.

Most articles say it takes some practice and I haven't done it yet but it
does sound a little funky. There is another way using low tension springs
but I didn't understand it fully. It's mostly done with high-lift cams and
rocker arms. My head was shaved .003" and the valves stick up a good ways
from the surface now. It probably looks worse than it is but better safe
than sorry.

Fred T. '65 S2

-----Original Message-----
From: Rich Boyd [mailto:***@***.***
Sent: Monday, July 12, 2004 2:16 PM
To: ***@***.***
Subject: Re: [LotusElan.net] Re: piston-valve clearance


I would like to try this "clay to test valve clearance" method also. But I
don't have a clue. So I will ask some simple questions.

Is the clay placed on the piston tops? Or, placed over the head and valve
surfaces? I assume the piston remains static at TDC (crank doesn't turn)
while one turns a single camshaft at a time. Does this sound correct? Only
one cam in place at any given time? Can you do two cylinders in one
"set-up," mounting the EX cam first, then the IN cam? Is there a "better"
procedure?

How thick of a clay layer should one use? More than the 0.060" than Brian
suggest (min allowable clearance), and less than the full chamber volume, I
would guess. But what is a reasonable thickness to start with? Or will this
become apparent as I start applying the clay?

Your advice is appreciated,
Rich Boyd

[Elan45]

Rich,

NO<NO<NO!!!!

You will surely bend a valve or two!

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



Is the clay placed on the piston tops? Or, placed over the head and valve surfaces? I assume the piston remains static at TDC (crank doesn't turn) while one turns a single camshaft at a time. Does this sound correct? Only onecam in place at any given time? Can you do two cylinders in one
"set-up," mounting the EX cam first, then the IN cam? Is there a "better" procedure?

How thick of a clay layer should one use? More than the 0.060" than Brian suggest (min allowable clearance), and less than the full chamber volume, I
would guess. But what is a reasonable thickness to start with? Or will thisbecome apparent as I start applying the clay?

Your advice is appreciated,
Rich Boyd


At 09:59 AM 7/12/2004, you wrote:

[types26/36]

--- In ***@***.***, Rich Boyd <richboyd@r...> wrote:


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

[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
(734)-971-5583
Cell (734)-604-9280
Email: ***@***.***
Too many Hobbies.... Too Little Time
1969 Lotus Elan....It's not a restoration, it's a never-ending adventure.