Rocker arm geometry [angles] ?
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  1. #1
    1000+ Posts Wildebeest's Avatar
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    Default Rocker arm geometry [angles] ?

    In discussion with a race car owner we had different ideas about how the rocker angles change when an engine is fitted with a modified camshaft. My opinion [not theory] was that it does and something has to be done to compensate. Obviously the extra lift of the cam will affect the travel [arc] of the rocker but does the geometry or 'purchase' of the rocker suffer?

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    When cylinder heads have been excessively machined I've always understood that to restore the rocker angle it is necessary to place spacer shims under the rocker shaft pedestals. The thickness of the shims being the same as the amount removed from the head.

    I put it to my race car owner friend that the same mods would need to be done to the engine with the modified cam.
    He dismissed my opinion as myth.

    I may be "building bridges where there are no rivers" with most of the above but this old head needs to get around it.

    Any race car engineers out there with pushrod race engine experience care to contribute?

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    1000+ Posts Frans's Avatar
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    Hi,

    Rocker arm geometry will be 100% ideal if: the valve is at 50% of its travel and at a right angle with the rocker shaft and the pushrod.

    When a cam is ground to give more lift, the base is machined away.( They will also cut a little to modify from the lead in and the ramp and maybe from the nose as well but that is for a specific character of the cam).

    When a head is skimmed the valves clearance will be smaller or too small if you leave the settings the same. When you add shims under the pedestal of the same amount the clearance will be back to normal with a standard cam. That theory is correct.

    But when a hot cam is installed at the same time the head is skimmed, the base of the cam will be ground away and the clearance might just be back to normal when assembled or you might need to tweak it a little without shims. I think then the geometry change will be negligible, or at least very close to ideal.

    Shaved head with standard cam you are right, shaved head with hot cam your friend is right.

    I hope I am right. Regards
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    1000+ Posts Fordman's Avatar
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    Quote Originally Posted by Wildebeest View Post
    When cylinder heads have been excessively machined I've always understood that to restore the rocker angle it is necessary to place spacer shims under the rocker shaft pedestals. The thickness of the shims being the same as the amount removed from the head.
    Here's something to help get the head around. This may seem out of step with other opinions, but food for thought.

    Talking typical pushrod engine, taking any amount of metal off the head surface makes no difference, because the tappet adjuster, or hydraulic lifter, is adjusted to suit. Think of it this way, the height of the top of the valve stem remains the same distance above the valve spring seat (head), and the centre of the rocker therefore remains the same height above the head to give the correct clearance (zero in the case of the hydraulic). The distance between the pushrod end of the rocker, and the "back of the cam" (ie, the combined length of the pushrod and follower) is reduced by the same amount as the amount machined from the head surface. Agreed? This is counteracted by unscrewing the mechanical adjuster (have you noticed that modified engines usually have a lot of thread sticking out of the locknut on the rocker - I mean the old tappet adjuster type). The hydraulic lifter will just close up, as long as it has enough travel. If it is getting towards the limit of travel (ie binding) then a shorter pushrod is required. Summary - if the camshaft hasn't changed, no change to the valve geometry by just machining the head.

    Now if you use a high lift cam then obviously the valve is pushed further down and the rocker travels through a greater arc (but the starting point is still the same!). It seems you can get away with this within reason, but to get the angle of the rocker at, say, half valve travel, to be similar to original, in fact you need to machine off the bottom of the pedestals, not put spacers in. IE, this lowers the pushrod end of the rocker down a little to get the angle sort of midway in its travel. Does that make sense?

