Stock lower end smog pistons with 781 heads

We got any engine builders here?

im keeping the stock lower end with the flat top smog pistons because they all look really good and and I’m not trying to build a 8000 rpm buzzer.

my question is will the stock 781 bbc heads in the photos work with the stock 1984 long block?

Will be studding the heads if that matters.

thanks

Could someone also clarify if these are open chamber or closed chamber heads and are the the peanut or oval port?

thanks

Closed chamber, oval ports. Sometimes referred to as large oval because they aren't the little peanut ports. Compression will be low, but it was low with the stock heads too. If you can afford it, I'd put pistons in it.

What's the reason for studs? A good set of bolts will do just fine.

Only reason for going with studs if because in the past when I have not used studs on older blocks and the head bolts have stripped in the block when torquing them down.

Less likely to have happen but still entirely possible with a stud being that the nutt threads are typically fine thread which will be a less aggressive pull on the block threads which are course.

Thank you for the clarification on the heads.

I've never had head bolts strip in a block? I've certainly never used a new or even a new-ish block. Just old junk for me, lol.

bucket said:

I've never had head bolts strip in a block? I've certainly never used a new or even a new-ish block. Just old junk for me, lol.

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of course you haven’t.
First time for everything bud.

buddy350 said:

of course you haven’t.
First time for everything bud.

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Lol, what's that supposed to mean?
My only point was, it's not a common thing to happen, especially when the bolts are lubed/sealed and properly torqued. I'm not trying to talk you out of studs, just more or less curious why you were so set on head studs.

Technically, 781’s are open chamber heads at 119cc. Those heads with stock pistons will put the compression at a soft 7.9:1.

nvrenuf said:

Technically, 781’s are open chamber heads at 119cc. Those heads with stock pistons will put the compression at a soft 7.9:1.

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Yes, I apparently misspoke about the chamber designation.

buddy350 said:

We got any engine builders here?

im keeping the stock lower end with the flat top smog pistons because they all look really good and and I’m not trying to build a 8000 rpm buzzer.

my question is will the stock 781 bbc heads in the photos work with the stock 1984 long block?

Will be studding the heads if that matters.

thanks

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Yes they will work, but a couple things to note, or ask. Have the heads ever been reworked? If not. i'd caution that the exhaust seats are probably worn out if they have many miles on them. Are you going to have them freshened up possibly? They are an open chamber with anywhere from 115-125cc's depending on who you talk to. Those pistons are fine for most run of the mill applications. What cam are you going to put in it if you are making a cam change? You never mentioned it, but I wouldn't get too wild if you are keeping those pistons and heads. Otherwise the engine will feel like a slug. I know you say you don't want an 8,000rpm screamer, but what are you going to do with it?

nvrenuf said:

Technically, 781’s are open chamber heads at 119cc. Those heads with stock pistons will put the compression at a soft 7.9:1.

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Got a set of 3964290 Heads that were basically given to me by a friend.

Was reading up on the 290 heads and it looks like they are closed chamber 101cc heads.
Do you think that will give me a little bit more compression with those flat top smogger pictons?

Kind of a waste of time for me because I had spent nearly two days of work on the 781 heads, lapped the valves nicely and cleaned everything up on the 781's.

Do you think the 290 heads will work on the flat top pistons? Will I need longer push rods perhaps?

Thanks

QBuff02 said:

Yes they will work, but a couple things to note, or ask. Have the heads ever been reworked? If not. i'd caution that the exhaust seats are probably worn out if they have many miles on them. Are you going to have them freshened up possibly? They are an open chamber with anywhere from 115-125cc's depending on who you talk to. Those pistons are fine for most run of the mill applications. What cam are you going to put in it if you are making a cam change? You never mentioned it, but I wouldn't get too wild if you are keeping those pistons and heads. Otherwise the engine will feel like a slug. I know you say you don't want an 8,000rpm screamer, but what are you going to do with it?

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My intentions are to use the motor in a squarebody 1980s suburban with a sm465 transmission with 8lug, 4 somethings gears.

just basically cruise around town and try to look cool.

likely never go off-road or tow.

here are the specs on the cam that is already in the block.

290 heads

You can use the closed chamber heads with the flat top pistons without issues. You will get a bump in compression as you're losing 15-20cc's of chamber volume, you can't use closed chamber heads with pistons designed for open chamber heads with a dome as they will cause interference. Any head change to closed chamber would require different pistons only if you have domed pistons for open chamber heads but since you have flat top pistons it doesn't matter. With the cam you are running currently it would make good torque and power numbers as it's designed for a low compression engine. Big cams with long durations require a bump in compression because the dynamic compression ratio changes and drops off quite a bit. That's why big cams usually state a minimum compression requirement. Because it lowers the dynamic compression ratio. If you're going to go the route of these other heads, you could almost go up a few sizes on camshaft to take advantage of the added compression ratio. But you'd also have to check piston to valve clearance as those smog pistons have shallow valve reliefs and more lift and duration will have an effect on the piston to valve clearance. But all things being the same, put it together and run it.


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