340 Piston Selection

[SS Lancer]

I have a 40 over 340 block 4.080 - and cant find pistons with compression level I want without having them custom made I wanted 11 or 11.50.1 compression and the most I can find are these but these slugs are heavy .
What gasket thickness do you suppose they are using to get these compression levels ???

 

[1Fast340]

The solution is probably to go for a modern flattop at zero deck Run the god old 0.039" thick headgasket and then cut the heads until your chambers are small enough to get the desired compressison.
But that depends on what heads you have and if they can be cut far enough to make that happen without getting into other isues.
If your problem is the 0.040 overbore,then i think i have seen forged Mahle flattops for that boresize.

Thats what im looking at doing with my .030 over 340 to get similar compression while avoiding to use those heavy old pistons,but im also stuck with the open chamber edelbrock 340 heads.

 

[rumblefish360]

It says right theee on the page, “We also have other pistons sizes.” So call them up and ask a few questions and answer there’s that they’ll ask you like, “How many cc’s is your cylinder head?” Which you didn’t tell us.

What ratio are you looking for and with what head and cc amount in the chamber.

The tech line should also ask you this exact question along with a few othwers like, “Are you going to run NO2?”

 

[SS Lancer]

The solution is probably to go for a modern flattop at zero deck Run the god old 0.039" thick headgasket and then cut the heads until your chambers are small enough to get the desired compressison.
But that depends on what heads you have and if they can be cut far enough to make that happen without getting into other isues.
If your problem is the 0.040 overbore,then i think i have seen forged Mahle flattops for that boresize.

Thats what im looking at doing with my .030 over 340 to get similar compression while avoiding to use those heavy old pistons,but im also stuck with the open chamber edelbrock 340 heads.

I have new
Edelbrock rpm 340 heads.

 

[SS Lancer]

The solution is probably to go for a modern flattop at zero deck Run the god old 0.039" thick headgasket and then cut the heads until your chambers are small enough to get the desired compressison.
But that depends on what heads you have and if they can be cut far enough to make that happen without getting into other isues.
If your problem is the 0.040 overbore,then i think i have seen forged Mahle flattops for that boresize.

Thats what im looking at doing with my .030 over 340 to get similar compression while avoiding to use those heavy old pistons,but im also stuck with the open chamber edelbrock 340 heads.

Yes the 4.080 is my problem and I also would like to get lite items if possible I will go on Mahle website and see if they have something thanks again.

 

[SS Lancer]

It says right theee on the page, “We also have other pistons sizes.” So call them up and ask a few questions and answer there’s that they’ll ask you like, “How many cc’s is your cylinder head?” Which you didn’t tell us.

What ratio are you looking for and with what head and cc amount in the chamber.

The tech line should also ask you this exact question along with a few othwers like, “Are you going to run NO2?”

65 cc on heads looking for 11.1 or 11.5
I checked all the major piston manufactures wedsite and plenty of 4.00 stroke 340 stuff but not so with stock 3.31 stroke.

 

[SS Lancer]

It says right theee on the page, “We also have other pistons sizes.” So call them up and ask a few questions and answer there’s that they’ll ask you like, “How many cc’s is your cylinder head?” Which you didn’t tell us.

What ratio are you looking for and with what head and cc amount in the chamber.

The tech line should also ask you this exact question along with a few othwers like, “Are you going to run NO2?”

65 CC Rpm heads they are new and I just dropped them off at Killians Porting Service in Warner Robbins GA have heads ported and tunnel ram port matched and competition valve job done . Im building a 340 for my 51 Henry J gasser to be raced not street legal so I need compression.

 

[replicaracer43]

I would, in your case, buy a custom piston, with the metric ring pack, and chevy pin size, and run a 6.125 or 6.250 chevy rod. MUCH lighter than any shelf 340 piston, with superior rings. Jmho

 

[SS Lancer]

I would, in your case, buy a custom piston, with the metric ring pack, and chevy pin size, and run a 6.125 or 6.250 chevy rod. MUCH lighter than any shelf 340 piston, with superior rings. Jmho

Thanks I just had CNC motorsports email me and SRP has some for my bore and they weigh in @ 541 grams but with 65 cc chambers it still only yields 10.0 compression !!!
My block has to be decked and if I ran a 0.27 MLS head gasket maybe that would jack compression up slightly?