    As they say - learn by your mistakes. I helped my bro-in-law rebuild a 5 litre V8 many years ago, including a mild cam. Murphy took control. The valve seats in the heads (cast iron, no inserts) were very recessed and we decided to put in new larger (GT) valves which allowed us the material to reinstate the seats with a good bit of protusion of the valves into the combustion chamber. Took off minimal material from the top of the seat then "throated" the port to get the narrow seat and increase the flow - looked beautiful, and we had plenty of meat then in reserve for a future seat cutting in years to come. Checked the hydraulic lifter clearance (in the collapsed state) and it was too much, requiring 1/16" longer pushrods which were available as standard parts. Engine running nicely, but first thrash around the block resulted in 2 bent pushrods! To cut the even longer story short, after about 6 bent pushrods had been replaced, we worked it out. Our good work in the valve seats had caused the top of the valve stem (where the rocker works on it) to be about 0.1" closer to the head surface (ie the valves were sitting lower). This caused the rocker to be angling down more than usual at the starting point, plus with the higher lift cam, at full lift the rocker angle was just too acute. The fix was to machine about 0.1" off the bottom of all the rocker pedestals (they were the individual pedestals with a fixed half-moon type pivot) which brought the back of the rocker down and we used the original shorter pushrods. Interesting that with the very common earlier indivdual rockers with the self-locking nut - you know, the ones where you wound them down 3/4 turn while running - this may not have been a problem - they are basically an adjustable pedestal.

    So many little quirks to think about, but I learned that the valve stem height (which is a given spec in the book) is very important.

    Good memories for me.
    Last edited by Fordman; 22nd June 2011 at 12:21 AM. Reason: Typo
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    Fellow Frogger! chez00's Avatar
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    For a reason that escapes me at the moment, I was instructed at trade school to check rocker geometry at 40% lift. at 40% lift the rocker arm should not pass the centreline of the valve tip (you make sure with bearing blue).

    I'll try to find the "why" in my text books if you're interested.

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    Quote Originally Posted by Fordman View Post
    Engine running nicely, but first thrash around the block resulted in 2 bent pushrods! To cut the even longer story short, after about 6 bent pushrods had been replaced, we worked it out. Our good work in the valve seats had caused the top of the valve stem (where the rocker works on it) to be about 0.1" closer to the head surface (ie the valves were sitting lower). This caused the rocker to be angling down more than usual at the starting point, plus with the higher lift cam, at full lift the rocker angle was just too acute. .
    hi, i have a couple of questions on this...
    first, why was it that it ran initially - i presume you mean it idled without bending the valves? - but did when driven at speed?

    second, i am not sure why the rocker angle was the problem. i would have thought that if the top of the valve was now sitting lower, the rocker would be rotating to a greater angle (as you said..), and that would mean it was pushing the valve less at the extremity of its movement. obviously you figured out and fixed the problem, so it is my visualisation of it at fault. if you can elaborate, that would be appreciated. or perhaps i am completely missing the point? apologies in advance; as you might note from my queries in Frans' thread, i dont have practical experience with these things.

    alexander.

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    Quote Originally Posted by alexander View Post
    hi, i have a couple of questions on this...
    first, why was it that it ran initially - i presume you mean it idled without bending the valves? - but did when driven at speed?

    second, i am not sure why the rocker angle was the problem. i would have thought that if the top of the valve was now sitting lower, the rocker would be rotating to a greater angle (as you said..), and that would mean it was pushing the valve less at the extremity of its movement. obviously you figured out and fixed the problem, so it is my visualisation of it at fault. if you can elaborate, that would be appreciated. or perhaps i am completely missing the point? apologies in advance; as you might note from my queries in Frans' thread, i dont have practical experience with these things.

    alexander.
    OK, its not easy to explain but I'll try using a clock face. Imagine the rocker to be the minute hand, and at 10 o'clock is the top of the valve. At full lift the valve is pushed down to 8 o'clock - nice movement with 9 being the central position. Also imagine a vertical line down through 9 - this is the valve stem.
    Now lower the valve so the top is at 9 - the valve movement is now from 9 to 7 o'clock and see how the rocker is now moving way off that vertical line (as the rocker tip moves to the edge of the valve stem).
    Now throw in a high lift cam and push the valve tip down to 6 o'clock - the angle gets ridiculous and the rocker is sliding off the valve tip. Ok, thats extreme, but hoping I made the point.

