Small block ports

[cudamadd]

So I want to break the cam in on the new engine .I have a 340 X heads and so on. My Question is are the exhaust ports on a J head the same as the X head ? The reason is my factory manifolds are away and being coated . So a friend was going to leaned me some headers .to break the cam if they are the same ,port size .And if they  bolt  up  Cheers 

[HP2]

Yes, you can use those headers for break in with out issue.

[cudamadd]

Thank you for your reply

[MoparLeo]

Ports are exactly the same size. Are the "X" heads rebuilt ?
 Did the engine come with them ? I ask because a lot of ppeople pay too much money for the "X".
The "J" has the exact same ports. The early "J" came with the exact same 2.02 valve sizes . The only difference being abpout 2-3cc's difference in combustion chamber size.
Normally the same size after a resurface for straightness.
 Even if 1.88 size, easily made to  2.02 with recutting the seats which will also restore the height of the valves.
"J"  Heads are more plentiful and much cheaper.

[cudamadd]

Thanks MoparLeo yes it's the factory engine to the car 69 dec build Night  shift   

[MoparLeo]

I meant did the engine come with those heads. Pre 73 heads should have the unleaded valve seats installed.
Otherwise the valves tend to sink into the seats. If the original heads need a valve/guide job, the seats should be fixed at that time. That is why I would suggest the "J" heads. Same power out put, newer castings and cheaper. These heads are now avg. 50 years old.
This is a major reason that given the cost of a complete head rebuild including machine work, aftermarket heads begin to make more sense.
Most are aluminum. New casting, Lighter, can run higher compression without detonation, better port flow, all new parts and the latest valve guides, valve seals, springs , retainers and valves/materials.

[cudamadd]

Yes my engine did come with the x heads   

[cudamadd]

Sorry I did not read your question correctly. Yes the heads came with my engine .And I have had them fully rebuilt.p.s not cheap but what is these days .