340 Build Decisions

[RT Dude]

I am currently rebuilding the original 340 engine on the 1970 A66 Challenger. The car is a numbers matching 340 with a 3 speed manual box.
The engine has already been bored 40 over, so it looks like I have to go to 60 over. This is due to the engine sitting for 20 years and it has some rust pitting on number 1 bore.
 I have looked at Speedpro power forged pistons, which appear to have about 1-2 thou clearance, as opposed to other forged pistons which have to run about 6-7 thou. I have been told that the Speedpro pistons are good, but the problem is that I cant seem to get any 60 thou Speedpro pistons.
There are a few conflicting storys over which is the best type of piston to use so I would appreciate some input on what you guys think.
I want to go with a slightly lumpier cam (for street use) but apart from that the engine will remain standard.
Any input on recommended parts replacement is appreciated.
 

[moper]

If it's only one bore, sleeve that one hole. A quality shop can do it for about $100/hole and it will be stronger than the original bore was. If you are looking for a quality piston, the TRW/SpeedPro stuff is cheap heavy and old design. Something like SRP or Diamond would be a much better part. Most modern forged pistons use an alloy that doesnt swell like the older stuff, hence the tighter piston to wall clearance. Normally nowadays it's around .0025-.004 depending on the manufacturer. If this is a street car or a resto, an less expensive and lighter option that will still do a fine job are the KB-Hypereutectics. Some guys don't like them, but I've had no failures and I run them on most budget street/strip deals.

[Topcat]

As far as pistons are concerned you have the option to consider getting custom made Diamond forged pistons.

When I did my engine, it was at .060 over already. Couldn't do much more. So ordered a .062 after an unfinished bore measurement showed it at.

Before you decide if you really want to go out to .060, you should get the block sonic tested. This makes sure that you have enough material to keep the block solid and strong enough to handle a bore out that far. Blocks vary on internal thickness so that's why it is a good thing to make sure. .060 you should be OK but it pays to make sure and leave no doubt in mind.

Once you decide what the unfinished bore will end up being at; you will need the deck height so you can get the corrected compression. This means you should fly cut the block first after sonic testing and final unfinished bore work is done. Now you have some specs to send in for custom pistons to order. 

[RT Dude]

Thanks for the suggestions guys. Yeah, thought about sleeving just the one bore, but have spoken to a few engine guys here, and they dont seem to recommend it. Thats why we are looking at 60 over.
If we can find suitable pistons, we will then make a decision on whether to bore the block or simply sleeve it.
It may sound a little backwards, but we cant just walk down to the nearest parts store and get the required parts to rebuild this engine. We have to order the parts over the internet, and import them from your side of the world. Its just not that easy to exchange parts if you know what I mean.
What is the favoured store for Engine parts for everyone, Summit, Mancini, etc etc. 

[Chryco Psycho]

just hone the cylinder & measure it , you might get away with a loose fit instead of boring ti

[RT Dude]

just hone the cylinder & measure it , you might get away with a loose fit instead of boring ti

We already tried that and unfortunately it will only just clean up at 60 over so it does require a rebore.

[boydsdodge]

I watch my machinist sleeve blocks all the time.
It costs but i's a new fresh bore.
I wouldn't go over 30 without sonic checking block.
I do have a brand new still on stand 68 casting 340 balanced studded, eagle rods, etc....
I won't be using it because I am building a stroker for more torque to get the Challenger moving out of the corners.

[RT Dude]

I watch my machinist sleeve blocks all the time.
It costs but i's a new fresh bore.
I wouldn't go over 30 without sonic checking block.
I do have a brand new still on stand 68 casting 340 balanced studded, eagle rods, etc....
I won't be using it because I am building a stroker for more torque to get the Challenger moving out of the corners.

Thanks for the info, thats good to know.
The engine has a "Power Pak" tag riveted to the block that seems to show it was rebuilt under warranty.
Its already at 40 over, but yeah I hear what your saying about the Sonic check.
We dont want to begin any machining or make any decisions without locating the correct pistons first.
Once we locate a set of pistons, we will make the decision.

[72bluNblu]

I have a set of KB hypers (KB 243's) in my .060 over 340 that I'm building. As long as your machinist follows the clearance specs for them you shouldn't have any issues. And as a bonus they're quite a bit lighter than the stock pistons. They won't take detonation like a forged piston will, but they also cost about half of what a forged piston costs, soooo there's always a trade off. I know some folks don't like them, and while I think they're significantly stronger than a regular cast piston they are not on the same level as forged pistons. But for naturally aspirated engines under 450 HP ( small block ) I think they're plenty adequate. Especially if you're not planning on racing the thing every weekend.

As far as the sonic check goes, its not a bad idea. But if you're not planning a high HP rebuild you should be fine at .060 over. But its a good piece of insurance to know how thick the walls really are before you spend a bunch of money on a rebuild.

[RT Dude]

I have a set of KB hypers (KB 243's) in my .060 over 340 that I'm building. As long as your machinist follows the clearance specs for them you shouldn't have any issues. And as a bonus they're quite a bit lighter than the stock pistons. They won't take detonation like a forged piston will, but they also cost about half of what a forged piston costs, soooo there's always a trade off. I know some folks don't like them, and while I think they're significantly stronger than a regular cast piston they are not on the same level as forged pistons. But for naturally aspirated engines under 450 HP ( small block ) I think they're plenty adequate. Especially if you're not planning on racing the thing every weekend.

As far as the sonic check goes, its not a bad idea. But if you're not planning a high HP rebuild you should be fine at .060 over. But its a good piece of insurance to know how thick the walls really are before you spend a bunch of money on a rebuild.

Thanks 72bluNblu
These pistons sound something along the lines of what i am after.
Do you happen to know what the clearance specs are and where can you get the KB Hypers from

[72bluNblu]

Thanks 72bluNblu
These pistons sound something along the lines of what i am after.
Do you happen to know what the clearance specs are and where can you get the KB Hypers from

The clearance specs are available online at http://www.kb-silvolite.com/index.html. If you go to "tech info" you'll see "installation instructions" and you can download the KB automotive instructions. Most decent machinists should be familiar with using them at this point.

As for where to get them, I just bought them from Summit. At the time that was the best price I could get, actually cheaper than through my machinist. I'm sure Jeg's and all the other major parts retailers have them as well to compare prices.

[RT Dude]

Cool thanks