Breaking rocker arms

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Tommy_v
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Joined: Mon Nov 28, 2005 8:56 am

Breaking rocker arms

Post by Tommy_v » Sat Aug 26, 2017 2:04 am

Hi everyone,
After a bit of advice if anyone may be able to help, so about 12monts ago I broke a rocker on inlet #4 initially I thought it had happened because the lashcap may have come off somehow and the pushrods become in contact with the rocker itself not the locating cup. Anyway thought no more of it and replaced the rocker and upgraded pushrods to manton series 5 and all seemed fine.... untill last week when the rocker on inlet #3 did pretty much exactly the same. 12months seems along time but the car has only covered probably 3-400 miles. I have checked for coilbind and there seems to be plenty of clearance on both inner and outer springs and there Is nothing contacting through the stroke of the arm. Did i just get unlucky and get a bad batch of forgings? I only just remembered after looking at the pics but on one of them you can see where it looks like the oil hole has broken through the side just next to the adjusting screw- this was the one that let go last week.
Engine spec is 2276 turbo efi. Street eliminator heads. Vw650 springs. Fk8 cam, manton series 5 pushrods, cb forged 1:4 rockers.


This is first rocker that let go
IMG_7303.jpg
This is pic I took at time of first rocker breaking showing small hole in forging.
IMG_7305.jpg
This is latest breakage. Note this is the above rocker with hole in it
IMG_7306.jpg
These are latest snappage
IMG_7310.jpg
IMG_7310.jpg
IMG_7309.jpg
IMG_7310.jpg
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66brm
Posts: 192
Joined: Mon Apr 12, 2010 8:55 pm
Location: Perth, Western Australia, The Southern Hemisphere

Re: Breaking rocker arms

Post by 66brm » Sat Aug 26, 2017 6:15 am

I would show these pics to cb, you should get some replacement rockers for nada. Definitely a machining issue causing a weakness there and also a lubrication problem for the rocker

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FJCamper
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Re: Breaking rocker arms

Post by FJCamper » Sat Aug 26, 2017 8:24 am

First thing to check is oil delivery through the pushrod. Even a few seconds of no oil will seize the rocker.

A floating bit of sealant can block a pushrod.

If the pushrod checks out okay -- free flowing -- then it is probably exactly what it looks like, a badly machined rocker.

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FJCamper
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Re: Breaking rocker arms

Post by FJCamper » Sat Aug 26, 2017 8:24 am

First thing to check is oil delivery through the pushrod. Even a few seconds of no oil will seize the rocker.

A floating bit of sealant can block a pushrod.

If the pushrod checks out okay -- free flowing -- then it is probably exactly what it looks like, a badly machined rocker.

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Jadewombat
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Location: Houston, TX

Re: Breaking rocker arms

Post by Jadewombat » Sat Aug 26, 2017 5:33 pm

Try giving it a little more shim clearance also. My friend's ghia was OK in low revs, when he stomped it and brought it up past 5,000 we had a problem of the rockers coming off though. We shimmed it a hair more and haven't had a problem since then.

66brm
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Location: Perth, Western Australia, The Southern Hemisphere

Re: Breaking rocker arms

Post by 66brm » Sun Aug 27, 2017 5:34 am

All the side clearance in the world won't save a rocker with all its oil pumping into the rocker cover rather than the bushing

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FJCamper
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Re: Breaking rocker arms

Post by FJCamper » Sun Aug 27, 2017 12:23 pm

I have seen shallow spiral cuts machined on rocker shafts (old school tricks) intended to aid oil flow. I have no direct knowledge if this weakened the shafts or not. The spiral cut guys also cut (notched, really) lifters to increase oiling.

Some even drilled the engine case main oil gallery larger. I was a bit skeptical about that, because the case top oil in/out holes for the cooler are the ultimate bottleneck in the VW oil system.

FJC

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4agedub
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Location: Johannesburg, South Africa

Re: Breaking rocker arms

Post by 4agedub » Mon Aug 28, 2017 1:21 am

Question is, do you actually need the CB 650 springs for your application? These springs add loads of stress to the valve train.

I tried the CB 650 springs years ago on my circuit racer (my old springs were deemed too soft by the professionals for 8000rpm), first event out it broke a rocker. Changed back to the normal CB dual springs and never looked back.
Superbug Racing Team
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Bruce2
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Joined: Sat Oct 13, 2001 3:01 am

Re: Breaking rocker arms

Post by Bruce2 » Tue Aug 29, 2017 4:49 pm

I bet that when you pull the head off, you find a bent valve and collision damage to the piston.

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