BROKEN ROCKER ARMS - Why??

ken180 · 02-16-2008, 10:31 PM

Hi All,

914-6 2.0 Stock motor - was not in car when I got it, so I have no history on it.

Opened it up and found the Exhaust Rocker Arms on # 4, 5 & 1, 3 are broken.

Checked the following:

Using a good rocker arm and setting #5 at TDC I turned the valve adjusting screw in 2 turns -.040"- and rotated the crank. The piston hit the valve during the rotation, had to turn the adjuster out about ¾ of a turn to get clearance. So the valve clearance is not desired .04" or better.

Checked CAM overlaps - 2.5mm which is Spec for this motor.

Pulled the heads and can see where the valve hit the piston.

Checked the Deck Height – solder method - and measured .106" between the exhaust valve and piston.

This engine had many miles on before it broke, so under normal operation .030" +/- valve clearance was probably enough.

Questions

With that deck height why is there not enough valve to piston clearance?

What could have happened to break 4 rocker arms?

What do I do to see it does not happen again?

Ken

Steve@Rennsport · 02-16-2008, 11:52 PM

Assuming that:

1) Piston to valve clearances are truly sufficient

2) Correct pistons are in the motor for the cams in use

3) Deck height is at spec

4) No broken valve springs

Valve float is usually the culprit for broken rockers (massive overrevs)

ken180 · 02-17-2008, 11:49 AM

The motor appears to have never taken apart.

No other broken parts.

I thought the deck height of .106" was a bit much. Is there a spec published for it??

I am going to add cylinder gasket spacers to get the valve clearence up to .040".

Ken

Steve@Rennsport · 10:29 PM

Ken,

Deck height in 911 motors should be .040 and thats accomplished using base gaskets. (thanks Andy!)

Valve adjustments are made at the rocker arm adjusting screws.

You might review your engine books.

Eagledriver · 02-17-2008, 05:57 PM

Steve,

I think you mean .040 (1mm)

-Andy

Ken,

The deck height is not the distance between the valve and piston. As Steve says your engine was most likely over-revved.

-Andy

cfc · 02-17-2008, 06:27 PM

those arms were designed to break if the engine was over reved.

cstreit · cstreit's Garage

...just in case it wasn't stated (haha) an overrev. The exhaust rockers often break when this happens, which is actually a good thing, helps prevent worse damage.

ken180 · 02-17-2008, 10:40 PM

Well I am glad the consensus is over rev. That means there should not be an undiagnosed problem when I rebuild it.
Regarding Deck Height, I used the "crush solder" method as the pistons are domed. There is a small flat spot at the edge of the piston - is that were I should check it?? I know on un-domed piston to check at the edge.

Steve - ment to say use base shims to set "Valve to Piston" clearence.

Ken

Plavan · Plavan's Garage

Just throwing this out there, probably not your case. We saw some rockers break from badly worn cams/rockers. This was due to the recent crappy oil formulation changes.

304065 · 02-18-2008, 08:37 AM

Quote:

Originally Posted by ken180

Well I am glad the consensus is over rev. That means there should not be an undiagnosed problem when I rebuild it.
Regarding Deck Height, I used the "crush solder" method as the pistons are domed. There is a small flat spot at the edge of the piston - is that were I should check it?? I know on un-domed piston to check at the edge.

Steve - ment to say use base shims to set "Valve to Piston" clearence.

Ken

Ken, even though we all might conclude that the engine was over-revved, that doesn't mean it was, or that you shouldn't check the other stuff.

ken180 · 02-18-2008, 09:29 AM

Thank John - Looking carefully as I tear it down.

Ken

304065 · 02-19-2008, 02:29 AM

Ken, post pics if you can of what the components look like-- it's entirely possible that the FOUR broken rockers are due to interference of some kind-- someone tried to use "S" cams with "T" pistons resulting in contact. We can in short order give you a sense of what is in there.

ken180 · 02-20-2008, 03:09 PM

Here is a picture of one arm. The other 3 broke at the same place. Engine is a stock 914-6 2.0.

Ken

forcerecon44 · forcerecon44's Garage

i have the same problem as ken 180 a broken top on my rocker adjuster just one on the left side rear of engine its perfectly split ! can i jb weld it back together ??? its a clean split ??? ??? what size allan keys to remove the rocker arm and i can remove it and put in a new one without removeing cam ???? please advise thanks

HawgRyder · HawgRyder's Garage

Are you joking about the JB weld?
You do realise the rockers get about 250# or more pressure ...right?
Get new rockers....and maybe springs too....don't cheap out and have to do a complete rebuild later.
Bob