Thread: Block and Head Casting Numbers help

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Hi All,
New member here.

My current project is a 1985 GMC 1/2 ton 2 WD. Super nice, rust free truck.

I've got 3 SBC engines in stock.

Block: 3970010
Heads: 397487
Appears to be a 350

Block: 3914660
Heads: 3947040
Appears to be a 327

Block: 3970010
Heads: 3932441
Also looks like a 350 pistons stamped .040 so its 40 over making it something like 357 CI

Any thoughts on which combination of these would be best for a mid range RPM daily driver would be very much appreciated.

Trans will be a 700R4

Rear ratio is 3.08

Thanks in advance.

You’ve left out some important information that is needed to make any really meaningful recommendations. That being what do you want the truck to be able to do once you get it built. Is it going to be an in town cruiser, is it going to be used on the interstate, etc. What kind of compression ratio are you looking for (do you want it to run on regular or premium fuel.

In generalities I would probably go with one of the 350s just for the cubic inches. Chances are both are going to need to be bored, so it probably really doesn’t matter which one you use.

As far as heads go the 397487 are a pretty good large chamber choice. I would stay away from the 3947040s which are a smaller chamber head that will make more compression, but the big down side on these heads is they don’t have the accessory bolt holes which may make finding bracket a problem and possibly pricey.

I like the 700R4, but would really relook the rear end ratio. With any of these cylinder heads you’re basically looking at 1960s cylinder head tech. Depending on the cam you choose, these engines have a happy spot when cruising of around 2700-3200 RPM.

There a few different OD ratios for the 700R4s, a common OD ratio is 0.69. With a 3.08 rear gear the effective rear ratio becomes 2.13. With that in mind and depending on the tire size, chances are the engine will be lugging when the transmission shifts into OD.

The deeper the rear gear you use, the more fun the truck becomes to drive (just an old hot rodders opinion LOL). From personal experience one of the last rods I built used an OD transmission that also had a 0.69 OD ratio. I was running 4.56 gears in that rear end and in OD ( effective ratio 3.14) with a lockup convertor it would comfortably run at 75 MPH with the engine turning around 3000 rpm. I’m currently building a car with an OD transmission that also has a 0.69 ratio and am planning on using 4.30 gears (effective ratio 2.96) and a 28 inch tall tire.

Just an old mans 2 cents, and welcome to the forum.


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Originally Posted by Nardulli

Hi All,
New member here.
My current project is a 1985 GMC 1/2 ton 2 WD. Super nice, rust free truck.
I've got 3 SBC engines in stock.
Any thoughts on which combination of these would be best for a mid range RPM daily driver would be very much appreciated.
Trans will be a 700R4
Rear ratio is 3.08 .

Welcome Aboard! Nice to see you here. We've got a couple guys that are really good with selecting Chebby parts.. Myself. I bleed Blue but I can offer you this much. There's no substitution for cubic inches so I'd start with the 40 over block. Then select the heads that can flow the best, cam? Depends what you want to do.. Towing? Cruising? Daily Driver? All the above??? Always a compromise right!?!?! Overdrive Tranny is always a good choice now and the rear gear ratio is alright, not my first choice.. I'd be more inclined to go with 3.25 minimum or more probably 3.5 / 3.7 but that would also depend on the use. If I had to drive a long commute each day.. Then the 3.08's look much better! Let's see what Glenn has to say, he's an ace with the SBC.

Welcome to CHR!
As indicated above there are several questions that should be addressed before getting too far along the engine build path. Could you tell us a bit about the truck? If memory serves me correctly, the 3.08 rear end was used for the 305 with a three speed manual transmission in both Chevy and GMC trucks of that era. Is that the combo that is in the truck now? Do you have a budget? (Ha, Ha, Ha – right….. I just thought I’d ask.)

Starting at the rear end, if it is in fact a 3.08, I would recommend swapping out the gears for a one of the following (Assuming your 700-R4 transmission has the standard ratios, i.e., 1st - 3.06, 2nd - 1.625, 3rd - 1.00, and 4th [OD], - 0.69):
3.90 - for cruising and all around daily driving and occasional freeway
3.73 – if you’ll be driving on a lot of 55MPH plus roads
3.42 – if you be commuting at or above 60 or doing a lot of freeway driving

The stock converter in the 700-R4 is fine if you keep the build at or around 300 HP.

