The Stovebolt Site Forums The Shop Area Engine Shop 261 Chev. -V- 270 GMC comparison

I am curious how really does the 261 Chevrolet 6 compare to the GMC 270, in both performance and also life expectancy, given normal care and driving?
I have never driven a truck with either engine so I have no reference on either engine.
Al

The only thing better than a GMC 270 is a GMC 302. Chevy can't hold a candle to a good GMC. The downside is parts availability and price. You could probably build at least two 261's for the cost of building one 270. Back in the 1950's 270's were a dime a dozen, mostly WW II surplus military engines. Round track racers and Bonneville speed record contenders snapped them up pretty quickly. Now they're very scarce and expensive to build.
Jerry

GMC engines are also about 3" longer that Chevies, so that needs to be accounted for if put in a Chevy truck. I replaced the 228 with a 302 in my pickup. Have a couple of spares.

Ed

I am a GMC guy and am running a factory original 248 in a dump truck. I have acquired a 302 that will replace the 248 some time in the future. In my opinion the 248 has better torque than a 235 (rightly so as it has a little larger CID). I also have a 1946 Chevrolet 1.5 ton that has the original 2167/235 engine. I have picked up a later full pressure 235 to replace that engine with. I have also been kicking around the notion of locating a 261 instead. Comparing CID between the two engines, the 261 should be a very near performance competitor for the 270. Is that performance comparison guess correct? Does the Chevrolet 261 have any inherit mechanical issues that I would need to recon with? I assume that the 261 would be a very easy replacement engine for the original design 216/235 that is currently in the 1946 1.5 truck.
Al

If it is a 1955-2nd or later 261, you would/should have to do a change in water pump, water-pump location, and maybe pulley.

Check the Tech Tips here.

Also, look at: http://chev235guy.blogspot.com/search/label/Water%20Pump%20Relocation%20Adapter

I have ran both the Chevy 261 in school buses and also the GMC 270. I liked both engines, and both served me well. With reasonable care they both will run many trouble free miles.

The 270 has stronger rods, smaller intake valves, larger intake ports (with the big heads).

Where is the 3" extra length from? The bore spacing difference isn't nearly that much.

When did the 261 come with full pressure oiling? Was it with the second series issued in 1955? If I were to hunt for a 261 I would want the full pressure oiled version.
Al

The 261 was introduced in 1954, and it was a high pressure engine. In that year (and later), the 235 was also a high-pressure engine.

Thanks Tim for your response. It sounds like any 261 built would be a "full pressure" engine.
Al

The 261 we built for John's 1 ton pickup about a year ago over at Stovebolt Headquarters had about 60 PSI oil pressure on a cold start, using 15W-40 oil. I think that would qualify as "full pressure"! Of course, it dropped a little after warmup. We intentionally built that one with some pretty close tolerances, including reconditioning all the connecting rods on my Sunnen hone. Right now I'm working on a buildup routine that should produce a spray-oiler 216 with about 40 PSI oil pressure at hot cruise RPM.
Jerry

In some ways the gmc “seems” more stout in design compared to the 261. Vertical exhaust valves on the gmc vs angled on the 261 and, non-Siamese bores on the gmc and the list goes on. But displacement is like splitting hairs. Each will last as long with proper care and maintenance. I just prefer the gmc for some reason but certainly will not scoff at a 261 or 235. Maybe that’s the reason I have 3 261’s on roller stands, but rebuilding a 302 and 41 gmc even with the little 228.

If I was into old school racing, hotrodding and performance, I would definitely lean toward the gmc

The deeper you get into your GMC, the more apparent it will be that it's a commercial engine- - - -designed to work hard all day, every day, and be rebuilt several times before it's scrapped. As mentioned above, the connecting rods are much stronger, and they're drilled their full length where the wrist pins get pressurized oil. The rods also have replaceable big end bearings, and bushed pin ends, which can only be accomplished with a pressurized crankshaft. The oil pump is at least twice the capacity of a Chevy pump. Ditto for pistons- - - -bigger, and heavier built. The cylinder head is better-designed, with bigger and less-restrictive ports. Chevy got pretty close with the 261, but they pretty much maxed out the design that began with the old 194 engine in 1929. GMC got over 100 cubic inches bigger with the 302, something the 216/235 design couldn't even approach.
Jerry

