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Old 01-10-2013, 01:46 PM   #14
Max Headroom
lost in the '70s
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Joined: Feb 2008
Location: New Zealand
Oddometer: 521
Originally Posted by supershaft View Post
Obviously? Where? The 336 bulletin I just read has specs at a 2mm (I was remembering 1mm earlier) lift check point "on the rocker arm". Is 2mm another typo? Why would that be on the valve side of the rocker arm when rocker arm lift ratio will completely change any reading? Our rocker arm design is notorious for having inconsistent lift ratios. Most all manufacturers list cam specs at the crank and not valve lift specs at the crank for that reason. Plus, you can always run different ratio arms on purpose! Valve lift specs vary. The cam lobes don't.
Billy, you've missed the clues given by every engine manufacturer for generations including BMW, when they give cam specs. The usual information given is valve timing specs, along with total valve lift. The term "cam timing" refers to the drive from the crankshaft to the cam, and the relationship between the two.

The cam is primarily there to open and close valves at the pre-determined times, when it's not operating an oil pump or ignition system. This data is based on standard rocker arms, for one thing. A change to the rocker ratio is outside the parameters of factory timing specs.

All cams have specs arrived at by using a degree wheel on the crank to determine the crankshaft position when the inlet valve opens and closes, and when the exhaust valve opens and closes. NOT when the rocker arm, pushrod or cam follower moves.

The 2mm checkpoint is partly to get the cam lobes away from the opening and closing ramps, and partly to avoid valves tangling with pistons.

Valve timing has everything to do with the opening and closing of the valves relative to the crankshaft, therefore it is critical to know the correct check clearance (2mm in the case of an airhead) and the exact crank angle when the valves open and close.

Correct me if I'm wrong but it's my understanding that an increase in the rocker arm ratio would affect valve lift rather than valve timing, using the same check clearance. Any change in valve timing would be minimal..

YMMV etc
'76 R90S silver smoke
'75 R90/6 hybrid

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