exhaust valve clearance ?

[Concrete-crusher]

hi what is the correct exhaust valve clearance. The manual quotes two sets of values depending on the engine number , but my engine number does not resemble the range quoted. (122E55403)

Also in Miles Wilkins book only one range is quoted 0.009 - 0.011 , my engine has the earlier cam cover and earlier front plate with only tdc 10 & 20 degree's

so I'm thinking 0.010 as a target , im currently at 0.003 - 0.005 so way off.

does it sound like i'm correct.

Thanks Steve

[mbell]

In my opinion you are correct. The lower numbers are for very early engines and in almost all cases is probably safe to ignore them.

Give those clearance on the exhaust valves I'd recommend you do adjust them fairly quickly.

[RogerFrench]

10 thou is fine, + or - 1 thou.

[Quart Meg Miles]

10 thou is fine, + or - 1 thou.

The earlier exhaust valves had a lower expansion coefficient so didn't need as much clearance. It won't do any harm even if you have the earlier type and set the wider gap.

[denicholls2]

If you consider what is going on inside the combustion chamber, you can make your settings accordingly. Think of the gap as determining how open (and more importantly, how closed) the valves get.

A wider gap will result in lower maximum lift. Which will affect horsepower, but not in a major way. If you are good at math, you can determine how much the airflow is restricted by the lower lift amount. Since the Twincam has directly actuated valves, the gap translates directly to lower lift in a 1:1 ratio. Rocker-actuated valves can have different lift ratios.

A smaller gap has the corollary of higher lift but with some attendant side-effects engines do not like: The valve spends marginally less time on its seat, which is how it gets cooled. When the engine is hot (Quart Meg Miles' expansion coefficient), a tight clearance may also mean the valve never fully closes. At this point, the power advantage of tight clearances gained by lift is more than negated because of the lost compression.

The biggest drawback is that tight clearances translate to burned valves. Which is why most people err towards loose. Loose clearances can make the valvetrain noisier, and at the extreme can bypass the gentle initial contact ramping that is part of the profile of the cam, causing more wear.

Reground cams get there by decreasing the base circle, magnifying the variations in transition of the cam profile. So it is more important to get these close than with the original base circle. (They will wear faster with the same loose clearances).

The correct gap also varies by expected average operating temperature. A racing engine is going to run hotter than a street engine in normal use, so needs more cold clearance to ensure the valves close. Which works against the power advantage of tight clearances.