History repeats itself. The turbo car is broken again and will be for a while..

The roller conversion itself doesn't change the pushrod length, or geometry, and doesn't come with pushrods. That would have to part of the assembly/mockup phase of the build, like any other. I think Jay just used stock length pushrods because the heads and block aren't significantly milled. Jay, is that right?

 

Whoops, that was in response to mhgs. Jim, the original holes are used, but that wouldn't cause any more variance than the stock setup. I think that while the castings vary quite a bit, the holes are coordinate drilled so as to have a consistent spacing. Any GM experts around to account for manufacturing methods?

If they aren't coordinate machined then you might be right on the money. With the holes in the shafts a fixed spacing, and the spacing in the heads varied, it could stress the bolt(s).

They would have to be off quite a bit though, or it wouldn't it be obvious on assembly?

 

The heads were slightly milled flat I would guess about .020 to .030 compared to a untouched set I have..

Hard to guess runtime.. Two weekends prior I drove 30 or so miles to the strip and made 8 passes and drove home. The shafts have been installed on the car for about 1.5 years and I drive it about every weekend..

 

Yes. I think that would be obvious as well..

 

Mark.. Do you think it would be safe to bend this back flat or not??

 

Yes the Plate itself wont change the geometry unless it is pushed either towards or away from the factory rocker center, which could be an issue....what will change it even more is if the stock base is either machined not enough or too much causing the need for a large change in pushrod lengths against the installed valve height

 

Where did you find the tensile strength?? They sure don't make it easy to contact them.

 

If you countersunk the holes so the bolt heads were flush, even a few thousandths misaligned would leave the head above flush, with the taper only dragging on one side of the hole. Plus, using the nylon lock screws, with a lot of drag going in, he may not have noticed a misaligned.

Hopefully we're getting closer to the answer.

Not an expert on GM's machining practices, but I would think they used a gang mill or drill press and drilled all the holes at the same time unless it was an NC or tape machine. They had millions(!!!) of holes to drill.

 

Man this takes awhile typing all this on a smartphone. :eek2: That's why my replies are a few messages behind.

 

heck...looking back at the pics, if you are using " old factory " pushrods the must look like hell aginst those guide plates by now.

 

http://www.astm.org/Standards/F835.htm.

Here's some info. Looks like 122 to 150 ksi.

 

Nope.. Look fine..

 

Here are some pics of the heads assembled when I first put it in the car..

It broke right on the driver's front..

 

The plates position the center of the rocker where the stock shaft would have been (as in toward or away from the valves). The base is not machined in any way that affects the location of the rocker arm. The plate could sit 1/4" higher or lower and have no effect on where the center of the rocker ends up, as the rockers are adjusted in the center. Their location is only determined by the valve tip, and pushrod length.

 

in the first pic of this last batch it looks like the plate is rolled towards the valve stems ( might be a picture flaw ? ). and it sure looks to me like those pushrods are too short .

 

What you're seeing is by design. Both SBB and BBB pedestals and hold down holes are not perpendicular to the valve cover rail when viewed from the end of the head.



Devon

 

Jay,if you find yourself wanting to go back to a regular set up while you try to fix this set of heads,I have a set of heads that I got on a trade a few years back that the owner said had been to the machine shop.If interested let me know,you can have them for $150.00.

 

That shold get me to the Turbo Buick Nationals in a few weeks. I'll PM you.

 

There is an interesting article in the June 2011 Car Craft magazine called "The Truth about turbo camshafts". In this article, they say the the typical cause of valve train failure in a Turbo pushrod engine is because the cam is trying to open the exhaust valve too soon. Because the cylinder is heavily pressurized, the force needed to open the exhaust valve it too great. This could easily explain your type of failure.

You could achieve a later exhaust valve opening event by either retarding the existing camshaft or have a cam ground on a narrower lobe center or both. Check out the article. I would degree your camshaft and discuss this with the Comp Cams tech department and compare your camshaft to the cam timing of some popular Buick V6 turbo cams for validation.

 

If you are sure of the cam, any chance random ignition events(arc'ing?) causing a spike in cylinder pressure creating the situation that speedtigger describes?