Clarification needed - Valve clearance adjustment

[pauldridge]

I'm a bit confused at the owner's manual instructions for adjusting the valves on my '53 SQ 4 MK II. First it gives the clearance of .006 and .008 for intake/exhaust, but then follows up with instructions to set one piston at TDC and then set clearance on an opposing cylinder at .010?

When I got the bike, virtually all of the clearances had been set very tight.. ..ranging from .001 - .002, and I am wondering if the P.O. misinterpreted those instructions (as well as misreading the specified clearance).. fortunately, he supposedly only rode the bike less than 1,000 miles (supported by odometer readings on previous title).

Can a wiser owner than me explain this seeming discrepancy? And, is there any problem with my chosen approach: turn the engine over until each valve is clearly closed, then set the .006 or .008 clearance and move on to the next?

[simon.holyfield]

As a Mk1 owner I'm reluctant to comment on the manual, but I know the MkII camshafts have 'quietening ramps' which will affect the way the clearance is set up.

I'm sure a knowledgeable MkII owner will be along shortly; meanwhile have you read Bruce Longman's piece on valve timing, downloadable from the 'articles' section of the document repository?

[paul.jameson]

I set each cylinder in turn to TDC on the compression stroke (cold), then set the valve clearances for that cylinder. With an alloy Square (Mk I or MK II) the clearances will increase as the engine warms up because the alloy pushrods are shorter than the alloy barrel. The steel cam followers expand less than alloy so clearances increase. The reverse is true on the iron engine Squares where the alloy pushrods expand more than the iron cylinder, thus reducing clearances. If you fit a MK II type camshaft with quietening ramps to an iron square (as I have done) the clearances to use are 0.023" and 0.025" which appears horrendous but - the quietening ramps work and the engine is none too noisy.

[pauldridge]

Thanks Paul.. you've confirmed my interpretation

[david.anderson]

The 4G mk11 owners guide states, page 26
“With the piston of no 1 cylinder at top dead centre and BOTH valves open, the rocker adjustments(tappets) should be set to give .006in inlet and .008in exhaust clearances for no 3 cylinder ……etc”
My owners guide does not mention a .010” clearance.
The reason being is that cylinders 1 and 3 are both at top dead centre at the same time, but only one cylinder is on the compression stroke. By checking that both valves are slightly open on no 1 cylinder (exhaust closing and inlet opening), then it means that no 3 cylinder is on the compression stroke.
David

[simon.holyfield]

... By checking that both valves are slightly open on no 1 cylinder (exhaust closing and inlet opening), then it means that no 3 cylinder is on the compression stroke.
David

Presumably this tells you that the cam followers are on the base circle of #3's cams - Like the 'Rule of 9' you can use on BMC A series engines (and presumably anything with a 1-3-4-2 firing order), where you can set Valve #1's tappets when valve #8 is open, #2 tappet when #7 is open, etc.

It would be interesting to work out a similar rule for the SQ4 engine. I must admit I find it difficult to get the crankshafts in the right place without taking the timing cover off and looking at the marks.

[paul.jameson]

For simplicity, use my system, as outlined in the post above. If the valves on the cylinder are both closed, and you find TDC on that cylinder using a screwdriver, you can't go wrong setting the tappets on that cylinder. Then move round clockwise, cylinder by cylinder, repeating the process and turning the engine over forwards using the rear wheel in 4th gear with the plugs out.

Do try to remember to select neutral again before starting the bike and riding off unless you want to wheelie down the drive.

[nevhunter]

Another way is to get one valve fully open (say the exhaust for example) and the diagonally opposite cylinder (on the sq4) is exhaust fully closed. For the vertical twin it's just the other cylinder. Have the bike on the stand with plugs out and in top gear. Mark each as you do it to avoid confusion. and make sure all are done. Some "stainless" so called valves expand a lot more than the original silchrome so a bit extra clearance on the exhaust won't go astray especially if you are a sidecar person. Nev