Mercruiser 260 Backfire Through Exhaust

[Scott06]

I have not had the time to test the block yet, but I got a quote from the machine shop and it was $1000 to repair my stock, plain Jane cast iron heads that came off an old SBC 400. That seems like I might as well buy new heads for that price? Can anyone recommend some good heads for a 355 with flat-tappet lifters? I want to eventually get a 383 with roller rockers so maybe heads I could eventually put on that? Would Vortec heads be an easy conversion or would there be too much involved in that switch. I know I would need a new intake and self aligning rockers, but would the valves have the proper lift? I have dished pistons.

@Scott Danforth would know

 

[Scott Danforth]

@Scott Danforth would know

brand new vortec heads (assembled) are $850 a set from GM (any GMPP dealer)

used/modified vortec heads (screw in studs, roller rockers, better valve seals) used to be $400 a set from any engine builder in the mid-west until the rule change about 10 years ago.

I ran vortec's on my 350 with a taller cam, 1.52:1 roller rockers and screw-in studs.

 

[Kola16]

I will have to see if my block is in good shape tonight to make a decision then. If it is, I might try to pick up some vortec heads, cause I am not paying $1k to get my stock heads back when I can pay the same price for an upgrade. I did not even find the crack in the heads yet, so I am not sure where the leak(s) is/are yet. They might not even be fixable if I can't even see the leak.

 

[nola mike]

I have not had the time to test the block yet, but I got a quote from the machine shop and it was $1000 to repair my stock, plain Jane cast iron heads that came off an old SBC 400. That seems like I might as well buy new heads for that price? Can anyone recommend some good heads for a 355 with flat-tappet lifters? I want to eventually get a 383 with roller rockers so maybe heads I could eventually put on that? Would Vortec heads be an easy conversion or would there be too much involved in that switch. I know I would need a new intake and self aligning rockers, but would the valves have the proper lift? I have dished pistons.

Do you know if the heads are even bad? Could pick up a set at the junk yard for $50.

 

[Kola16]

Do you know if the heads are even bad? Could pick up a set at the junk yard for $50.

I do not. All I know is that when I did a pressure test after removing the leaky manifold, I could hear hissing in the number 1 cylinder. When removing the plugs from cylinders 6 and 8 (side with the good exhaust manifold), they were either wet or oily. I could not put my ear up to the even-numbered cylinders since the exhaust manifold was in the way. I will try to pressure test the heads after I pressure test the block. I'm going to have to do a lot of manufacturing of block-off plates haha.

 

[Kola16]

A second look at the cylinders and heads reveals that cylinders 6 and 8 are oily, not wet like I thought. Rookie mistake. I suspect the intake manifold gasket was to blame since one of the bolts to the intake manifold gasket pulled the threads on the head, and this bolt was located by cylinders 6 and 8. In other words the intake manifold gasket was not tight in that spot like it should have been. Also the ports on the intake manifold near cylinders 6 and 8 were oily.

I still had that hissing in the number 1 cylinder though so I still think new/rebuilt heads are a must. I will see tonight hopefully.

 

[Kola16]

Tested the block. No leaks. Tested the heads. NO LEAKS!!! Thank goodness I might be able to go fishing after all. Since I heard a leak around cylinder 1 when I originally put it back together, I either did not get a good seal with the head gasket, or my mating surfaces are warped. It could be either or. I will check the flatness of my surfaces after my weekend trip.

 

[Kola16]

Forgot to update this. So after overheating the engine because the water pump belt got thrown from misaligned pulleys, I blew a head gasket and cracked an exhaust manifold. Head gaskets are now replaced, pulleys are aligned, and manifolds are replaced and it is running great. Got to take the boat to the ocean one last time before the season ended. Thanks all!

 

[Lou C]

Glad you were able to get it back together too much difficulty. The simple and cheap to repair small block Chevrolet is a wonderful thing isn’t it?