poly 318 with LA heads.

Posted By: kams

poly 318 with LA heads. - 07/29/14 05:02 AM

Can you put LA style small block heads on a 318 poly motor?

I was thinking of boring it out to 4.04 and using 915 340 J heads 2.02 1.60 valves.

Any ideas.

Posted By: jeebis44

Re: poly 318 with LA heads. - 07/29/14 05:20 AM

I'm pretty sure the geometry's all wrong on many levels

Posted By: shinnery

Re: poly 318 with LA heads. - 07/29/14 10:22 AM

I think if you lay a LA head gasket on the poly block you will find no way to put intake pushrods in it.
Bryce

Posted By: therocks

Re: poly 318 with LA heads. - 07/29/14 02:14 PM

I agree they are different.Rocky

Posted By: Andrewh

Re: poly 318 with LA heads. - 07/29/14 04:33 PM

someone here or maybe another board used to say it would work, but I don't recall actually seeing pictures, or anyone else trying.

Posted By: Twostick

Re: poly 318 with LA heads. - 07/29/14 04:34 PM

I was always told the reason push rod angles suck on an LA is because they never changed the lifter angle from a poly.

Kevin

Posted By: HotRodDave

Re: poly 318 with LA heads. - 07/29/14 06:06 PM

LAs go on a poly easy enough, it is the other way around you don't want ot get involved in

They have the same deck height, rods, crank, pistons, timing cover, timing set, lifter angle, oil pump... just the poly block has clearance for the intake PR to go at a weird angle and the LA does not, if you try to notch you LA block for poly PR angles you will hit water.

Posted By: cudaman1969

Re: poly 318 with LA heads. - 07/30/14 01:26 AM

Quote:

I was always told the reason push rod angles suck on an LA is because they never changed the lifter angle from a poly.

Kevin

Correct, Chrysler was saving a dime from retooling. The pushrod angle was designed for the polyhead and they needed a smaller v8 package for the new a-bodys so a smaller head was designed for the block they had. The block had proven to be very good, all about money. I think the poly head would have been a better perfprmance head in the long run(canted valves).

Posted By: cknight

Re: poly 318 with LA heads. - 07/30/14 01:42 AM

Don't forget to use an LA camshaft, as the valve layout on the LA heads is different than on the poly. Regards, Chase

Posted By: kams

Re: poly 318 with LA heads. - 07/30/14 01:55 AM

I just want to use the poly block so I can bore it out to a 340 size and make a cheap motor.

Posted By: 70AARcuda

Re: poly 318 with LA heads. - 07/30/14 03:12 AM

Are you sure that the Poly 318 3.91 bore can go .130 to a 4.04 bore...?

Posted By: dogdays

Re: poly 318 with LA heads. - 07/30/14 06:19 PM

UT is your friend. There is no guarantee that any 3.91 block will go to 4.040.

Yes, the LA (Light A) block is lighter than the Poly, but where did the weight change come from? Hasn't been researched, AFAIK. Mopar castings were all over the map, early or late.

R.

Posted By: HotRodDave

Re: poly 318 with LA heads. - 07/30/14 06:48 PM

They were way thicker in general but core shift was worse so the odds are better but still not good, especially considering 60 years of corrosion from not being serviced properly...

BTW I have a 71 318 block in the shop right now that is 4.070 bored and was a running motor with 12.5 compression, no block filler and was being dirt track raced till it developed a rod knock... I am scared to reassemble it

Posted By: dogdays

Re: poly 318 with LA heads. - 07/30/14 07:28 PM

It'd be interesting to sonic check it.

R.

Posted By: HotRodDave

Re: poly 318 with LA heads. - 07/30/14 07:53 PM

I sonic tested it by tapping the bores with a screw driver and they sound like glass, the block weighs 122# and a 68 std bore block (same casting number) weighs 165# on the same scale

Posted By: dogdays

Re: poly 318 with LA heads. - 07/30/14 11:51 PM

43 lb is 43 / 0.258 = 167 cubic inches of material. Calculating a 6" high cylinder 4.040 inches diameter, subtracting a 3.91" cylinder 6" high I get around 48 cubic inches, so your numbers work out.

I bet you don't have any cylinder wall with more than 0.100" thickness.

R.