340 heads on a 350

**randyrhr** · 10-12-2009, 07:47 PM

I know this one has been batted around somewhat but wanted to relate a motor combination I just got in a project car . This had a '75 350 block with '66 340 heads, intake and exhaust bolted on. I never did try to run it - didn't care as a 455 is going in. Looked like it had actually been running. I'm thinking that if the 340 cam were installed it might run. I haven't tried to compare all the detail dimensions to see if it would work. The lifters looked like they lined up well on the cam lobes (all were just slightly off center) and none of the push rods looked "non-parallel" so I think the lobe/valve spacing was probably the same or very close.

Anybody had any experience with a combination like this or any thoughts about it. Just curious - not interested in trying to run it.

Randyrhr
67 GS 4spd convert project underway

**bob k. mando** · 10-13-2009, 07:21 PM

I'm thinking that if the 340 cam were installed it might run.

yes, it would certainly require a 340 cam. there doesn't appear to be any issue with the center pushrods interfering anywhere?

word has been that on a 340, the center pushrods are spaced further apart in order to get around the siamesed intake runners.

the 340 cam would also require adapter bearings. on the 215/300/340/Rover, the journals get smaller as you move to the back of the block. on the 350, they are all the same size.

Anybody had any experience with a combination like this or any thoughts about it.

we have been bandying this idea around quite a bit in the Buick community, especially since TA announced they were going to produce "Rover style" aluminum heads.

not interested in trying to run it.

dang, i know there are quite a few people who would like to see you do it though.

**randyrhr** · 10-18-2009, 10:18 PM

Sorry about the long delay - had a Class Reunion to attend - and a great time was had by all!

Anyway - to answer your question about the push rod alignment - I only checked it visually before pulling the heads but they all looked really, really, "right" .

Now you have me thinking about putting the heads and manifold back on and trying to at least start it - just have to find the time! BTW it had the correct orig '66 carburetor on it.

I'll do a little actual measuring and post the results sometime after the first of Nov (another short trip out of town getting in the way).

**bob k. mando** · 10-19-2009, 10:19 AM

any information that you can give us on this hybrid will be greatly appreciated.

if you'd rather not, it might be possible to locate a Buick fan in the NW who'd be interested in playing with it.

**randyrhr** · 10-19-2009, 03:31 PM

Just in case I've misread something - here are the numbers I took off the parts as I disassembled it - maybe you could double check my info:

350 Short block - casting no. 1241748 - 1975. There is an extremely small ridge and the bore is standard (3.80"). Virtually no rust or scoring and no visible cracks in the cylinder walls. The motor looks pretty good overall but it's been sitting for about 25 years.

340 Intake Manifold - casting no. 1374559 - 1966 4bbl

340 Heads - casting no. 1376330 - 1966 these look pretty good, they were bolted onto the 350 block.

340 Exhaust Manifolds - casting no. 1375901 - L, casting no. 1375912 - R.

**bob k. mando** · 10-19-2009, 04:27 PM

the 340 heads/intake/exhaust are easy to identify.

the 340 siameses the center intake passages while all of the exhaust ports have discrete runners. the stock 215/300/340/Rover exh manifold looks almost like a set of cast shorty headers.

the 350 has siamesed center exhaust runners and looks quite a bit like a big block Buick manifold.

There is an extremely small ridge and the bore is standard (3.80"). Virtually no rust or scoring and no visible cracks in the cylinder walls.

it appears that you have the engine in some stage of disassembly. if it has a stock type 340 cam in it, the cam bearings will have to be non-standard adapter bearings. all of the 350 bearing journals are the same diameter. the 340 was designed with journals that get smaller in size the further towards the back of the block you get. so, bearing thickness on the rear journal will have to humongous in relation to bearing thickness on the front journal if you have a 340 cam in a 350 block.

if they somehow custom ground ground a 350 cam to a 340 pattern, all of the bearings will be standard 350 bearings, ie - the same specification for diameter and material thickness.

**randyrhr** · 10-19-2009, 06:05 PM

Yes, I have the heads off but have not pulled the front cover (or cam of course). And from the heads up this is obviously 340 hardware.

I just went out and set one head back on with push rods in place. Measuring with a dial caliper and my eyeball here are some numbers for the center pair (340-intakes)

The top of the push rods, fore and aft spacing, at the rockers - 2.720
The lifters - fore and aft spacing - 2.550

Each of the actual intake/exhaust pairs are slightly wider at the rocker arms than at the lifters.

The head gasket push rod holes line up perfectly with the lifters and not with the rockers. The gasket pattern on the block looks like it fit the block just right around the cylinders and based on these two observations I believe they used a 350 head gasket (And it looks like it fit just right on the underside of the 340 head too) . Note too that the head alignment dowel pins are a perfect fit.

Now that I'm really looking closely, the push rods are definitely visually skewed a little but none of them touch any of the clearance holes through the head or the clearance holes in the gasket (which are actually spaced correctly for the lifters and not the rockers). The ends of the push rods, the rocker "sockets" and the lifter "sockets" don't show any abnormal visual wear patterns (I actually only cleaned up and looked at the center two so far).

The center two are skewed the most and each intake/exhaust pair is also skewed a little (spread out at the top as noted above). Interestingly, the third rocker in from each end looks to be lined up just about perfectly with it's corresponding lifter. All the others get a little further off as you move either toward the center pair or toward the front or rear of the block.

When I get a little more time I'm going to pull that cam and see what the bearings/journals look like - that will have to wait several days. I'll keep you posted.

Do you know of a reference that would have some useful dimensional info - like cylinder spacing, valve spacing, cam lobe center-to-center distance, etc that might help here. Bore and stroke and the bearing sizes are easy but some of the other stuff I haven't found yet.

Also - if you think of anything else I might look at or measure, let me know. I'm way too interested now so I'm going to get some gaskets and put this thing together enough to try starting it one of these days soon.