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Are 318 Magnum pistons different from 318 LA?
Posted By: 69L78Nova
Are 318 Magnum pistons different from 318 LA? - 09/08/11 08:19 PM
I'm in the middle of my little budget 318 build and I've hit a snag. The 95 Magnum 318 short block I picked up has a small crack in one of the cylinders. I already bought a new Mahle ring set for the Magnum pistons. I have a 75 318 in immaculate condition needing nothing more than rings, bearings and resealed. My question is, can I use the Magnum pistons in the LA block for a little compression bump, or are they basically the same except for the metric size rings. I was going to use the K6900 cam and Magnum heads from my current 360 in the 318 LA. I don't have the spare dough to have the Magnum block bored and buy new pistons, so I was looking to do it on the cheap. Or should I stick with the 75 318 pistons?
Posted By: chrgr340
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 09:15 PM
I don't recall what the exact difference is, but I just read somewhere a day or two ago that LA and Magnum pistons are different. I'd probably stick with the LAs.
Posted By: Moparnut426
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 09:23 PM
the width of the rod is it. the LA rod wont fit between the wrist pin bosses.
If you run a scat I beam they fit. Pluss will be full floating.
Kasey
Posted By: GO_Fish
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 09:47 PM
If the Magnum bore is the same size as the LA's bore(both virgin, both +.020,etc), you should be able to swap the magnum crank, balancer, rods and pistons over to the LA block and keep moving. Use the Magnum head bolts to bolt the Magnum heads on the LA also.
Posted By: 69L78Nova
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 09:48 PM
Whoops, I'm sorry. I should have been more clear. I mean I was thinking of using the Magnum pistons AND rods in the LA engine. Would that work, or is the rod different on the crank end as well? The only reason I ask is because the magnum engines are rated as being 9 to 1, and the LA is 8.5 to 1. I don't know if the magnum heads on an LA block would boost compression, or if the magnum chambers are around the same size as the old 70s 318 LA heads.
Posted By: Moparnut426
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 09:50 PM
They will go in, I recomend balancing the whole shebang though. Also you shouldnt have any piston to valve clearance issues, still way shy on compression to be worried about that. The chambers on the LA heads are bigger than the nice chambers on a magnum head.
Kasey
Posted By: RapidRobert
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 09:53 PM
360 piston ~584 grams pin 154.6 rings 56.6 comp height 1.590". Mag piston 470, pin 134, rings 40 grams CH 1.662". As said th LA rod small end is 2 wide but can b narrowed but that'd b $$$ plus then you'd need to weigh all the pieces 4 the balance factor & you're doing a budget build. Not sure what I'd do but that's what I could come up w
Posted By: 69L78Nova
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 10:02 PM
Could I use the whole 1995 rotating assembly in the 1975 block? Crank, rods, pistons, balanced and flexplate? I would just like to use the newer stuff if I can.
I wasn't aware the old 70s 318 two barrel heads were open chamber. I guess ya learn something new every day!
Posted By: roe
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 10:10 PM
wouldn't the magnum heads on his LA 318 give him a nice bump in compression, especially with a thinner head gasket. If the LA is in such nice condition why not re-ring, new bearings, magnum heads and go with the LA 318?
Posted By: 69L78Nova
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 10:13 PM
That's what I planned to do. I wasn't sure if the compression height of the Magnum pistons was the same as the LA pistons. If the difference is minimal or not worth it, ill just run the LA stuff
Posted By: roe
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 10:28 PM
Im definately no expert on this. Somebody a few posts back said that you can swap over the entire rotating assembly. If that true what king of issues could he run into? Wouldn't that be like slightly stroking the 318 to about 349 or so? Would he have to start worrying about clearance issues with those pistons/heads in the 318?
Posted By: 69L78Nova
Re: Are 318 Magnum pistons different from 318 LA? - 09/08/11 10:50 PM
No, both are 318s. One Magnum, one LA
Posted By: GO_Fish
Re: Are 318 Magnum pistons different from 318 LA? - 09/09/11 01:59 AM
Quote:
Could I use the whole 1995 rotating assembly in the 1975 block? Crank, rods, pistons, balanced and flexplate? I would just like to use the newer stuff if I can.
Yes, that is what I was saying in my earlier post. Use the whole 95 rotating assembly so you do not have to rebalance anything. You would still have a 318, but the compression should be the same as a Magnum. You would essentially have a magnum less the roller cam and lifters. You'd need hollow pushrods from Summit or other to finish the conversion.
Posted By: 69L78Nova
Re: Are 318 Magnum pistons different from 318 LA? - 09/09/11 05:06 AM
Ok, good to know. I already have the complete Magnum top end on my 78 360, I would just transfer the heads, manifold, carb, and K6900 cam/lifters into the 318. Looks like that's what ill do.
Posted By: patrick
Re: Are 318 Magnum pistons different from 318 LA? - 09/09/11 02:01 PM
Quote:
Could I use the whole 1995 rotating assembly in the 1975 block? Crank, rods, pistons, balanced and flexplate? I would just like to use the newer stuff if I can.
