Reground Cam Rocker Arm Geometry

[glemon]

I am rebuilding the motor of my 100 BN1, I had the cam reground to M spec, neither the cam shop nor my machinist mentioned this, but the our local sports car mechanic/guru said I should do something about the rocker arm geometry change.

Should I?, I assume shaving the pedastals a hair would be the best way to put the geometry back to where it had been before.

 

[Dave Russell]

On a reground cam, the lobe diameter on it's base circle must be reduced to get the changed timing & lift.

Since the engine has adjusters on the push rod end of the rocker arms, there "should" be enough adjustment range to compensate for the material that was ground from the cam. If so, nothing else needs to be done.

If there is not enough adjustment in the push rods, longer ones would be in order. Changing rocker stand height to lower the rockers would screw up the rocker arm to valve stem geometry.

You absolutely should also use properly hardened new or refaced tappets on the new cam.

I would not install anything until a good machine shop has checked things for proper hardness, correct crown radius on the new tappets, & correct taper on the cam lobes. Both the cam & tappets should be hardness checked for RC 55 or higher.

Check previous posts on cam break in & oils. It's surprisingly easy to have something go seriously wrong if everything is not perfect.
D

 

[glemon]

Thanks, the lifters were refaced as well by Delta Cams, and I have read a ton of stuff on oil, ZDDP, etc, I am going to break in using machinists suggestion of straight 30W to which I am going to ad GM EOS.

Yes there is enough adjustment in the rocker arms, I also got the same advice from my sports car nut brother so will just do a normal valve adjustment, I have also liberally lubed the cam and lifters with cam specific lube.

 

[GregW]

I also got the same advice from my sports car nut brother

Are you twins? /bcforum/images/%%GRAEMLIN_URL%%/jester.gif

 

[glemon]

I also got the same advice from my sports car nut brother

Are you twins? /bcforum/images/%%GRAEMLIN_URL%%/jester.gif

No, I am an enthusiast, my brother, who in addition to having three LBCs, has 3 BMWs, and a 911, and was trying to talk me into going to Elkhart Lake to catch the vintage races Sunday (at 7:00 P.M. Saturday and it is 600 miles away) is a nut.

P.S. Is every other guy on this forum named Greg???

Greg L.

 

[piman]

Hello Dave,

this is an interesting point.
Going on mechanical principles, levers such as a rockers should be at 90 degrees to the valve stem at half lift to give the most accurate application of the cam. So if the total lift is raised that situation will no longer be true.
I don't know if cam designers take this factor into account in their design, or whether it is academic anyway and not significant from a performance point of view. There has to be a certain deviation in normal circumstances as valves and seats wear (and re-ground to compensate for that)

I know of a U.K. Triumph 2.5 specialist (Chris Witor) who sells a cam with different lifts for the inlet and the exhaust and he also sells a longer stem inlet valve to compensate as the rocker tip tends to go too close to the edge of the valve stem on full lift.

Alec

 

[GregW]

Ah yes, I see the distinction. Doubt I would have been talked into a journey like that. There does seem to be a lot of Gregs here, much more than Team.net.

 

[MarkA]

Not knowing how or who reground your cam, you might want to find out if they first built the cam lobes up. Some cam grinders will build up the cam by welding additional material to the lobes before the grinding. This keeps the back side of the lobes at their original height or close to it. This is not to say that your geometry will be correct, but it may be as it was stock. As Dave pointed out, if you are trying to get better geometry, you can change the length of the pushrods to get better angles without a lot of major work to the other components.

With all that said, it is probably best to talk with the cam grinder to get his opinion/advice. He should know...

Mark

 

[Dave Russell]

Hello Dave,
Going on mechanical principles, levers such as a rockers should be at 90 degrees to the valve stem at half lift to give the most accurate application of the cam. So if the total lift is raised that situation will no longer be true.
Alec

Hi Alec,
Very true. The original 100M cam grind limited the valve lift to around .420", an increase of .030" over the original standard cam. The milder after market Healey Four cam grinds tend to limit valve lift to not much more, or even a little less lift than the stock cams.

If the lift IS increased very much, the block must be relieved for valve head clearance. Also the rocker arm pivot might need to be raised or valve stem lengths changed.

Since the cam being discussed makes no mention of needing valve pockets in the block, & it is stated as a "mild" grind, it's my belief that the cam does not have enough lift change over the original to require valve/rocker geometry changes from original.
D

 

[glemon]

It was ground down on the back to get increased lift, not built up, made to M spec. as I think about it I don't remember any changes to pushrods or rocker geometry with the Le Mans conversion, not that time and knowledge can't march on.

 

[piman]

Hello Dave,

my thought was of a general nature, rather than a specific in this instance. I can imagine if a racing engine was being built that such detail would become more relevant.
I know that there is a commonly perceived idea that a skimmed head requires shims under the rocker pedestals but that is a different matter and erroneous.

Alec

 

[Dave Russell]

Hi Alec,
As you say, raising or lowering the head has no actual effect on rocker arm geometry. It would only affect the length of the push rod needed & then only if the adjuster were on the valve stem end & not on the pushrod end. I think the attached pic pretty well shows what is needed for correct rocker arm geometry.
D