Squish clearance

See my former post 'Head gasket?bigger, bigger, bigger!'

Have just spoken to the company that modified my heads, and find that they did not skim the heads, and mearly honed them (cleaned them off).

Reason given, is that they wanted to leave as much meat on the heads as is possible to allow the squish clearance to be measured and then have the heads skimmed accordingly to provide an optimum squish of 0.030"

I thought squish clearance was applicable to 2 strokes only, but they advise otherwise.

Has anyone ever done the squish clearance measurements ? and was there any benifit???

[stephendell]

Reason given, is that they wanted to leave as much meat on the heads as is possible to allow the squish clearance to be measured and then have the heads skimmed accordingly to provide an optimum squish of 0.030"

When were they planning on doing that?

No I have to measure the squish, then sends heads back to them for the skim, or use a local.

[stephendell]

OK

I thought squish clearance was applicable to 2 strokes only, but they advise otherwise.

Definately advantages for fine tuning squish for 4 strokes too.

But does not measuring the squish accurately require the head to be flat as a pancake to start with?

Cheers

It would help
Did they not mention this when you picked up the heads or has this come to light now you've started asking them questions ?

Came to light when I started asking questions, said he had mentioned it before!!

But this is not some backstreet outfit that worked on my heads, he is reputed to be the wizard!! see http://www.cylinderheadshop.co.uk/

Sad thing is it looks like I will now be taking engine back out of car.

The squish height is measured @ the block isn't it ?TDC to liner top + the compressed gasket dim - the liner protrusion So the head doesn't come in to it until you want to change it IYSWIM

Yep you must measure it with 'all' bolted up, the following extract seems the way to do it, however, if (by chance) 'Squish' is about 'right' I still have my weeping head gasket issue, so my heads still have to come off!!!

Your Cylinder Head Squish Clearance...


The "squish" band is a somewhat narrow angle cut into the outer periphery of the combustion chamber that comes into a close clearance proximity with the piston crown outer edges at TDC (Top Dead Center) of crankshaft rotation. Measuring it accurately so as to make decisions about what modification(s) should be made to an engine's compression ratio and combustion chamber is a critical reference step in planning a high performance motor.
To determine your actual squish clearance...
You will need about a 10" length of solder of a diameter that is thicker than your current squish clearance. Common available diameters are approximately .038", .065", .090" and about .140". Start with the .065" (in tightly set-up race engines of smaller displacement you may need only the .038") and if it is not thick enough because your squish clearance turns out to be greater, just go up to the next size. IMPORTANT! IF YOU USE SOLDER THAT IS THICKER THAN NECESSARY TO MEASURE THE EXISTING CLEARANCE (Example: Using .090" thickness solder in a cylinder with .055" squish clearance, when the proper solder thickness to be used would be .065") YOUR MEASURED CLEARANCE WILL BE INACCURATE!!! If you get inaccurate squish clearance impressions, everything that is deduced from them will be seriously flawed. THIS IS A BIG DEAL, SO TREAT IT WITH PROPERLY DUE ATTENTION!
Cut the solder tips squarely with a single edge razor blade so each tip is cut perfectly perpendicular to the solder length. This is also VERY IMPORTANT to get accurate indications from the very edge of the combustion chamber limits! Bend the solder into an upside down "U" shape with the "legs" about 1/2" apart and parallel to eachother. Now cross the "legs" over eachother. You should now have an inverted "U" of solder with "legs" crossed in an "X" pattern.
Remove spark plug from your engine. Bring the engine up to approximate top dead center (follow the piston up the bore with another piece of solder stuck into the plug hole.......). Now back the motor up opposite normal direction of rotation until the piston has dropped about an inch back down the bore. Insert your two "legs" of solder into the plug hole while holding the solder by its' "U". Push and allow the "legs" to splay left and right across the piston dome INLINE WITH THE WRIST PIN AXIS. Do NOT push the "legs" fore and aft (front to back) in the bore or the potential for the piston to "tilt" on the wrist pin in its' cylinder wall clearance could give you false readings.
You'll be able to "feel" the tips of the solder as they move left and right out to the cylinder wall limits. When you feel them touch the cylinder wall, stop pushing and hold what you've got with one hand while rotating the motor up to TDC with the other hand by its' flywheel or similar. If the solder is thicker than your squish clearance you will feel resistance as you reach TDC. If the resistance is too great to roll past TDC, select another length of solder of the next thinner diameter.
Roll the engine through TDC and allow the piston to descend enough down the bore to easily retract the legs of solder back out through the plug hole. IMPORTANT! Keep track of which tip was at which side of the bore so you'll know which is which if there's a difference in squish clearance from one side of the bore to the other! Differences in clearance from one side to the other are common and can be caused by a "tilted" squish band in the head, "tilted" piston crown, bent rod, cylinder bore not cut perpendicular to the head mating surface, etc.. Noticing that there is a problem and which direction the "tilt" is will alert you to track down the culprit.
Now measure the compressed tip ENDS (it is fairly common for the squish band angle in the head and the radius on the piston crown to diverge slightly, thus your "tightest" clearance will be right at the very ends of your solder) and record them. This is your actual squish clearance at TDC.

bigger The head guy should have made sure the heads were flat though

yep 'bigger' is the word, the head work was not cheap, what gets me is that he would spend so long modifying my heads and not even establishing prior that the heads are 'good'.

I am going to de-torque tonight, and then re- torgue and pump the water system up to 1 Bar, if they weep still then 'here comes Johnny'

I'm rebuilding mine @ a toolmaker mates so can clock ,check and if needed machine a lot of things during the rebuild If anyone wants to know specifics give me a shout and I can measure up the bits .

What is the minimum height the heads can be machined to and what can be associated problems other than increased compression (not so bad) inlet manifold alignment and timing case cover protrusion where your rocker cover sits? Maybe timing chain length!

Set my 'squish' for 0.04" not sure though how much has been taken off the heads now, as they were skimmed (first time) before my ownership, and then surfaced and then finally then skimmed after squish measurements.

Had problems with the chain cover as the bolts would no longer line up (with the head, so elongated the holes, the rocker covers also stood proud, so again machined the top of the chain cover to bring them flat, took some 0.020" off so this may give you an indication of how much the head has been skimmed.

Chains were no problem, though cams will have to re-timmed and I intend to use offset dowels

What is the minimum height the heads can be machined to ----- inlet manifold alignment ------!

Sorry forgot to cover the inlet manifold, yes if you still have a 'one piece' inlet then you may have to machine this. Not a problem though that I incurred as I have Triple Webers, so two seperate manifolds