340 engine repair/build questions

[Illahe]

Seems the aftermarket likes to shorten compression heights from the original design - I ran into that with the Silv-O-Lites in the 327/275HP SBC too. It was originally 10.25:1 compression ratio, but now it's only got 9.2:1 - and that's thanks to milling the heads!

Early 340s were 1.840, although some sources say 1.820. So that 1.802 compression height will put the piston near zero deck to .020 in the hole. Stock 340s have the piston .018" out. It will drop compression ratio about by about .8.

I know...... it's a street ride and compression ratio is less of an issue.

Agreed. I'm finishing up a Silv-O-Lite equipped 340 today; and with .03 off the heads, .005 above deck and thin head gaskets its working out to be 9.8:1. The last time I built a 340 was 30 years ago and the sealed power forged were available.

The other issue with the Silv-O-Lite pistons is the radial valve clearance of the intake valve relief. It has been cast for 1:88 intake valve relief. I mocked up the engine with a set of EQ 318B heads, 1.92 intake valve and it made contact with the radius of the valve relief...

 

[Inertia]

Can you post the compression height of the pistons for reference.

Very sorry for delay, totally forgot.

I get a measurement of 1.350 from top of pin hole. (.980?)

I mentioned no .060 pistons for 340 to my son.
He went to Summit, found Hypers? 4 sets remaining, .. (I didnt see it.)
Cheers

 

[Illahe]

Very sorry for delay, totally forgot.

I get a measurement of 1.350 from top of pin hole.
Don't have a pin.
I mentioned no .060 pistons for 340 to my son.
He went to Summit, found Hypers? 4 sets remaining, .. (I didnt see it.)
Cheers

Good luck with your project.

 

[CFD244]

Agreed. I'm finishing up a Silv-O-Lite equipped 340 today; and with .03 off the heads, .005 above deck and thin head gaskets its working out to be 9.8:1. The last time I built a 340 was 30 years ago and the sealed power forged were available.

The other issue with the Silv-O-Lite pistons is the radial valve clearance of the intake valve relief. It has been cast for 1:88 intake valve relief. I mocked up the engine with a set of EQ 318B heads, 1.92 intake valve and it made contact with the radius of the valve relief...

I used the Silvolites in my 1971 340 build. They ended up being .017 out of the hole so I used .050 (approx) head gaskets to keep it below 10:1. IIRC my combustion chambers were around 70cc and I had no contact issues.....Engine runs great.

 

[FlamesAreWicked]

I talked to an engine builder Monday of this week and I kinda have it in mind that it’ll need new slugs. I’ll probably go with some KB pistons. At this point in the game, it’s tempting to go with a roller cam and aluminum heads lol… def don’t need it but.. if we’re honest, I don’t need the car much less more go fast parts lol.

 

[cookietruck]

Call autotec and see what their pistons would run. Im sure they could a .040 over piston.

 

[FlamesAreWicked]

Call autotec and see what their pistons would run. Im sure they could a .040 over piston.

Lookin like $700-$800.. the KB set will be just fine.. I will eventually go with some aluminum heads and a little bigger cam while swapping to hydraulic roller.. I have no desire to add any boost or spray to this. Just a street car that’s super fun.

 

[RustyRatRod]

I talked to an engine builder Monday of this week and I kinda have it in mind that it’ll need new slugs. I’ll probably go with some KB pistons. At this point in the game, it’s tempting to go with a roller cam and aluminum heads lol… def don’t need it but.. if we’re honest, I don’t need the car much less more go fast parts lol.

Why? They're still cast. IF you go with the KB pistons, make SURE you are generous with the ring gaps. Knowing what I know about them now, I'd put a little more with it and go forged. You won't be disappointed.