Inliners International Forums General Discussion Engines GMC 270 Head Question

I am cleaning a 1947 GMC 270 Short Block that I recently acquired. It was bored to .060 and came with flat top pistons.

I see that there were two heads that were available for this earlier 1947 270: 2136477 and 2193417 (casting codes).

I have a rough but original 228 with dome top pistons that has the 477 head. Yet, I also acquired a military 302 with .020 flat top pistons with this same casting code (477).

My questions are: Which head would be compatible to use? I can rebuild the 477 head from the 228 and install on this 270. Or should I search for another head that would best be suited for flat top pistons?

Is the 477 compatible with flat top and dome top pistons? There is also a more common 2194819 head - same would apply? Can I use this head too?

Thank you.

My guess is that your 270 is later than '47, likely a replacement. There are 4 heads. #1 domed chamber with small ports. #2 H head domed chambers with large ports. #3 302 head D chamber for flat top pistons and has large ports . #4 Later ( around '55) 270 head different D chamber for flat top pistons and has small ports. #4 is probably what you need. Piston for the late 270s are hard to find but if yours are good find a head. These heads are the least desirable for performance but are fine for a street build.
Domed pistons are easier to find and you can use either domed head. Again for a street build the small ports work well. The large 1 3/4 ports can cause flow issues at low to mid throttle.

My 270 is a '57 and has the small port D chamber head. I built the engine in 1978 when pistons were easier to come by. It is in a 1/2 ton '53 Chevy pickup with a T-5 trans and a Dana 44 limited slip with 354 -1 gears. It will cruse at 70-80 all day.

Thanks for the information. I am learning a lot. I measured the two 477 heads - and the port sizes are different on each.

The 302 head has 1.75" intake and central exhaust ports. The end exhaust ports are 1 3/8".

The (1946) 228 477 head has 1 3/8 intakes and 1.75" center exhaust ports. Same 1 3/8" exhaust ports on the ends.

So there are two variations of the 477 head - a large port and a small port head.

My intuition says that the larger port would be best for a 270 bored at .060. But you say the small port head will be fine.

Perhaps the small port head would be fine for street use. Can you help me understand this?

You may be right - it could be a 1957 270. This engine had a cast iron mount that was fit to a machined flat boss on it for a compressor and a 3/4" drain hole drilled into the block just forward of the fuel pump.

Cast code is G247. Serial number is 270 829597.

Pistons, rings and bearings are new. Wasn't used after rebuild. Just put away in a barn as a short block, and now I am cleaning it up.

The number you posted looks like the one stamped into the flat by the distributer. The first three digits are the displacement of the engine. the next digits tell a use code. That is helpful but the cast number is on the driver side of the block block near the middle just above where the pan bolts on. Check that number on the casting number chart and you can see what you have and what head went with it. I think '55-'63 and you'll want the small port D chamber head.

Bill Fisher, in his book, recommended the small port heads for the street. While the large port heads were best for all out racing most driving is not done at wide open throttle and the small ports provide plenty of flow for normal driving even with a heavy foot and a lot of mods. They all had the same size valves so the large ports can flow more than the valves. I think you can go all the way to 2' intakes but this would lower velocity of the fuel mixture and possibly allow the fuel to separate. Al least it is inefficient. The smaller ports keeps the volicity up.
You can go to 1.87 or 1.94 intakes and 1.50-1.60 exhaust valves and get a good flowing street head. Small block valves and springs work with some work. I think they are a little shorter. If you have the stamped and welded rockers they are plenty strong. You can shave the head about .035" if it hasn't already been done. The block can be decked but I don't remember how much. This will raise the compression maybe to around 9 to 1 or a bit better. The pistons you have are most likely stock replacements and quite heavy by today's standards but will be fine on the street. That is what mine has.Two one barrel carbs and headers will bring it alive. Find a good cam that will do what you want. It is important to be honest with yourself about what the engine will REALLY be asked to do. A truck does different work than a Bonneville racer. I suppose you will be somewhere in between.What is your engine going into? What will you use it for?

Go to the "casting numbers" window at the left of the forum page and find your number.

I had that book, and another - until I loaned it to a guy about 20 miles from here and I have not seen him since. For what I am doing - a 270 is better than a 228 and will be fine for the hills around northern New Hampshire. Either going into a 1950 Suburban or 41 GMC half-ton. I like the old school hot rods and these GMC' 6's. Thank you.

A '50 Berb with a strong 270? What a cool and versatile rig! '41 1/2 ton is cool too but the Berb has so many possibilities for hauling, camping, locking up your gear. I had a '50 3/4 ton panel for a while in high school when we were surfing. Too slow on the highway but we could live in it. A 270 and some gears would have made a lot of difference. My first GM six was a 216 in a $55 '48 Chevy fastback. It was the least of these '39 and later engines but it spoiled me forever.

Interesting. That is our plan with the Berb. Yes, hauling out of the weather, access, and locking up gear. Think I will add the 3.55 ring and pinion, this 270 bored at .060, a water tank heater running off the motor for a portable shower, roof rack, 12V, A/C conversion, and a tow behind kegerator on wheels. The tool box under the front seat? I'm converting it to a humidor for my cigars.

Ya know .... i have a 302 GMC ready to go too... bored at .020 but i am thinking the 270 has the power and easier on the gas for traveling in the Burb. . Think about a 302 in a little short bed 41 half ton. That should git me some groceries in a hurry. Ideas welcome.

I like your plans. These will be fun trucks and deserve to be used.
It is a shame that no one has written a new GMC speed book. Bill Fisher's is from 1952 when 302s were new and Small block Chevys didn't exist. There is quite a bit of new tech that can be and has been applied to these engines since then.