I have posted this question on other forums hoping to get something to work with. I don't know if there is a single definitive answer, but hoping to gather enough answers to learn something.

I have an on going problem of bending #7 & 8 piston rods. This has happened with three different engines. The engines were 468, 496, and now a 598 BBC. The three engines had just a few things in common. 14:1 C/R, Solid roller cam with .792/789 lift, and "H" beam rods (the first two had Manley, this one has Scat)

My engine builder is convinced that the cylinders are hydralic-ing, and I don't have the expertise to disagree with him if I wanted to.

I can understand it happening with the first two engines, but this engine has twin 1050 dominators on a Dart tunnel ram. Each carb throttle bore is directly over it's own runner, and any leak down would drop directly into the runner. Why then is only the rear two cylinders being hydralic-ed?

I shut down the fuel pump early to give the bowls a chance to drain, and try to prevent any raw fuel from leaking down into the cylinders, but don't know if it is even possible to prevent leak down 100%.

Would using the rev limiter contribute to this problem? Has anyone had this type problem, and if so, how did you solve it. Or, does anyone have any ideas why this is happening.

This time the engine will be repairable, but I don't know if I can continue to deal with this.

I certainly could use some help and guidance on this one.

Ken Gasbarri

what parts are incommon with all three engines? block? heads?

What manifold setup was on the other engines? Tunnel rams can if there is enough fuel in the plenum fill the back two cylinders.

Ken....

When are the rods getting bent ?
What other damage if any is happening ?
Did all 3 engines bend the rods under the same situations ?

OK, now that I think about it, all incidents did have the "Isky" cam and "Dart" 320 heads in common, along with the CR (14:1)

The first two engines had a "Dart" single plan intake, which I can understand contributing to dumping raw fuel into the rear cylinders, but would think the tunnel ram would tend to reduce that possiblity.

I don't really know exactly when or why the rods are bending, or why only the rear cylinders are being bent. I am very careful about dumping fuel into the cylinders, but I can not say that no fuel is reaching the cylinders. I do not know of anyone else that has this problem. Most of the guys I race with have some what milder engines with lower CR's, so they may not be a good source for data.

The second time the rods bent, we caught it early because we had opened the engine up after a leaky gasket milked the oil. When the rods do bend and break, it takes the other rod with it, smacks the valve, and generally makes a mess of the rear part of the engine.

It has been suggested that I pull the spark plugs if I should suspect fuel in the cylinders. I can do that, but knowing when to do it may be somewhat of a guessing game.

Again, thanks for any thought you may have.

Ken Gasbarri

You are at 14:1 compression.
could detonation be bending the rods?
I don't understand why it would be in the last two cylls though. Is the compression higher on those?
I don't have any experience with H-beam rods, but I have see a lot of pictures of h-beam rods broken. Also, you say at one point you had milky oil. That is water or anti freeze in the oil. It could have leaked a head gasket and hydrauliced the cyllinder, bending the rod.

Just thinking out loud.

Can't tell you what the problem is but I can just about guarantee your engine builder is correct that it is hydro-locking.
I have never seen a performance rod bend unless the cylinder got partly filled with a fluid or some other uncompressible item,,, like the head of a valve :D

Check that the heads are flat. Since they have been on all the engines and the problem follows them to the same cylinders, they would be my first suspects. I don't know BB heads but if these are aluminum I'd have them pressure checked. It would seem odd that both heads would be warped over the same cylinders, though.

Ken ...
Are you still using one of the same carbs from the single carb set up ? if so is it the rear carb ?
I went through a problem a few months back with the fuel being siphoned out of the float bowls after I shut it off.
I had raw fuel running out of a header at one point.
My problem was partially that the carb had the wrong metering block gaskets and it covered the holes for the idle air bleeds.
Another friend had the same issue with both his carbs on his 2x4 set up and it was the airbleeds got clogged.
Just somethings to look for

Whatb BLK64SS said. I've a few instances where a carb leaked after shutdown. One had a warped sedondary metering block. It leaked the entire bowl full of fuel out everytime it was shut off. It was leaking from the secondary idle feed hole below the throttle plate. Pretty hard to see unless you get lucky, which I did. With 14:1 your chambers are very smal at TDC and it won't take much to hydraulic the engine.

DO this: remove the secondary accelerator pump rocker/lever. run your engine long enough to get good and hot. Shut it off normally, wait a sec and carefully open the secondary throttle a little. Have good light handy. Closely inspect the areas around the secondary idle feeds and transfer slots. If you can, open the sec throttle all the way and look down in the manifold to see if there's any dripping.

How much fuel pressure do you have? Does it go down to zero when you shut it down? Does it maybe take awhile? If it takes a while the pressure could be bleeding off into your carb/manifold somewhere.

If you have a big carb you might be able to find one of the little mirrors on a telescoping rod that will fit through the venturi and provide a way to see the bottom of the carb.

Tom

Ken, are you sure it's leaking fuel into the cylinders or is it maybe water? Did you pull the plugs out of the problem cylinders and turn it thru a couple of revolutions by hand, (takes two people), and see what kind of liquid comes out of the holes? I'd check it after it has set for quite a while, then pull the plugs and check either cylinder. Has this been on the same Block? All the variables have to be checked but it's more likely water than gasoline as gas would evaporate overnight more than likely.

Racers;

Thanks for all the ideas. Been on vacation for awhile, so now it's time to get busy on this problem.

I hope to be breaking down the engine sometime later this week. I know I lost two pistons, two connecting rods, at least one exhaust valve, and a cam. The crank is damaged, but can be repaired. The block may (probably will) need sleeves in 7 & 8. I will probably "heli-coil" all the rocker stud bosses, and maybe the intake manifold bosses as well since they seem to be getting chewed up.

It doesn't look good for getting back out there this season. I can live with that if I feel that I can prevent this from happening again.

If I should happen to find something I can put my finger on for a cause, I will certainly let you all know about it.

Thanks again for the help.

Ken Gasbarri