I would also concentrate on the 350 blocks. All things equal, if the one that has been bored .040 over is in good shape, use it. I would only use the other 350 if it is a 4 bolt main (or the bored one is bad). Completely disassemble the engine (don't forget to mark/label the rods and their respective caps) and have a competent machine shop hot tank and magna-flux the block. If it’s good, have them measure the block for being square. If it has never had the decks cut, it should measure 9.025”. If so, have them square it at 9.010”. This will give you a good base to build from. E-bay the other engines as cores.

Assuming the rods are good (they almost always are), have the machine shop clean them up and shot peen them.

Have your machine shop check the crank and only if necessary, regrind. In either case – buy new rod and main bearings from the machinist.

If the bores are in good shape without excessive taper, buy new .040 over cast flat-top pistons and a good quality set of rings from your machine shop and have them press the new pins into the pistons.

Have your machine shop install new soft plugs, all threaded plugs and new cam bearings. Buy a good double roller timing set from the machine shop as well.

Cam and lifters are a bit of a “personal taste” thing. I’m okay with about any brand but for anything except extreme builds, would recommend you invest the money (+/- $1,000) to convert to hydraulic roller lifters and cam. It’s a bit of work, but the safest way to build an engine that will continue to perform well over 100K miles. For street I like to use 110 degree lobe separation and stay at or below 270 degree advertised duration with lift no greater than .525”. If you use a bigger cam, it will begin to sound like a flatulating elephant and you’ll embarrass yourself in front of the rice rocket crowd (as well as have bad performance). Howards (and others) make complete kits that include new distributer gear and fuel pump rods. If your machinist isn’t familiar with such, call the tech desk at Summit and talk through what you’re doing and make sure you get all the bits and pieces. I like the cast aluminum front cover when installing a button on the cam (to keep it from “walking”).

I’m assuming you have 3973487, or 3973487X. This 487 casting was found on the ‘71-‘72 350 LT-1 engines and has 1.94/1.50 valves, 76CC combustion chambers and accessory holes. The 487X has 2.02/1.60 valves. The 487 heads will flow as well as the old factory “Camel Hump” head. The big problem is they are open chamber have poor combustion and burn characteristics compared to the earlier closed chamber style heads (or any modern after market head). If you have the 487X heads, 2.02/1.64 valves, they would be worth considering after a cleanup and magna-flux. Note that these are some of the last heavy castings and are not as prone to cracks. The 487X also got screw-in studs and guide plates. The reason I recommended cutting the block to 9.010” is it will help a bit with final compression ratio. Bear in mind that with a roller cam conversion, you would need to install new valve springs and use different push rods. Use new head bolts.

3947040 - They are 64cc with 1.94/1.50 valves but do not have accessory holes. E-bay them for cores - ($50.00 plus shipping).

The 441 head was probably the most popular of the 76cc castings. It was used on a ton of '69-'70 250/350HP 327 engines. Not a bad head, but it has 1.94/1.50 valves and will never flow as well as the 487 casting due to the small 155cc port volume. It does creates great torque and excellent drivability (think trucks, big Impalas, and Caprices), but wheezes after 5,000RPM. E-bay these as well.

Okay – after all that, here’s a couple other things to consider:

Aftermarket heads – aluminum heads are reasonable and already setup for roller lifters/camshafts. Again, a call to the tech line at Summit or Speedway telling them what you’re doing would help you get a good set of heads that are ready to bolt on and go.

Pullout engine/transmission from a late model vehicle - Done correctly, this is a drop in that can be very satisfying and brings your technology forward 30 plus years.

Final note/thought - buy what you can from your local machine/speed shop. We need to honor these men and women by using their services and they are good people to go to when a problem arises or you have a question. There is great value in a set of grease stained hands that resemble a pair of well worn leather gloves.

Probably too much information – others will chime in I’m sure

Best,
Glenn

Welcome to CHR! I won't try to add more details, but I will echo what's been said about choosing your rear end ratio. Decide on the tires you're going to run, confirm your 700R4 OD ratio (likely 0.69), and select your cam to give you a fairly flat torque curve. With your tire OD you can run the numbers to find rear gears that at highway cruise speed sets your RPM towards the bottom of the flat part of your torque curve for good economy.
When I was doing my coupe (Tremec 5 speed), 30" tall tires) the tranny guy suggested 4.11 gears and I told him he was crazy, I wanted a highway car, not a drag racer. He told me to run the numbers, I'd be turning 1995rpm at 70, and getting up to 70 would be like a rocket! On my Ford 347 (302 bored & stroked) my torque curve is steep from 0 to 1800, starts flattening to about 2000 and stays there until it rolls over at about 5800 to 6000.

Good luck on your build. Glad to have you here, and we LOVE pictures of the build process!!