340 magnum roller conversion

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plugger340

plugger340

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Someone has a new 96-2002 magnum head conversion for sale, for small block Mopars 67 and up. The heads have been checked and rebuilt, and the guides cut for a larger lift cam and bee hive springs #250 seat pressure. It has Mopar performance conversion push rods and comp cams roller tipped magnum rockers on studs. Would this be a straight install on a 340 block without any block machining if I use the Hughes roller lifters that are designed for LA blocks?
 
Further to this ... the guy selling it said that he couldn't use the cam that he had so that's why he's selling the heads. Does anyone see any driveablility problems using this on the street?
 
I'm sure people will chime that have done this.
 
What year 340? If it's a 70 or older you may have to run a real thick head gasket cause their pistons came out of the hole pretty far and magnum heads are closed chambers. Check with Hughes to make sure those lifters oil through the pushrods. LA engines oil the rockers through a drilled port in the block to the rocker shaft. Magnum heads oil the rockers through the pushrods.
 
Just thought of one more thing. Magnum head chambers are much smaller than LA heads were so if your engine is a 70 or older the compression might be up there pretty high even with thick head gaskets. That's the only driveability issue I see.
 
Thanks, does anyone have recommendations for a cam? I don't want anything too lumpy. The seller recommended a purple cam but he didn't say what model. The block is a 68. Are old unrebuilt 340 x heads worth anything?
 
plugger340 said:
Thanks, does anyone have recommendations for a cam? I don't want anything too lumpy. The seller recommended a purple cam but he didn't say what model. The block is a 68. Are old unrebuilt 340 x heads worth anything?
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I know flat tappet cams pretty well but haven't done enough roller cams to know what's what so my suggestion is to call some manufacturers (Comp cams, Bullet cams, Lunati, Hughes engines) give them your specs and intended use and see what they say. They'll know their product the best.

Sure X heads are worth something as long as their not cracked. In reality their mechanically identical to the early J and U heads but they have that big X on them that some guys need or others just can't live without. Take some pics and post them for sale for a fair price and they'll move.

BTW: You'll almost always be money ahead selling un-rebuilt heads cause hardly no one trusts machinists or sellers anymore. Case in point, I had a set of X's for sale that I had personally rebuilt with all the best parts available (stainless REV valves, Comp dual springs, 10 degree retainers and locks, PC seals, bronze guides, hardened seats, milled, blueprinted, etc...) and even did some minor porting on them and had them flowed so I had proof of what they flowed. I put about 8000 miles on them when I decided to build a stroker. Figured I'd list them for sale for $750 since you can't do a basic rebuild (stock parts with no porting) on a set of heads properly for less than that and I had a little more than that in them. Didn't get a single inquiry but I watched a couple other guys sell old worn out X heads for half that. Doesn't make any sense at all :wack:
 
Whatever cam you get just make sure it has a cast iron gear on it
 
fishy68 said:
Just thought of one more thing. Magnum head chambers are much smaller than LA heads were so if your engine is a 70 or older the compression might be up there pretty high even with thick head gaskets. That's the only driveability issue I see.
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Just to add to this comment.
If is is a 68 340 the pistons will stick out of the block at TDC. The magnum heads are closed chamber. The pistons will hit the heads. Even if they don't quite hit the heads the squish area will be way too close and the compression will be through the roof!
 
340duster said:
Just to add to this comment.
If is is a 68 340 the pistons will stick out of the block at TDC. The magnum heads are closed chamber. The pistons will hit the heads. Even if they don't quite hit the heads the squish area will be way too close and the compression will be through the roof!
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Yep. 68-70 all stick up above the deck. I mentioned that in my post above the one you quoted.
 
Would different pistons help the clearance issue? I'm trying to figure out what route to take: to get the x heads rebuild with hardened seats (which sounds pricey) or buy after market heads or buy these 360 heads and modify them or change the pistons. If all 360 heads have clearance issues on the older 340 blocks can they be modified (machined) or would that be too pricey?
 
How about ordering a set of eddy 340 Alum heads? You will have to clearance the Pushrod holes that's it.
 
plugger340 said:
Would different pistons help the clearance issue? I'm trying to figure out what route to take: to get the x heads rebuild with hardened seats (which sounds pricey) or buy after market heads or buy these 360 heads and modify them or change the pistons. If all 360 heads have clearance issues on the older 340 blocks can they be modified (machined) or would that be too pricey?
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Yeah different piston would help if you get some that don't stick up too much but remember changing pistons can alter the balance so your looking at a lot of work doing it that way. If your already rebuilding the engine it's not a big deal but if your just trying to make these heads work it seems to me a lot of effort. The first thing I'd do is measure to see just how much of a clearance issue you actually have. With Ma Mopars inconsistent machining from the factory you may not even have a problem, or it may be real bad. If you can get .035" clearance you'll be fine I'm sure. That'd be real easy to do with thicker head gaskets. Cometic gaskets are expensive but top quality gaskets that you can get in different thicknesses. In fact they'll build you a set in whatever thickness you want for the same price as off the shelf gaskets. Or if you shop around I bet you can find lesser expensive gaskets that are thick enough to give you the clearance you need.

After going through rebuilding a set of X heads which needed everything (which most do) I'll never again consider doing it. I'd buy a set of RHS or Edelbrock heads. More performance right out of the box for very little more than a total rebuild of X heads. I say leave the X heads to the guys that need them for a concours restoration but that's JMO. RHS heads are also closed chambers so you'd run into the same clearance issue as the magnum heads but you can get open chambered eddy heads.

BTW: are those magnum heads 100% guaranteed to be crack free? I ask because everything I read about them is they were terrible about cracking between the valves.
 
The motor has to be rebuild so it probably will need new pistons. The 360 heads were magnafluxed and they checked out. Are all aftermarket heads drilled to take magnum intake manifolds? I have a fairly new LA intake manifold that I would like to use but I can always sell it if I can't use it.
 
If that's the case swap out them junk pistons. Just make sure and get the assy. balanced cause piston weight affects that a lot. I think you can get most any aftermarket heads in either configuration. I bought some of the first EQ heads (pre RHS heads) and got them with the LA bolt pattern.
 
Your welcome and good luck with the project.

BTW: ignore my last comment about your pistons being junk. I was talking to another guy about his low compression pistons and that comment was meant for him but I accidentally posted it here.:banghead:
 
The factory 340 pistons are very very heavy. When selecting pistons for the rebuild pay attention to weight. Less weight will make the engine rev quicker and relieve the crank and rods of a lot of stress. Like fishy says you will have to rebalance. Some KB hyper pistons would work well assuming you don't plan to use NOS. Also keep in mind a lot of pistons can be slightly milled so they do not stick up above the block surface. Edelbrock had an article on line where they used the "360" aluminum head and milled the replacement pistons so they did not sitck up.

And yes '71 340s had the high compression pistons. '72 was the first year for low compression. Kind of funny cause GM and Ford went low compression in '71 but chrysler did not.
 
As fishy mentioned, Piston sticks out deck, put a thicker head gasket on it to get your quench distance. It's not hard to solve.

I wouldn't use the open chamber alum heads, use a closed chamber head and solve the quench distance with your head gasket. Most of those early 340's the piston is out .012-.018, get a .052-.060 thick head gasket and you're good.
 
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