 

[SS Lancer]

Thanks I just had CNC motorsports email me and SRP has some for my bore and they weigh in @ 541 grams but with 65 cc chambers it still only yields 10.0 compression !!!
My block has to be decked and if I ran a 0.27 MLS head gasket maybe that would jack compression up slightly?

 

[dano]

I have new
Edelbrock rpm 340 heads.

So you have the ones with the chambers milled open for the 68-71 "above deck" pistons.

 

[SS Lancer]

So you have the ones with the chambers milled open for the 68-71 "above deck" pistons.

Yes sir I do !!

 

[SS Lancer]

So you have the ones with the chambers milled open for the 68-71 "above deck" pistons.
Chambers are machined 0.60" for clearance with early high compression engines .

 

[rumblefish360]

65 CC Rpm heads they are new and I just dropped them off at Killians Porting Service in Warner Robbins GA have heads ported and tunnel ram port matched and competition valve job done . Im building a 340 for my 51 Henry J gasser to be raced not street legal so I need compression.

YES! I remember through the conversation.
Trust me I am excited to see some pictures of this. I’m thinking totally BAD ***!

I have new
Edelbrock rpm 340 heads.

With the piston relief in it. Can the head be milled as per rules? It’s a lot I know....

Yes the 4.080 is my problem and I also would like to get lite items if possible I will go on Mahle website and see if they have something thanks again.

Better to call.

65 cc on heads looking for 11.1 or 11.5
I checked all the major piston manufactures wedsite and plenty of 4.00 stroke 340 stuff but not so with stock 3.31 stroke.

Man! It seems to be getting hard these days what used to be easy is no longer such and so.
Did I mention member Ramm?

 

[SS Lancer]

YES! I remember through the conversation.
Trust me I am excited to see some pictures of this. I’m thinking totally BAD ***!

With the piston relief in it. Can the head be milled as per rules? It’s a lot I know....

Better to call.


Man! It seems to be getting hard these days what used to be easy is no longer such and so.
Did I mention member Ramm?

Sorry no you didnt mention him thanks Chris

 

[yellow rose]

If you use that piston you need the piston out of the deck .050 to get your compression. That's the way it should be.

Don't be afraid to stick the piston out of the bore.

If your engine builder won't do it, get a new engine build
Ooo

 

[SS Lancer]

YES! I remember through the conversation.
Trust me I am excited to see some pictures of this. I’m thinking totally BAD ***!

With the piston relief in it. Can the head be milled as per rules? It’s a lot I know....

Better to call.


Man! It seems to be getting hard these days what used to be easy is no longer such and so.
Did I mention member Ramm?

If you use that piston you need the piston out of the deck .050 to get your compression. That's the way it should be.

Don't be afraid to stick the piston out of the bore.

If your engine builder won't do it, get a new engine build
Ooo

U referring to the SRP piston ???
Is it different versus this one shown in photo I thought they were similar except one is much lighter?

 

[rumblefish360]

The one pictured appears to have a dome. I’m not familiar with that piston, or about it’s fit with your heads. Or if it can be out of the hole because of your heads.

YR, IMO, I believe was talking about a flat top slug sticking out of the hole.

 

[SS Lancer]

The one pictured appears to have a dome. I’m not familiar with that piston, or about it’s fit with your heads. Or if it can be out of the hole because of your heads.

YR, IMO, I believe was talking about a flat top slug sticking out of the hole.

No problem thanks my heads are designed to accommodate the high compression pistons so either one
SHOULD function properly
Both pistons are for the 340 specifically so I guess It just comes down to weight and I could buy
0.027 MLS Cometic gaskets and bump up the compression to a more suitable level ?

 

[rumblefish360]

Well, yes, but, even though the pistons are for a 340, not all pistons have the same features. Domes, valve reliefs, height, weight, etc...

Don’t get to hung up on the compression ratio. 1pt. Is roughly 3-5% in power. I know it is a big help with large cams.

Speaking of which, have you a cam for this?
Any ideas on its duration?

 

[SS Lancer]

Well, yes, but, even though the pistons are for a 340, not all pistons have the same features. Domes, valve reliefs, height, weight, etc...

