|View single post by Esprit2|
|Posted: 02-10-2020 08:10 pm||
|Q1 - No, a performance cam wouldn't be the reason for 'tight' clearances.
Q2 - Use the clearances given in the Workshop manual...
0.005" - 0.007" Intake
0.010" - 0.012" Exhaust
Q3 - Yes. Measure each valve's clearance, and subtract that from the MAXimum clearance (TOP of the spec range).
Add the difference to the current shim's measured thickness to determine the 'gross' thickness for the new shim.
Finally, subtract the film thickness for the sealant that will be used between the cam carrier & head.
The final total is the 'net' thickness you should use for the next shim.
The sealant that is applied between the cam carriers and the head has a squished/ cured thickness. If you carefully shim all the valves to the tops of their ranges using 'dry' trial assembly steps, then when the sealant is applied during final assembly, it's film thickness will 'float' the cam carriers that much off the head, and increase the valve clearances by a similar amount. Typical film thicknesses are:
0.0015" - Loctite 504, original sealant - still available, but better options are no available.
0.0010" - Wellseal or Permatex Aviation Form-a-Gasket - pretty old-school.
0.0005" - Modern anaerobic sealants, like Loctite 518, Permabond A136, Permatex Anaerobic Gasket Maker (518).
The anaerobic sealants have been Lotus' spec since around 1980. J-H owners would be well advised to use the later sealant.
"Dry Assembly" means a trial assembly done without applying any sealant.
"Dry"-shim the valves to less than the top of the specs by the film thicknesss of the sealant you will be using. I strongly recommend using a modern anaerobic sealant, and my personal preference is Loctite 518. In that case, the film thickness is 0.0005", so Dry-shim the valves to:
Dry Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . Final Result
0.0065" Dry, plus 0.0005" film during final ass'y = 0.007" final clearance - INTAKE
0.0115" Dry, plus 0.0005" film during final ass'y = 0.012" final clearance - EXHAUST
The valve stem will hammer a depression into the center of the shim. So the shim's "working" thickness is measured with a micrometer WITHIN (!!) that depression. A micrometer's round 'rod' anvil will fit into that depression and give an accurate measurement. The jaws of a caliper will bridge across the depression, and incorrectly measure the thicker part of the shim surrounding the depression. Measuring the thicker outer perimeter, or measuring with a caliper will give a false reading. USE A "MICROMETER" to measure the center of the shim.
If you re-use old shims (perfectly acceptable), make certain that the side with a depression goes down against the valve stem. If the depression is installed 'up', then the tappet will bridge across it and result in an excessively large clearance. If the depression is installed 'down', the valve stem will fit into the depression and use the shims 'working' thickness to produce the correct valve clearance.
Valves that are clearanced to the tops of their ranges will run with a certain amount of "clatter"... they're not quiet. That's NORMAL for all solid-lifter engines. As the clearances close-up with wear, the valves start to run more quietly. That may sound nice to your delicate ears, but "quiet" is telling you there is something wrong in the valve train. Check it out and fix it. The valves need to be talking to you.
It's best to shim the valves to the top of each range. That will give the longest service interval (time or miles) before the valves need to be re-shimmed again.
But doing accurate work, and getting all 16 valves set to their max spec is laborious, time-consuming work. Many mechanics split the range, and figure that any clearance between the mid-point and the top of the range is good enough. But each range is only 0.002" wide, from Max to Min/ Service Limit. If they shim the easy/ lazy way, and send you down the road with only 0.001" of wear remaining before the valves need to be re-shimmed, then they're cutting the service life in half before you have to come back and pay them to do it all over again.
If you're doing the work yourself, I strongly recommennd that you take the time to shim all the valves to the tops of their spec ranges... 0.007" Intake and 0.012" Exhaust. Done that way, you should have something like 18,000 miles before you'll have to do the job again.
The valves live in a very hot environment... especially the exhaust valves. Their only opportunity to cool down is while they're closed and in contact with the water cooled head. The "closed duration" is very important to the valve's longevity. Valve clearances influence the time-duration that the valves are closed, and tighter/ smaller clearances mean the valves are closed/ cooling for LESS time... the valves will run HOT.
In normal use, the valve heads tend to hammer their way up into the valve seats... it's called Valve Recession. As the valve recedes up into the head, the upper end of the stem gets closer to the cam lobe, so the clearances get smaller. That's normal wear, and the valve clearances are supposed to be re-shimmed to spec periodically. Check the clearances at each scheduled major service interval, which is specified at every 6000 miles. Adjust the valve clearances as required... meaning 'IF" the clearances are below the minimum spec. If the valves are correctly shimmed to the TOP of the spec, then it will not normally be necessary to re-shim at every 6000 mile service interval... more likely every third interval, or 18,000 miles. But regardless of how long it has been since the last time the valves were clearanced, if you measure the clearances as at or below the spec minimums, then it is now time to re-shim the valve clearances... no argument.
At your measured 0.003" - 0 .004" Intake, and 0.006" - 0.008" Exhaust, all of your engine's valves are w-a-a-a-y past due for being re-shimmed. Repeating... the spec clearance is:
0.005" - 0.007" - Intake
0.010" - 0.012" - Exhaust
In each valve clearance Spec range, the smaller number is the "Service Limit", and the clearances are not supposed to below that. At 0.005" INT or 0.010" EXH, it's time to re-shim the valves. Allowing the clearances to close up any further than that (as your valves are now) is putting the valves at risk of over-heating or burning. Valve clearances also affect the points in crank degreess when the valves begin to open, or close. That alters the effective cam timing, and affects how the engine runs. A "serious" engine tune-up should start with clearancing the valves first, then tuning the ignition timing, and finally tuning & balancing the carbs.
Last edited on 02-10-2020 08:29 pm by Esprit2