    Now, in my opinion, the perfect movement with a high lift cam would be from 10.30 to 7.30 on the clock face. Assuming the top of the valve is still at its standard height, then the only way to achieve this is to lower the pivot of the rocker - ie, machine off the pedestal.

    Putting spacers under the pedestals may be a visible solution to obtaining valve clearance with a high lift cam, but IMO it really should be done with shorter pushrods if there is not sufficient adjustment. And to actually get the angularity correct, to lower the rocker centreline, ie, shorten the pedestal.

    What actually bent the pushrods I am not sure, but yes, it was when the engine was revved higher - one quick squirt would bend the pushrods - no real damage but a very rattly engine! Maybe the ball end of the pushrod was binding in the ball socket of the rocker due to the extreme angle, or there was some dynamic load due to the angle at the valve tip.

    BTW, I think the clock face example also explains the reasoning in Chez00's post above, the imaginary vertical line should be in the centre of the movement of the rocker end across the valve tip. What Chez00 is saying makes perfect sense to me.

    Wildebeest, you might even remember the car I used to work on at Wanneroo in the early 70's - Graeme Ibbotson's clubman sports with a 1500 MkI Cortina pushrod engine. Over a couple of years we kept grinding out the combustion chambers for gas flow, and maching off the head surface to keep the compression ratio up. Then we started getting head bolts bottoming in the block, and realized we had by now machined 0.250" off the head face! (the combustion chambers were now like saucers). The only relevance of this to your post, is that we had 0.250" machined off the pushrods (which were solid) because there wasn't enough adjustment at the tappet adjuster on the rocker, and it compensated for the head machining.

    I'll have to stop now - I could talk about this all day!
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    Quote Originally Posted by Frans View Post
    Hi,

    Rocker arm geometry will be 100% ideal if: the valve is at 50% of its travel and at a right angle with the rocker shaft and the pushrod.

    When a cam is ground to give more lift, the base is machined away.( They will also cut a little to modify from the lead in and the ramp and maybe from the nose as well but that is for a specific character of the cam).

    When a head is skimmed the valves clearance will be smaller or too small if you leave the settings the same. When you add shims under the pedestal of the same amount the clearance will be back to normal with a standard cam. That theory is correct.

    But when a hot cam is installed at the same time the head is skimmed, the base of the cam will be ground away and the clearance might just be back to normal when assembled or you might need to tweak it a little without shims. I think then the geometry change will be negligible, or at least very close to ideal.
    Interesting points, and I agree, except I think the shims are only covering a problem which should be addressed in other ways, as the shims do actually make the angularity of the rocker worse but we get away with it most of the time.

    And your point of the cam grind compensating for the head machining is a good one - its the whole package which needs to be looked at.

    Also it is why it is considered good practice to use a cam machined from a NEW blank, rather than grind an existing cam which reduces the base diameter to get the higher lift, and this explains the difference in price between a "new" cam and a reground cam.
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    Quote Originally Posted by Fordman View Post

    What actually bent the pushrods I am not sure, but yes, it was when the engine was revved higher - one quick squirt would bend the pushrods - no real damage but a very rattly engine! Maybe the ball end of the pushrod was binding in the ball socket of the rocker due to the extreme angle, or there was some dynamic load due to the angle at the valve tip.

    y!
    many thanks, that was an excellent description and all is revealed: even though you said 'pushrods' i was thinking bent valves, when i read your initial post.

    ta.

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    1000+ Posts Wildebeest's Avatar
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    Fordman,
    Thanks for contributing to this thread. With a bit more time I'll fully mull over your earlier posts.

    The subject of pedestal shimming. I'd been led to understand that with extreme head machining it restores the rocker to its original "at rest" angle. The rocker angle and lift remains the same.
    This being the point at which the manufacturer originally designed it.

    Your comment on the cam grinding plus head machining package does require further thought.