I wasn't aware the old 70s 318 two barrel heads were open chamber. I guess ya learn something new every day!
yes, and is what I'd do.
compression height is marginally higher than the older pistons, and the metric ring packages are the big differences between LA and magnum 318.
Posted By: Moparnut426
Re: Are 318 Magnum pistons different from 318 LA? - 09/09/11 03:44 PM
You might already know this, but make sure you match the lifters to the lobes. If not, the cam and lifters are certain junk.
Kasey
Posted By: roe
Re: Are 318 Magnum pistons different from 318 LA? - 09/09/11 09:13 PM
Quote:
No, both are 318s. One Magnum, one LA
Silly me, not paying attention
Posted By: daniel_depetro
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 12:57 AM
Supposedly the Magnum engines have a lighter rotating assembly weight too.
Posted By: GO_Fish
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 02:22 AM
That is true (lighter Magnum rods and pistons). That is why the whole Magnum rotating assembly needs to move to the LA block to maintain correct balance without a trip to the machine shop for custom balancing.
Posted By: HotRodDave
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 04:13 AM
The differance in piston height is about .080 witch will make a nice bump in compression. However just swapping magnum heads onto a 318 LA with magnum heads will not yield you as much compression as a factory 318 magnum because the 318 magnum had a lower deck height also by about .015.
Posted By: 69L78Nova
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 04:18 AM
Yeah I'm just going to swap the whole magnum rotating assembly into the LA block. Plus, I already bought the rings and they weren't cheap. Does it matter what year bearings I use since the cranks interchange? Every listing I've seen lists pre-80, and 80-newer for rod/main bearings
Posted By: 1_WILD_RT
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 04:20 AM
I know nothing about the differences but I'll throw this out... Most late model engines have gone to a one piece full circle rear main seal... For that to work the back of the crankshaft is different from an earlier style engine...
Posted By: 69L78Nova
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 04:21 AM
They are both 2-piece seal blocks
Posted By: 1_WILD_RT
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 04:25 AM
Cool..
Just a thought...
Posted By: HotRodDave
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 03:23 PM
The only bearings that may make a differance is the thrust bearing, or the rear main bearing, the flange on the thrust bearing may be different (bigger diameter)and I think one bearing fits all but the other will not fit in the other block so if your thrust bearing goes in your block it is good. The rear main bearing one was wider than the other and again I don't remember the year break or witch one is universal.
Posted By: daniel_depetro
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 03:37 PM
Quote:
The differance in piston height is about .080 witch will make a nice bump in compression. However just swapping magnum heads onto a 318 LA with magnum heads will not yield you as much compression as a factory 318 magnum because the 318 magnum had a lower deck height also by about .015.
Interesting...
Is there a difference in deck heights between:
1971-88 LA360
1989-92 roller camshaft LA360
1993-2002 5.9l Magnum
?!?!?!
Posted By: scratchnfotraction
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 03:57 PM
Quote:
The only bearings that may make a differance is the thrust bearing, or the rear main bearing, the flange on the thrust bearing may be different (bigger diameter)and I think one bearing fits all but the other will not fit in the other block so if your thrust bearing goes in your block it is good. The rear main bearing one was wider than the other and again I don't remember the year break or witch one is universal.
I didnt think of the thrust bearing..but IIRc all the magnum engine used the 360 size rear main bearing or main cap size and 360 oil pan.
not sure if the bearing would have changed or the main cap size for the pan seal
something you might want to look at before the swap starts 
Posted By: Yellow Fever
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 04:05 PM
Don't forget that you'll have to balance your flywheel or torque converter for the magnum balance unless you neutral balance the assy, which is the route I would take.
Posted By: RapidRobert
Re: Are 318 Magnum pistons different from 318 LA? - 09/10/11 04:25 PM
Quote:
neutral balance the assy, which is the route I would take.
X2
Posted By: HotRodDave
Re: Are 318 Magnum pistons different from 318 LA? - 09/11/11 06:36 PM
The 318 and 360 magnums both have slightly shorter deck heights than the LAs did. TBI and carb had one deck height and the MPI magnum had the shorter one, I am not sure why they made one ity bity change like that
Main bearing diameter on ALL 318 polys, LA, and magnums are the same includeing the rear main. The differance in the magnum and earlier ones is the thickness of the cap, it was changed so you could use one pan for both motors.
Posted By: dogdays
Re: Are 318 Magnum pistons different from 318 LA? - 09/16/11 06:19 PM
To recap: Use the whole rotating assembly AND the Magnum heads on the LA block, this will give the best results.
As you're swapping a neutral balance 5.2 assembly for a neutral balance 318 assembly you don't have to do anything babout torque converter or flywheel balance. ALL 3.91 x 3.31 V8 engines are neutral balance.
OR, you could spend $100 and have the 5.2 block sleeved and finished to your existing piston size. Then it would be all 5.2 and you'd have the roller cam to boot.
R.