Kemo Sabe, et all,
Lash settings are determined by:
1/ Height of the clearance ramp ground into the cam blank, ie, the portion of the cam grind between the base circle and the opening/closing ramp . . . .
2/ The rocker ratio, where larger/higher ratios permit more clearance . . . . . (OH cams with direct action and therefore a ratio of 1/1,
REQUIRE LESS clearance for proper
operation . . . .
3/ Valve stem/pushrod growth (in length) due to the heat of operation, which in efffect
reduces the working clearance . . . .
(heat one of your exhaust valves & measure the length
HOT Vs.
cold. You might be surprised . . . . )
4/ The "stiffness" of the valve train for OHV (pushrod) engines . . . .
5/ Etc, etc . . . . .
6/ This is one area where the customer
should follow the clearance specs set forth by their (reputable) cam grinder, since he/she knows how much
"ramp" is ground/designed in.
Although I don't have the ramp specs for the custom "Midget" cam, I typically find that I can't get myself in too much trouble by following St. Dema's instructions. He has designed/supplied me with dozens of specialty cams for "weirdball" applications over the last 30+ years. I have
NEVER been disappointed by his product/work. It is why I continue to use & recommend his products . . . . . .
By now it should be clear to everybody why you just can not use any dimension for the camshaft clearances,
OR, compare between OHV (pushrod) & Direct Action (OHC or Flathead!!) valve trains. (Yes, I know I left out the exceedingly complex OHC with rocker followers, please don't PM me about it . . . .) The purpose of the clearance ramp is to gently take up the lash &
PREVENT an excessive instantaneous load to the valve train. If the follower is exposed to the flank without a ramp, the loads are very high and almost instant, quite possibly exceeding the strength the the materials in question with extreme contact pressures . . . . . (this also includes roller followers . . . . .)
Some numbers for cold lash:A/ Let's say we have a cam with a .016" ramp height, and an OHV engine with a 1.5/1 rocker ratio. So then:
.016" ramp on cam * 1.5/1 R/Ratio = .024" MAX permissible valve lash. I would probably run .021"/.022" Ex. and .019"/.020" Int. for starters, UNLESS the cam mfg.
suggested otherwise . . . .
B/ Let's say we have a cam with a .010" ramp height, and an OHC engine. So then:
.010" ramp height * 1/1 Ratio = .010" MAX permissible valve lash. I would probably run .009"/.010" Ex. and .007"/.008" Int for starters, again UNLESS the cam mfg.
suggested otherwise . . . .
Some advice from experience:AA/ For a typical OHV engine, (.020"/.025" recommended lash), you can typically run .004"/.006" tighter than the cam mfg. states. If your engine picks up power with less
lash, you
probably need more cam.
BB/ For a typical OHC engine, (.008"/.012" recommended lash), you can typically run .002"/.003" tighter than the cam mfg. states. Comment as per above.
CC/ For either engine type, if your engine picks up power with a lash setting that exceeds the mfg's recommendation, you
probably need less cam.
DD/ Running with excessive lash will typically beat your valve train to death from the high loads imposed . . . . . .
EE/ Take the time to ask your cam grinder how much clearance ramp is ground into your cam, so you can calculate intelligent choices . . . . .
If you are not familiar with camshaft terminology refer to this site:
http://www.camtechniques.com/Pages/term.html Fordboy