    The subject of a cam being machined from a billet is preferable to grinding the original cam. My race car owner agreed with this after finding his cam had considerable machining from the cam base along with relieving the surrounding material.

    Certainly Graham Ibbotson's name is familiar. My connection with motor sport apart from local speedway ended in the mid '60's at Caversham.

    Franz. Alexander and ChezOO have been appreciated and absorbed
    also. Keep them coming.

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    1000+ Posts Beano's Avatar
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    Quote Originally Posted by Wildebeest View Post
    Fordman,
    My connection with motor sport apart from local speedway ended in the mid '60's at Caversham.


    Not against a wall, I hope

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    1000+ Posts Fordman's Avatar
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    Quote Originally Posted by Wildebeest View Post
    The subject of pedestal shimming. I'd been led to understand that with extreme head machining it restores the rocker to its original "at rest" angle. The rocker angle and lift remains the same.
    This being the point at which the manufacturer originally designed it.

    Certainly Graham Ibbotson's name is familiar. My connection with motor sport apart from local speedway ended in the mid '60's at Caversham.
    Re shimming: I've thought of another way to explain. Take (or imagine) a standard head with valves installed, sitting on the bench, removed from the engine. Install the rocker gear, pedestals bolted down and rockers touching the valve tips. Now imagine machining 3mm off the head gasket surface - does it change the relationship of the valve and rocker ? - not at all.

    Only when you bolt it back on the engine your pushrods need to be effectively 3mm shorter, and this is achieved by backing off the screw adjuster 3mm, or in case of hydraulic follower, compressing the piston in the follower by 3mm. Only if you run out of adjustment (or the hyd follower is going to bind up) then shorter pushrods are required - OR - you could shim the pedestals up say 1.5 mm (a 1:1 rocker would then give 3mm more at the pushrod end) but then you ARE changing the relationship of the valve/rocker, which is less desirable, and magnified if a high lift cam is used.

    Don't worry, it took me years to get my head around this, and only after really playing with things a bit, and stuffing up a few things I might add, because some of it is not obvious at first glance.

    Re Graeme Ibbotson (the late G I unfortunately), he did start racing his home built clubman sports at Caversham some years before I met him in 1970. I think racing moved to Wanneroo in 1969? Also he did have a bit of a fling at Claremont Speedway in the early-mid 70's, he was one of a few guys running Minis for a while. He was a bit of a character, and raced at speedway under the name Rick Shaw (true!!) because in those days you lost your CAMS licence if you ran at speedway, if I remember correctly. It was all a bit of a laugh for him, really.

    Cheers.
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    Quote Originally Posted by Wildebeest View Post
    The subject of a cam being machined from a billet is preferable to grinding the original cam.
    Only if you don't have the mechanical aptitude to correct the geometry from reducing the base circle. There's oodles of solutions out there for the Chev and Ford V8's. Just duplicate them. All that matters is what's happening at the tip of the valve. Whether you use cam profile, rocker ratio or whatever to achieve it, is up to the engine builder.
    Last edited by PeterT; 22nd June 2011 at 09:50 PM.

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    1000+ Posts Frans's Avatar
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    Hi All,

    It looks like Fordman is right. I did a quick dwg on graph paper and found that the geometry doesn't change at all. Interesting subject of you Wildebeest.

    I have made a schematic dwg of this issue and shows that nothing changes. Have a look and note that the pedestal and valve length and pushrod length remains the same in all 4 dwgs. When the head is shaved with 2 blocks all the components still remain the same length and the cam position doesn't change either. With the adjuster doing the job the clearance is achieved and the geometrie remains the same. This will go for the complete travel of the valve. See how the adjuster takes care of everything.

    The same goes for the bottom 2 dwgs with a hot cam. Maybe I have overlooked something?

    Comments?

    Regards
    Frans.
    Attached Thumbnails Attached Thumbnails Rocker arm geometry [angles] ?-geometry.jpg  
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    1000+ Posts Wildebeest's Avatar
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    Default Rocker geom...?

