View Full Version : 318 bored to 340?

05-17-2008, 09:58 AM
Ok been seeing a lot of 318's that have been bored to 340 spec (4.04). The stock bore on the 318 is 3.91. Boring the 318 by .13 seems crazy to me. Obviouslly this depends on the block - The later 318 thin wall casting will not handle this overbore. I suppose one could bore and sleeve the 318 - but I still don't know how reliable that would be.

I have heard that some of the early 318 blocks that can handle this overbore. Specifically to identify those they have 4 motor mount tabs on each side of the block (instead of 3 on one side). I happen to have one of those blocks that is a 1968 casting.

Any racers out there that have done this and which block did you use?

05-17-2008, 12:54 PM
If you could build it to 340 that would be the only worth while notion.
Otherwise build a 360, but...
If you wanted you could turn down the mains on a 360 crank and drop it in a 318 thats .060 over =355cid , .040=351cid , .030=349cid , .020=347cid, It will live in the 6500 range you will also be lightening the crank a lot due to way less piston weight, could even lighten the rod a bit.
Pistons will cost you a lil though.

Ray Bell
05-17-2008, 01:36 PM
I simply don't see this as possible unless Chrysler cast special blocks for some purpose...

We've sonic tested several Poly 318 blocks and none of them are fit to bore to 4" without offsetting the bores... and they're not the thin wall castings.

05-17-2008, 05:58 PM
I agree Ray. just not possible. IF you want to use the 318, put a stroker crank in it and make a 390 out of it or build a 360 and be done with it. :king:

05-18-2008, 12:48 PM
could you destroke a 360 by putting in a 340 crank or a 318?? also can you stroke a 340 by putting in a 360 crank?? this has been some things on my mind as well

i've heard some say that the 68-69 la 318s you could bore .090 over because of the steel that it is made out of and the walls are thicker.

05-18-2008, 02:22 PM
Does a 340 sound better than a 360, due to rod/stroke ratio ?

Perhaps it would make more sense to build a 340 from a 360 (main bearing shims or new crank) because both the name and sound is better (though I have a "cuda 360" so I like 360).

One of the mopar performance books found long rods (up to 7" in 318/340) made more power with a restrictive intake (2 barrel). The article was based on circle track magazine findings.

Wonder what a 318/340 would sound like with 7" rods - Nascar engine ?

Ray Bell
05-18-2008, 06:12 PM
Don't know about sounds... I hardly think it's the most important thing...

I do think that there's something 'right' about the bore/stroke ratio of the 340, along with the rod length, though I'd not be convinced of that without someone doing some very expensive testing.

There would be reasons that the 7" rods worked with that restrictive intake, to do with just when the piston acceleration was highest and that sort of thing.

You can get short stroke cranks with the bigger 360 main journals, I'm sure.

05-18-2008, 07:06 PM
Longer rod=Longer dwell time, Less skirt load/side thrust,,less piston weight, more milage ect...

05-18-2008, 07:19 PM
Dart, I think you answered your own question - taking .13 inches out of any 318 bore will require sleeves, assuming there's enough meat left to anchor them. You would need to get your 68 block sonic-checked to determine the actual thickness of the cyl. walls. The real question would be "Why should one spend all that money?"

05-18-2008, 07:27 PM
I agree...I may have answered my own question...

I think maybe people are confused with .040 overbore on a 318 calling it a 340...lol

05-18-2008, 07:45 PM
D The real question would be "Why should one spend all that money?"

Yea, And still not have the strength of a real 340, [Main web & Caps']

05-19-2008, 02:08 AM
Im going to do it, and I have a buddy who has done it and that was 15 years agop and it is still running to this day, in a ramcharger.
I have gathered every 67, 68 318 block I can get, and if I didnt have two, almost three motors getting built right now, and my coronet was a little closer to paint, I would have it done already. I am confident it will work. Just by knocking a freeze plug out and measuring there has given me the confidence to try it, but it seems I have been in the planning stage for three years now, it is on the low priority, but all this talk on here about it gets me thinking again.
Remind me again what the safe thickness is to have. I know 40 over 360 blocks are pretty thin, someone needs to try this, or is it your waiting on me to test the water for sharks, I see how it is! LOL
I will never build another stupid 360, I dont care for them.