Are cams available for either the 261 or the 270? Or do we find a cam grinder to renew the cams?
Al

A brand new cam that duplicates the timing and lift of the original 261 is available from several vendors. Elgin Industries makes one that only differs from the OEM cam in the lash adjustment specification. The 261 cam was the same one that was used in the 6 cylinder Corvette in 1953 and 54. GMC cam cores are a little scarce, but I believe a few custom cam grinders still offer regrinding services. Once again, most of the available supply of GMC valve gear was used up by racers in the 1950's and 60's. What's left is E$$$$pensive!
Jerry

Thanks Jerry for your responses.
Al

Please place a Want Ad in the Swap Meet (not in here).

Interesting discussion. I live in the Memphis area, and know where three or four fully intact GMC trucks are that are for sale. The bodies are essentially scrap, but the engines and transmissions are there and have been under sheet metal. Not sure the size but the trucks range from 1/2 ton up to a GMC 470.

If you have the space and money those gmc’s might be worth saving definitely if a 270 or 302. But many parts from the 228 248 270 and 302 interchange. Not sure that a 228 crank, head, and exhaust manifold marries to a 270 or 302 but 90% of the rest will fit. Good engines!

I'm in the process of doing some horsetrading with a neighbor for a 51 GMC half ton rolling chassis with a SM 420 and a 270 engine. He's putting the sheet metal on a S-10 frame, making a V8 powered rat rod out of it. He says "The engine is locked up- - -it's junk. I'm not going to tell how many locked-up engines I've resurrected in the past 50 years!
Jerry

Hey Jerry, Tell us the "Paul Harvey" version of the story on this new project you mentioned.
Al

Which one? At the rate I'm going I'll need to live another 73 years! I'm not really impressed with how many 216 engines are going to scrap because people think they're some kind of fragile antiques. Spray-oiler engines aren't all that difficult to keep running, and with a few upgrades, they can come very close to the performance level of a later model 235. Let's look at some of the downsides to running a completely original 216:

Cast iron pistons- - - -heavy, rough on rod bearings, and they limit top RPM.

Babbit rod bearings- - - -these things are a really good design, and they can be adjusted with shims to maintain close oil clearances, but the old geezers like me who remember how to work with them are getting long in the tooth!

Low oil pressure- - - -Chevy just didn't put a big enough oil pump in the 216, so once the bearings wear a little, the oil pressure goes away.

Rear main bearing rope seal leaks- - - -just like other stovebolts, the performance of rear main seals are mediocre at best!


Now- - - -here's the plan- - - -Machine the rods for insert bearings- - - -not the pricey, scarce OEM-replacement ones; use Massey-Ferguson tractor bearings intended for several sizes of Perkins Diesel engines. They're plentiful and cheap, and it doesn't take much work to make them suitable for spray-oiling. An added advantage is the fact that these bearings have a crankpin diameter .060" smaller than stovebolt connecting rod journals. That "worn out" 30 undersize 216 or spray-oiler 235 crankshaft can be reground for a std. diameter Perkins bearing instead of being welded or scrapped.

Next- - - -bore the 216 for std. diameter 235 aluminum pistons. The wrist pin height and diameter are identical to the 216, and the .062" overbore brings the displacement up to 224 cubic inches. Aluminum pistons are much easier on rod bearings and the engine can rev quicker and run faster without damage or vibration.

Oil pressure- - - -fabricate a spacer about 1/4" thick to fit between the oil pump housing and the cover and run a set of 235 pump gears- - - -about 20% more volume, more pressure, and no problem maintaining 30-40 PSI at cruise RPM.

Leaky rear main seal- - - -machine the block and rear main cap for a full circle lip seal- - - - -no more drippy bellhousings!

Add a HEI distributor if the truck has been converted to 12 volts, or put a Torrington needle bearing into the top of an OEM point type distributor, and eliminate all the top end shaft wobble that messes with spark timing between cylinders.

All the above upgrades can be incorporated into a routine major engine overhaul with only moderately more cost than building the engine like they did in the 1940's! Other than the HEI, the engine will still look original, also!
Jerry

Just a tip here. Over the years I was into WWII military jeeps. There is a forum I went to virtually every day, G503.com. G503 was the government number for WWII Jeep. Anyway, long story short, there were other military trucks on the forum, including Chevrolet and GMC. According to those guys, 350 pistons can be used in the GMC 302. Seems like there was other parts interchangeability but I can’t remember. Me personally I’d go with a GMC engine.