    Quote Originally Posted by Frans View Post
    Hi All,

    It looks like Fordman is right. I did a quick dwg on graph paper and found that the geometry doesn't change at all. Interesting subject of you Wildebeest.

    I have made a schematic dwg of this issue and shows that nothing changes. Have a look and note that the pedestal and valve length and pushrod length remains the same in all 4 dwgs. When the head is shaved with 2 blocks all the components still remain the same length and the cam position doesn't change either. With the adjuster doing the job the clearance is achieved and the geometrie remains the same. This will go for the complete travel of the valve. See how the adjuster takes care of everything.

    The same goes for the bottom 2 dwgs with a hot cam. Maybe I have overlooked something?

    Comments?

    Regards
    Frans.

    Frans. [With an S].
    Well, what a can of worms we have here
    I've studied your schematic, concentrating on the upper drawings of the valve gear in relation to std and shaved heads.
    The std layout is obvious but with a shaved head installed the now 'longer' pushrod is now angling the rocker to where it is bearing on the valve ie no clearance.
    Adjustment will of course restore the clearance but the rocker angle is now changed from its 'at rest' position. This where I keep banging on about the necessity of shims to achieve the status quo.
    I think that somewhere a rocker leverage advantage is lost without shimming, as distinct from rocker ratio.
    We may need to call in a leverage expert to contribute.

    A lot of my wild theories have been influenced by my copy of Phil Irving's book Automobile Engine Tuning, bought from Alberts Bookshop years ago. Remember them Fordman?
    It is claimed that I've been seen walking around like Moses and the Ten Commandments with Irving's book tucked under my arm

    The subject of rocker tip to valve contact has been spoken of previously, something not to be ignored.
    Last edited by Wildebeest; 23rd June 2011 at 01:33 PM. Reason: stuff

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    1000+ Posts Wildebeest's Avatar
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    Anyone out there?
    After making my opening post and taking in the 12-14 replies I will humbly accept all theories put forward.
    What an interesting week it has been, a thoroughly meaty subject for a change.

    Fordman,
    Checking back on the race results put up on Terry Walker's Site for the '70's I see where "G.I." had a few good results in the Jomax.

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    1000+ Posts Fordman's Avatar
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    Quote Originally Posted by Wildebeest View Post
    Fordman,
    Checking back on the race results put up on Terry Walker's Site for the '70's I see where "G.I." had a few good results in the Jomax.
    Thanks for that post, I haven't looked at that for a long time. I was helping GI in 1969-1970. I didn't realize we had so many DNF's! The Jomax was a GI home built clubman sports chassis, with running gear (including finned alloy drum brakes) from a Fiat 1100. Engine and driveline was Ford MkI Cortina 1500cc pushrod engine and gearbox, and I think it was a BMC rigid rear axle. Read into the lap times: at end of 1969 my mate and I basically refused to help him any more until he put disc brakes on it, and didnt see him for a couple of months. One day about 4 weeks before the 1970 season, he tore into my (Mum's) place with a boot full of discs and calipers from a wrecked Fiat 124 Sports. Enthusiasm regained, we machined up the hubs to fit the discs, made caliper mount brackets and dropped the lap times by about 3 secs first time out. At one stage he obtained a Hewland 5 speed gear set, which also dropped lap times when it didnt break down. The crowning glory was the 1970 Six Hour, where we weren't classified as finishers, but won the "hardest triers" trophy, after engine lost oil pressure after 45 mins, GI refused to give up, drove back to South Perth to get bunky standard engine from back yard, while we pulled the engine at the track. When he got back with engine, we put in the hot cam, head, etc, and got back into the race with about 45 mins to run, and he went berserk, putting up some of the fastest times he had ever done, which looked spectacular because at that stage everyone else was slowing down to last to the end. What a laugh! My Mum said she even sat down from doing her housework when she kept hearing the car's name on the TV, as it kept crossing to the pits showing us working on the car to get it going.

    cheers.
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