318 head on 340 block??

[71roadrunnersassy]

hi all,
i recently purchased my rr and just discovered that the 340 engine block has a 318 head on it?
im wondering if that can even work and why it would be there in the first place.
the engine is supposedly a 73 340 but how can i confirm this?
sorry if these are dumb questions but i am new to all this thanks.

 

[a68postcar]

on the drivers side of the block below the head you will find a line of numbers. write them down and post em up on here. as far as the head goes, it will run but not the way it should by any means.

 

[71roadrunnersassy]

its 2780930-340-5 and then has a big 4 with a faint circle around it
the head casting number is 2843675

 

[RustyRatRod]

Yeah, that's a 340. Your biggest differences would be probably valve sizes and port sizes. I imagine if your other head is true to the 340 it will have a J cast close to a spark plug hole and have the last three casting numbers of either 915 or 587. If so, endeavoring to find a match will result in more 340 like performance. Also, if you're going to go to the trouble to swap one out, now's the time to get them both reworked so that you can see what you have and maybe even do some more work like a possible recam and the like.

 

[71roadrunnersassy]

Yeh I have worked that oout my main problem was that I wanted to know if the block was a 73 or earlier but I'm also assuming that they are are very similar or the same. I also know that the 340 x heads have larger inlets at 2.02 compared to the 72-73 heads 1.88 so would it be possible to use the 340 x head or will this affect the pistons? I also have a cast crank which points to a 73 block and may be designed for the j heads if I'm correct?
If I am wrong please correct me.

 

[RustyRatRod]

You can use the big valve heads with either piston. The pistons are where the difference is between early and late short blocks. The early ones were 2 eyebrow with a positive (above the deck) deck height. The later ones were slightly negative deck height with 4 eyebrows. But the big valve heads will work fine with either.

 

[71roadrunnersassy]

Well that is some most helpful info
I guess I better start searching for a new head then thanks for the help

 

[RustyRatRod]

You're certainly welcome. If you'll post your 340 head castin number, I bet somebody here might have one.

 

[71roadrunnersassy]

I will do that then although being is Australia makes things more difficult which sucks
Thanks again

 

[Cranky]

IF your engine has the earlier pistons that stick up above the deck, then you might have a problem with that 318 head. Those have a smaller diameter combustion chamber than the 340/360 heads and will over hang the edge of the 340 bores. With the later 340, there's no worry about that but the over hang of the 318 heads will impeded the exhaust flow from the cylinder slightly.

 

[71roadrunnersassy]

Well i am unable to check that at the moment because the engine has been stripped and mostly been pulled apart and the crank isn't even in. I have new pistons and the engine has been bored but still needs honing so I have some work to do before I get it anywhere close to running.

 

[Cranky]

Well, if you look at the 318 head and if the engine had the higher deck pistons and it had the steel shim factory type head gaskets, the pistons were probably 'kissing' that head and you will be able to see it on the pistons and head. Btw, big blocks have a date code cast onto the side of the block and I'm pretty sure the small blocks do too.

 

[71roadrunnersassy]

Yeh I'll have to look at that although I'm doubting its going to have the factory gasket there but ill have a look.
where abouts would this casting of the date be if there is one?

 

[RustyRatRod]

I will do that then although being is Australia makes things more difficult which sucks
Thanks again

Well if it makes you feel any better. I ain't skeered. I'll ship you a head if somebody else doesn't want to. You can have them ship it to me and I'll send it to you the cheapest way if you cannot find one there.