05-19-2008, 02:25 AM
We did that in the eearly 80's. Didn't know anything about these engines, my brother was working in a machine shop and someone brought a 340 ther for a rebuild. The original std pistons were in good condition, and since we happened to have a'71 318 engine, we figured that it could be bored for the 340 pistons. And that's what we did, 0.13" overbore. While putting the engine together, we installed a wrong, too long bolt in the drivers side lower timing cover bolt hole. Then, once the engine was ready and we poured water in, we started to wonder how much water it really would need. Then, a check from the dipstick, and we had quite a lot fo oil in the engine. :D The timing cover bolt had gone right through the cylinder wall without anyone noticing, it was...., well, pretty thin. Anyway, that didn't scare us at all. since the hole was below the ring travel, we just plugged it with a brall plug, put the thing together and started driving. It worked just fine, even produced some power. We used it for some five years, and after that it went in to an "econo gasser" race car, run low 12's at best. That's not propably the best way to do it, but it can, and has been done.

05-21-2008, 08:41 AM
I had also a -68 318 block overbored to 340 in my ยด68 Charger for about 10 years, then suddenly there was a tiny tiny little hole in one cylinder wall and the engine had to go. I remember that I had some problem with cooling until I installed a bigblock radiator. Otherwise the engine was great..

05-21-2008, 12:40 PM
Someone asked what the minimum thickness to go with is, well, according to my latest machinist, he said 0.200 at the least on the thrust side. I had asked because I have 3 340 blocks all 0.030 over, and very loose. I had one rotating assy balanced and all, ready to go back in, when I decided to see if I could bore the block to 0.040 over and go with new pistons, and a 4" stroker crank. That particular 340 block was cast 06/67, with no vin tab on the block. My machinist sonic checked it for me and told me I had at the thinnest point in the block 0.210, so I was good to go to 0.040, so now I will end up having a 0.040 340 stroker, somewhere around 421 cubes now, I believe. Big block power from a small block. Patiently waiting for some custom pistons to be made....

I dont know if that number of 0.200 is an accepted standard or just my machinists preference, I suspect his pref tho, as I read an article by Steve Magnante in Hot Rod building a 383 or a 440 where he stated that a minimum of 0.080 was good enough for a big block. ( I asked Hot Rod if that was a misprint but never got a reply) One of my 340 blocks was sonic checked by my old machinist, and told me that I had several cylinders at 0.135 and that he wouldn't build an engine with that unless it was just to idle around the fairgrounds. Now what do I do with a couple of thin 340 blocks? :crazy:

05-21-2008, 12:48 PM
"read an article by Steve Magnante in Hot Rod building a 383 or a 440 where he stated that a minimum of 0.080 was good enough for a big block."

MISPRINT. .200 is a good rule of thumb. -Bob

43 purestock
05-23-2008, 03:35 PM
I saw one first hand a while back, 68 318 overboard to 4.040, with dome pistons and a solid lifter mushroom cam, was in a dirt stockcar in the early 70s, owner told me they would turn it 7500-8000 most nights. Yikes!! Il try to get a picture of it next time im out that way. Oh yea, its got a bent rod in it, but the block looks fine!!

05-23-2008, 03:57 PM
Ray Barton told me he had bored Magnum 5.2L to 4.003" and they worked, but he doesn't reccomend it because of the tolerences in casting the blocks, you can strike water or oil real easy. Very few factory blocks have all 8 cylinders at a true 90* to each other either. When you straighten up the bores, and make them truely round, you may not have much metal in places. You can buy a new Magnum block designed to be a 340. Stronger than the original 340, with extra ribbing in the block, thicker decks, better mains. Put a 4.13" stroker crank and you've got a long rod 426 CID small block that'll put up HP and torque like a elephant Hemi. Ain't gonna be cheap, but it'll sure work. Hughes Engines or Ray Barton can build you one in a couple of weeks...just send money.

5th Ave Sleeper
05-23-2008, 10:51 PM
sounds like a waste of time and resources. you will have more money wrapped up in sleeves and balancing/turning everything than you would just by just building a 390 stroker, strong 360 or a 408 stroker for that matter!!
350 hp out of a 318 is easy anyway. the right combo of not so expensive parts will get you there easy.
my advise is find a late model 360 block, get it all cleaned and machined, buy a 408 stroker kit for it. stick a roller cam in it and some magnum heads on it, waste every slomaro and 5.slow and tell em its a 318!

05-23-2008, 11:42 PM
Cageman asked "or is it your waiting on me to test the water for sharks" Yep!!:)