Don’t get to hung up on the compression ratio. 1pt. Is roughly 3-5% in power. I know it is a big help with large cams.

Speaking of which, have you a cam for this?
Any ideas on its duration?

544/558
304/310
Crower solid flat tappet.

 

[rumblefish360]

Duration @.050?

 

[SS Lancer]

Duration @.050?

I - 250
E -254

 

[1Fast340]

Ok i will admit that i have not done the whole math thing on this ,but we are both in kind of the same boat and i did some research on this years ago before i took along break from this so my math is not great on this and i have not figured out if the valvepockets would end up being a problem in this case,the only real difference here is that i havre fresh.030 bore and a somewhat similar cam bit mine is a roller.

Your heads will probably not be 65CC anymore when the headportingshop is done with them,they will probably be alitle bigger.

I would not go with a popuppiston that sits above the deck for a whole bunch of reasons.

I would do a zero deck flattoppiston and cut the heads until i get the desired compresion.

I have read in the past that RPM340 heads are supposed to be possible to cut until they are closedchamber without isues and at that point they will be closer to 55CC but i have not done the math on this,and at that point you will have ended up needing to cut the intakeface of the head aswell a bunch to make any intake fit again,this is just what i have read,i have not measured or calculated this and can not say for sure if this is correct.

Another problem that may show up at this point after cutting the heads 0.060" or so is that the valvepockets in the pistons may not be deep enough anymore but i cant say for sure.

Sure you could get there with either a set of those heavy TRW/Speedpro/SealedPower #L2316 whatever they may be named nowadays pistons or a custom popup piston but at that point if you want to upgrade your heads at any point in time in the future you are screwed again and need to change pistons or go for a ridicilusly thick headgasket.

The other option is some kind of domepiston and at that point you have either ancient TRWL2322 pistons with huge domes (if you can find a set,they show up from time to time for sale brand new )that fits a stock head but will not fit a edelbrock chamber im sure those domes can be reshaped to fit a Edelbrock head since they can be cut down to flattops but i bet its a ton of work to get right and you are once again stuck with a popuppiston that wont fit any other heads except open chamber garbage. Ross makes an of the shelf dome piston aswell(atleast in .030 over)but once again its made for a stock head and the dome wont fit an edelbrock chamber,they may be possible to cut and reshape to fit your heads but who knows. They TRW L2322 piston is quite heavy but not as heavy as a L2316 piston,the ross domepistons are a bunch lighter

I think i have read that Diamond might have done domepistons for RPM340 heads but i have not seen them in the catalog.

This is the reality of pistons for the 340RPM heads its nothing wrong with the head itself its a rpm head but the whole option of that openchambercut makes them a damn headache to anyone who wants to do something hotter with them than bolting them to a stock shortblock and have fun. I have a very well ported set ontop of a 345cui shortblock in my garage and i regret not getting the closedchamber version everytime i look at them it was a damn stupid mistake.
The thing is that you will end up having to cut quite alot to make them closed chamber and get some kind of squish/quench or whatever you like to call it.

There are other opinions in this thread and i think more people could have more and better opinions and possibly options,but i would not go for another popup piston build,i would certainly go for a zerodeck flattop and cut the heads until compression is where it needs to be,and possibly cut deaper valvenotches,im not sure about that part.

Darn it,i was wrong on Mahle,found the place where i saw .040 over pistons for a stockstroke 340 and they where made by SRP

 

[yellow rose]

U referring to the SRP piston ???
Is it different versus this one shown in photo I thought they were similar except one is much lighter?

View attachment 1715194276

If that part where the dome should be (the bottom of the piston when it's installed) is higher than the flat above the valve notches then yes, it needs to come out of the hole.

I never EVER deck the piss out of a head to get compression. One, it can be a flow killer. Two, it's a waste of heads. The engine is DESIGNED to have the piston positive out of the deck. That's the way it should be done.

You are running enough cam that I'd be damn sure (and the ONLY way to be damn sure is to measure the volume by doing the 1/2 inch downfill and then doing the math) I had at least a measured 11:1 and if you get to 11.25:1 I damn sure wouldn't worry about it.