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 Post subject: Rockers aint rockers
PostPosted: Mon Jan 17, 2011 11:49 am 
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Joined: Mon Jan 17, 2011 3:32 am
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Location: Sydney, Oz
OK, so this is one of those, "Bugger me, I didn't know that...", moments.

When I was rebuilding/resurecting my 650 pantah about 7 years ago, there were a couple of spares in a box.. I didn't pay much attention to them till it came to the point of doing the heads.

One of the rockers already in the head was a bit scuffed, nothing nasty, but I thought I'd like to do better. So I looked in the spares box and found another opener that looked very nice. Then I looked closer.. damned thing was completely different.

I had a chat with my Guru, Gowie, and he taught me the very basics that he thought every young, impressionable, naive, ignorant, clueless, "wouldn't know shit from shinola", recent desmos dromos convert should know. And I was all those things. My lucky day!

The forward exhaust (real) pantahs came with two (eventualy three) different types of rockers.

The first type are a full width rocker, it fits from one side of the cambox to the other and is shimmed for axial play. The closer is easily identified with it's club foot that has a wide notch machined on the underside for valve seal/guide clearance. I guess that since this is the first type made/used, it would need to be called 'normal'. This set is visible at the top of the image. They are perfectly suitable for road work where maximum revs are not held for extended periods on standard heads.

The secont type is the 'racing' rocker, it also fits one side of the cambox to the other and is shimmed for axial play. It was sold under the NCR banner as a 'racing' replacement part (NCR was a 'front' for the factory race shop of course, but we can discuss that bit later if you like). Below you can see the 'beefier' set in the bottom of the image. The 'spine' of the opening rocker, and of the closing rocker, is much taller. The curve of the arm to the cam lobe pad and the valve tip is also more pronounced. The fork of the closer is abreviated/re-angled and is not machined like the 'normal' unit for guide clearance. Overall these rockers have a taller cross section.

Obviously the engineer(s) designed this shape for a much stronger rocker when heavier valves or higher revs, or both, were employed for 'go-fast' work, i.e. racing. I can't pass comment on the mass differences of the two types, I've not weighed them.

I mentioned that both the above were full width rockers, which brings me to the third type. The clipped opening rocker. In discussing this below I'm only referring to flat out, heavy load racing situations. I have not heard of any rocker or pin failures in normal road use... (anyone?)

This type was introduced in the mid to late 1980's and should also be considered a road use only rocker. As it was explained to me, once the rocker is no longer 'full width', that is, the clip system is used to ease opening shim servicing, weakness is introduced in the rocker/pin assembly.

When a full width rocker is asked to open a valve the force (or strain) generated - that's the resistance of the opening rocker, the valve and the closing rocker, to being accelerated - is carried by the openers' pin. That load manifests as a shear moment, vertically, in the space between the ends of the rocker and the wall of the cambox. The pin is very strong in 'shear'.

Now use a clipped rocker, half of its pin support is a bit of pressed tin. When it is asked by the cam lobe to open the valve, the same force now manifests as a flex moment. The area available to flex is around 15- 20mm (I've never measured it). The pin is not strong in 'flex' and the flex moment can occur 4 or more thousand times a second.

Going racing with high revs and big valves? Which set of rockers do you want in your heads?

As usual, feel completely free to correct me if you think I'm way off the mark.

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Garry.

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 Post subject: Re: Rockers aint rockers
PostPosted: Mon Jan 17, 2011 9:22 pm 
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Joined: Sun Nov 07, 2010 10:54 pm
Posts: 425
Location: Brisbane
There goes the good nights sleep tonight !

What's shinola?


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 Post subject: Re: Rockers aint rockers
PostPosted: Mon Jan 17, 2011 11:26 pm 
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Joined: Mon Oct 04, 2010 7:47 am
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Garry,
That is correct that there are differences in some of the rockers, but having used all three different types in racing applications for the last 10 years the rockers design is not that critical. If the heads are set-up proper with rockers in good condition and the cams that are used don't have interference in their clearences then any of the rockers will work fine. If you have cams that have a great deal of clearence changes per revolution such as some of the Vee Two cams, you will first see issues with broken half rings. One thing I have found to be very important on the rockers is not only the condition of the cam contact surface but on the closer rocker you want to make sure that were the closing shim rides on the rocker that the surface is in good condition and no grooves worn into it.

I sure like the newer style rockers from the 900s with the clips and have never had any problems. I have broken a closer which was the heavy duty type, luckily it was the last corner on the last lap and I had the closer springs still installed.

Palmer


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