milled head/milled pushrods?

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Feb 25, 2008
Hello to all. I am reviewing my Winter project list for my 70 fastback. One possible issue involves the cylinder head. I had the head shaved .030 during an overhaul. I asked the machine shop to take off .030 from the pushrods. The caps proved to be reluctant to remove and I settled for the answer that leaving the pushrods alone only slightly changed the geometry of the valve train and was not really necessary.

Was this wise?
How do you remove the pushrod caps without scoring them?

Thanks, Mike 8)
Mike, considering Norton did nothing when they milled the Combat head 040" you will get away with it but the rocker contact geometry will change. It could change for the better, depends where on the valves the rockers originally contacted.
To remove the rod ends you need to use a little heat and they should twist off but it does mean a soft jawed vice for the main tube and a vise grip on the ends, just tight enough to grip but not so tight that the grip tightens on the pushrod end! Any marks can be polished out after assy. Use a medium bearing fit adhesive or threadlock when replacing.
Thanks. I will look at the contact point with the valves but it will probably be fine. I suspected that heat was involved in the end cap removal process. It usually is!
Mike :wink:
I had 40 thou taken off the cam followers as they were worn anyway as the fix on my Combat.
Thanks for the response. I had not considered the followers dimensions. I have installed a Megacycle 560 NR cam and had the original followers resurfaced by Megacycle also. That might have given me a little more clearance. Anyway the bike runs great and there is probably no sense in fixing something that is not broke :wink:
The issue with leaving the pushrods too long is rapid valve guide wear due to side loading of the valve caused by the altered rocker geometry. Combat heads were shaved .040" and that was one of the problems they were notorious for.

Hi Deb. Valve guide wear! Not good! Maybe I should pull the pushrods this Winter and find a way to remove some metal. Don @Acme told me that he could not get the ends off! No signs of guide wear yet but that could be anytime.
Thanks, I will add it to my list.
Removing a fixed amount equal to the head mill is only helpful if:
1. this is the only modification
2. the geometry was perfect before (no wear, aftermarket replacement parts, seat recession)

If the cam is changed, in theory the pushrod length should be shortened (in addition to any correction made to cylinder head or barrel height) by 1/2 of the added lobe height (not lift).
Example only:
Original lobe .350"
New lobe .400"
Difference .050"
Shorten pushrod .025"

However, this is only true if the base circle (zero lift) is unchanged.
Since the cam manufacturers, when asked "do you reduce the base circle by 1/2 of the added lobe height to correct the geometry?" answer (quoting, literally) "................................................ what?" you won't know what you have unless you either mike the new cam for both base circle and lobe height and compare it to an NOS or perfect cam, or (better) assemble the engine with the new parts and analyze the geometry.
The welded hardface cams such as Norris and Web-Cam probably use the stock base circle - but I don't know this as fact, it's just more likely.
Reground cams (no weld) cannot use the stock base circle, because the new lobe profile and height are produced by removing metal from the base circle - there's no other way. A change of duration, lift, centerline or any combination of these will always reduce the base circle diameter.

Warning: analyzing the geometry is one of the most misunderstood, badly diagnosed problems on the net. Everybody and his dog thinks it's simple to understand, the original factory stuff was fine, and that it can all be handled with adjustments and/or pushrod length.
Wrong, wrong, wrong.
Rather than give you 10,000 words (no, I'm not kidding) if you assemble the engine some of the easiest things to look for are:
1. adjuster runs off the tip (duh)
2. can't get an adjustment
3. angle for valve closed (lift = 0) is completely different from valve fully open (lift = 100%)
4. the adjuster only moves in one direction during the lift
5. adjuster does not pass through the center of the stem tip during lift

The "correct" geometry is perceived to be when a(n) (imaginary) line drawn between the adjuster contact point and the rocker shaft centerline is at 90° with the valve stem axis at 50% lift. This is called the "mid-lift" or "Miller" method, and it's not the only method, not necessarily the best method, but it works and is the easiest to plan and measure.
If this is correct, the adjuster's "scrub path" or what pattern it makes on the stem during lift, will start slightly inboard (closer to the shaft) of the valve stem tip center at 0 lift, walk across (away from the shaft) until 1/2 lift, then walk back. If it does not do this, it's wrong.
The positions need not be centered (far less important, and cannot be cured by adjustment anyway), but stopping and reversing at mid-way is key.
If you can't actually measure this, anything that shortens the length of the scrub path is good.
You can simulate a shorter pushrod by shimming the rocker box up, use the smallest increment shims your patience will stand (.005" is a waste of time, .060" is too big).
You can simulate a longer pushrod by placing a small piece of metal scrap between the tappet and pushrod end, or pushrod and rocker end. I suggest aluminum, since you can easily dent it with a punch to match the curvature. Cut a small circle, then saw a slit from 1 side to the center to make it easier to dent.
Hi Panic.
it was nice to hear that, cause when I received a pair of Johnson's cams(360 &380), I had ordered as well a pair of steel pushrods, and when received they were a bit longer than the Cdo std ones , say approx same lenght as Atlas ones , after asking to the supplier , he replied that they should be OK and many customers used them into Norton with that kind of cams , and added that they even had slightly longer ones , so as I supposed as you said they are working by grinding the base , that should be the reason , of longer pushies??? however I know now I must used the "Miller " way of checking when installing them.
it's a long way before knowing all the tricks, and hopefully this forum is a great source of help, thanks for beeing there.......Pierre
It's a bit worse than that. With a regrind, there are 2 compensations working in opposite directions.
If (round number, not actual data, example only) the original lobe is .400", to increase the lobe height to .450" (with no other changes) the base circle must be reduced by the difference in lobe heights: .050". If the profile is changed, even more reduction is needed.
The base circle reduction alone requires an equal increase in pushrod length: +.050".
The added lobe height requires a reduction in pushrod length equal to 1/2 of the change in lobe height: -.025".
The net change is the total of both factors, in this case +.025".
It's important to recognize that some geometry is controlled by factors designed in to the engine, and are not amenable to adjustment by adjustment, shimming, or simple machining.
The rocker arm itself is designed for a specific lobe height and lift (not merely rocker ratio). The 2 levers (arms) are at an angle ("Delta" = difference) to each other:
1. the pushrod lever is horizontal at 50% lobe height if the tappets and pushrods are exactly vertical - some aren't (Triumph is slightly inclined).
2. the valve lever is inclined at the valve stem angle (26°, etc.) at 50% lift (lobe height × rocker ratio × .5).
As you can see, if the lobe height changes the corrected pushrod length will bring the pushrod lever back to "normal", but the valve lever cannot be corrected, since the Delta angle is changed (it's only neutral if the ratio is 1:1, and the error is magnified by larger rocker ratios).
This means that in theory a new rocker arm, with a new Delta angle, is needed for any significant change in lobe height - although this is never done (high-end NASCAR items made by Jesel etc. may be closer to correct).
Yes, re-angling the valve stem axis (Dunstall) also changes the geometry.

In summary: avoid any simple explanations or suggestions on how these parts function. There's more, but I don't want your head to explode.
panic said:
In summary: avoid any simple explanations or suggestions on how these parts function. There's more, but I don't want your head to explode.

I can appreciate that.
I do want to thank you as you have made this easy to understand.
It is obvious to me these are the things that turn a good motor into a great one and a great one into a race winning one.
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