valve lash


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By stuey - 12 Years Ago
hi there

this is one for Ted or anyone else who knows

cam info says 18 thou valve clearance  i'm using Johns ally heads and i remember reading one of Teds articles that the heads grow somewhat increasing valve clearance by 4 thou

do i set initial clearance to 14 thou? i need reassurance please.

stuey

England

By Ted - 12 Years Ago
Setting the cold lash at 0.014” is expected to net you 0.018” hot valve lash.  If using iron rocker arms instead of aluminum, then the cold lash value may vary slightly.
By pegleg - 12 Years Ago
Ted,

      What difference do you find between Tubular, or solid pushrods?

By stuey - 12 Years Ago
thanks Ted

this may seem a daft question and i can take the ridicule but why do we have Lash (tappet clearance)?

if the lash grows as the engine warms up why not set cold to 1 thou so as not to hold valve open knowing it will increase to 5 thou as engine warms?

stuey

By Ted - 12 Years Ago
pegleg (2/8/2012)
Ted, What difference do you find between Tubular, or solid pushrods?

Frank.  All my data is with tubular pushrods.  It’s not very often I deal with the solid pushrods and then actually do hot and cold measurements with them.  But I suspect only 0.001" as the amount of change between them.

By Ted - 12 Years Ago
stuey (2/9/2012)
thanks Ted

this may seem a daft question and i can take the ridicule but why do we have Lash (tappet clearance)?

if the lash grows as the engine warms up why not set cold to 1 thou so as not to hold valve open knowing it will increase to 5 thou as engine warms?

stuey

Besides too many temperature variances, flat tappet cams are built with opening ramps on the lobes and those ramps only have a small amount of latitude in adjustment before abnormal lobe wear starts taking place.  And if setting for 0.001" at 55°F for the aluminum heads, then at 30°F the valves would be potentially cracked open and the engine wouldn't start.  On the EMC engine, 0.014" hot lash was the make or break point.  At 0.014", the exhaust valves got hot enough to be -000 lash at wide open throttle and you could hear a misfire taking place at the top of the rpm scale.  The brake specific fuel numbers also showed this in the data.  At 0.015" lash, that engine was fine at the same rpms.  When the engine ran at the EMC competition, it was with 0.015" lash as a result of what was found during the testing.

On the flip side of that, I recently ran a hydraulic camshaft in a 390 on the dyno using solid lifters.  That particular camshaft did respond to minimal lash being used versus the flat tappet camshaft that was used responding to increased lash settings.  Just food for thought.