Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Valve Lash
Thread: Valve Lash
View Single Post
  #9 (permalink)  
Old 10-04-2007, 11:49 PM
Registered User
Last photo:
Join Date: Aug 2007
Location: Tucson, Arizona
Posts: 86
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
TDC base circle

I realize this is my germanic heritage speaking - but my dear departed uncle who taught me about the 4 cycle engine - would only set dead engine lash on a cylinder that was at TDC. He told me that was the best guess a man could make at being on the base circle (with no dial indicators).

He (I) would pull all of the plugs and mark which hole they came from. Turn the engine to the compression stroke of cylinder one and stop with your damper and indicator aligned on the zero mark. Put marks on the damper at 90 intervals - measure the circumference if you have to, and divide by four to get the distance between them. With this method - you adjust number one cylinder - then turn 90 and adjust the next cylinder in the firing order and so on for two complete revolutions of the crankshaft. Its a dance to do this but you will get consistent results with a variety of cams and manufacturers. It works for solids and it works for hydraulics.

That gets you on the base circle, hole for hole - next you get experience with finding "near zero" lash. Making the lash tight enough to feel the "right" drag in the pushrod with your thumb and index finger is a tough learn. The polished seats for the pushrod ball ends and the lubricant will work against you. It is purposely designed to have minimal drag and what you have already found is that the pushrod will spin fairly easily even when preloaded. Zero lash is where the pushrod is on the lifter, the rocker is resting on the pushrod - and there is no clearance at the valve tip. Work at getting down close to zero clearance at the valve tip - the rocker ratio is working for you. For example, with a 1.5:1 rocker a .002 tip clearance is only about .0013 over at the pushrod end. A consistent (very) minimal clearance is better than preload.

You are trying to get consistent low clearance at the valve tip on every valve - even if you are wide by say .002 - it can't hurt. Because the next step is to turn down the rocker nut to compress the hydraulic lifter and apply the preload. Whether you turn the rocker nut down 180 or 360 - whatever - you are using the thread pitch of the stud as a sort of micrometer to establish the compression of the plunger. If you had .002 static clearance at the valve (as described above) - instead of a perfect zero - this compression will only be off by .0016. If you error by preloading the pushrod while trying to guess at the rotational drag the difference can and will be, a much higher value. There are preload schemes that deliberately set the valve tip clearance even higher than .002 - and then correct with the turn of the rocker nut.
Reply With Quote