GMC Inliner question Diff between 270 and 302

what is the difference between a GMC 270 and a 302?

Is it just bore or is the head different??
Do they share the same Crank?

Thanks.

 

Isnt' the 302 a V6 or was that a different one, 305 maybe?

 

All 32 extra cubes came from a larger bore. The crankshafts are dimensionally the same, but the 270 crank weighs 8-10 pounds less than a 302 crank; GMC racers used to use a 270 crank in a 302 block for less reciprocating mass, which helped revs. While a 270H (large port) head and a 302 head will physically interchange, it would result in ultralow compression on the 302. The 270H used a round combustion chamber and domed pistons, while the 302 used a D shaped combustion chamber (looks much like the chambers in an early '50s Olds V8 head) and flat top pistons. Some daring individuals bored 270 blocks out to 302, but that was in the day when GMC blocks were both plentiful and cheap. Most 270 owners stopped at 292 cubes, leaving a little meat for saftey's sake.

 

Sounds like the 270 may be the way to go. With a big bore, and go with lightened rods and pistons so it will rev a little more.

I thought the GMC combustion chambers were flat and you changed compression with the deck height, the Chevy has the combustion chamber in the head?????

 

No, the GMC engines all have a conventional style combustion chamber in the head. Some of the older engines had a huge chamber that required what looks like a high compression domed piston just to get it to 6.5:1. On the upside, they were cast quite thick and you could mill the hell out of them. Years ago, I bought a pile of GMC junk from an old guy that won at the dirt tracks with them in the '50s. In the pile was a 270H head that had been milled around 3/16 of an inch.

 

I realise this is an old post, but I am interested in reviving it.
I recently milled 100thou off a std 270 head. I am still using the standard bore (3 25/32) .
Does anyone know the volume of the pop up on the 270 piston? I am trying to calculate my final comp ratio..
I know the final combustion chamber volume is now 125cc, but I need to subtract the volume of the pop up. (Engine is now reassembled and running, so I can't measure it)

 

No dome info, but will share this casting/forging number chart with you.....
http://www.oldgmctrucks.com/Photos/GMC_Numbers.htm

 

One differance not mentioned here is that the 270 and smaller GMC motors have the crank centered under the cylinder bores and the pin offset in the piston as is normally done. The 302 crank is offset and the pins are centered.

 

why ya asking? anything new going on?

 

Now's that is a bit of trivia! 302's also have larger wrist pins,maybe just the later ones?

 

Tony, i thought all the GMC 228, 248, 270 & 302's shared a .990 wrist pin diameter with BBC's

 

I had a 1957 302 I built.The wrist pin bushes are something like 1.060.I had custom bushings made so .990 Big Block Chevy pins could be used.Actually custom wrist pins would be cheaper the the cystom bushings

 

Wrist pin size changed in 1956 ...

 

You guys Rock! ha ha, so who's got the stash of 270h heads? How about a complete 302?
PBRmeASAP, I have a strong 270 in the Hot Rod, but there are a few Kiwi's starting to look at HA/GR's. My 270 has been the 'practice' engine, with regards machining, and we are looking to take the tricks we have learned and build a race motor. S far we have found a set of Rods (jag) that should work, and I am on the hunt for suitable off the shelf pistons. Obviously with my std 270 head I need something up to 4" (Will bore to suit) that has a popup. I have a line on a set of McGurk 302 +0.125 but my block won't go that far without looking at sleeves(?)

Thanks RichFox, did not know about the offset pins. If I use custom or sbc pistons, I may have to watch rod to bore clearance?

 

Never had any any grief with mine. Used a number of bore diameters, rod lenths and cast and forged pistons. They all worked.

 

After some investigation,this statement ain't true I'm told

 

Yeah. Who told you that?

 

Well,I thought about it,228-302 all use the same timing gears.If the crank was off center in the 302 only,it would require a different timing gear unless the cam center was also moved.The cam wasn't moved because the front timing plate is the same for all engines.Except for external mounting holes on some.On my 302 I was actually using a Chevy 235 gear set.I might be wrong of course,but what's your explanation?

 

It seems like if you moved the crank centerline you would need to change the timing cover or at least the mounting holes to keep the crank snout centered in the seal. Don't think there is any differance in covers that i have used. So that would mean the holes would need to be moved. That would be OK. If the cam was moved to maintain it's relationship with the crank I guess that the covers would interchange. Then the lifter bores would need to be moved to maintain thier relationship with the cam. And theres the distributer oil pump drive. Still I went to Army school in '61 on 302s in 2 1/2 ton trucks and set my first GMC record on the salt in '78. Still hold Bonneville and El Mirage records with a GMC. And through all this time I have been told that the crank is moved. I believe it and don't know why the cam and everything else couldn't have moved with it. I used to think the Moon was made of green cheese, so I have made some mistakes. I don't think this is one. Or maybe they just changed the bore centerlines and left all the rest where it was. That way the crank centerline is moved off of the bore centerline and you don't need as much tooling changed. i bet that's the answer you wanted.

 

The cylinder bores moved to one side .060 sounds better than the crankshaft got moved over.

 

30!

 

Let me guess. Your a laywer?

 

maybe a little off topic, but whats the distance from one intake runner to the other. are they the same for a 270 and 302, i heard it's 8.5" for a regular 235, is this different?

 

I can measure an intake later, but I think GMC heads are more like 9 from center to center.

 

Yes a manifold that fits a 270H (Big ports) will fit a 302. The heads will bolt on to either block. 302 engines that I have seen have a D shaped combustion chamber while 270 engines that I have seen have a large open chamber and a pop up piston. 235 is smaller in port spacing I believe. The whole engine is shorter so it seems like the manifold would be also shorter. Never tried it though.

 

Chev & GMC intakes do not interchange without cutting/welding.

 

I measured my McGurk dual GMC intake tonight. Center of port to center of next port is almost exactly 9 inches.

 

Hey guys, now I know this thread is about 100 years old but it's the closest I've found so far for what I'm looking for.

I have a 228 in my 52 Suburban. I want to know if the intake port spacing is 9" across the board from 228 through to 302?

And if anyone has a dual intake that isn't gold plated for sale that would be great.

Cheers

 

Spacing is the same, but there's big and small ports. 228 is small port, 270H and 302 were big. You can use adaptors for intakes to suit port size.

 

OK, so I'm looking at this manifold. It has 302 stamped on it but the ports measure approx 9 5/8" - 9 3/4".
Picking it's for a bigger GMC 6?

Sent from my SM-G980F using The H.A.M.B. mobile app