It sounds like somebody was shoveling some fertilizer there - - - - -the 302/350 bore size is right (4 inches) but things like piston weight, wrist pin size, and a few other dimensions like compression height will be WAY off. It might be possible to adapt a 350 piston to a 302 six, but it's going to take a very skilled machinist to do it.
Jerry

Jerry, Your "Paul Harvey, the rest of the story" was great as you speak about the technical improvements to the 216 that raises the performance and reliability bar for that engine type. It almost has me convinced to do that on one of the 216 cores I have laying around. Here is a question, does that amount of overbore (.062) offend or make marginal the block in some way to set the table for an overheating problem. Fellows I know, back in the day, were quick to overbore the 283 to try and get more cubes and end up with a 301. Most after that had snappy little engines but also had to deal with a heating issue. I am going to chew on your suggestions.
Al

It was pretty routine in the 1950's for places like Sears, Montgomery Ward, and a few of the other mass merchandisers to offer mediocre-quality rebuilt engines with 30/30 reground crankshafts and 80 overbores. Most better-quality rebuilders considered 60 over to be an acceptable bore size, and 20/20 cranks were very common. There's a lot more block material available if long life isn't a concern- - - -Dad's round track flathead Ford V8's had .250" overbores, 3/8" stroker cranks, 11:1 compression, and ran an alcohol/Peroxide fuel mix at 6,000+ max RPM- - - -all on 3 main bearings! A well-built engine might last a full season of dirt track racing, while others did some epic hand grenading!

I believe a 60 over 216 should last a long time, particularly the way they're run now, in weekend cruisers and car show trailer queens. I wouldn't hesitate to use an engine I'd built myself like that as a daily driver.
Jerry

Jerry,

Thank you for all of the valuable information that you provide us everyday, here on Stovebolt. I know that I speak for all of us, when I say we really appreciate it. You have forgotten way more than I have ever learned, when it comes to engines. Your post on how to rebuild a more reliable 216 engine is no exception. When my original 216 starts to falter, I am going to look to rebuilding it, in the manner that you described. That sounds like a perfect balance for us that likes the original engines, but would like to improve them, to meet our needs.

Again thank you for your time and efforts.

Rugo

Thanks for the kind words! Like the Blues Brothers, "I'm on a Mission- - - - -" only this one is to save as many old 216's as possible from the scrappers! I've got several stashed away, and I'm building the machining fixtures needed to do the upgrades I've described. In the not too far distant future, I should have one of those resto-mod engines ready to do some dyno testing.
Jerry

Jerry,

You are welcome. Glad that you are on a mission. Keep us posted. I am very interested in the results, and in eventually in making your improvements.

Rugo

Here is another curiosity question. If I were building a Salt Flats car, (which I am not), and wanted the cubes of 261 or 270, and weight is now a serious consideration, what is the dry weight of the GMC 270 as compared to the dry weight of a Chev. 261?
Al

261 was between around 610 lbs and around 630 lbs, depending on equipment (such as clutch assembly parts)?

Just a SWAG, but I believe the GMC is a bit heavier. However, the better gas flow through the GMC cylinder head would probably make the added HP and/or torque gain of the GMC over the Chevy offset the weight advantage of running the Chevy. Chevy ports suck like a Hoover upright vacuum cleaner, with no practical way to improve the flow. There are still a few custom-cast 12 port heads around for the Jimmy engine, if your last name happens to be Rockefeller, Vanderbilt, etc. and you can afford to buy one.
Jerry

Serial number on GMC engines starts with displacement, i. e. 302xxxxx. There are 4 different GMC heads, identified by numbers CAST in top, seen without removing valve cover, come to fit certain pistons; therefore all heads will not work with all pistons. 2 have small intake ports. Some rare stock intake manifolds come with 2 barrel carbs(military?)

Patrick's catalogue has Casting numbers, if you have one of his old catalogs, can't find mine now.

Ed

GMC 270 = 790 lbs subtract 120 lbs for flywheel, clutch assembly and bellhousing gets you in the range of 670 pounds. Don’t ask if wet or dry lol! Of course add in 15 for generator and brackets . Two inches taller and 3” longer than chevy block and head. From Inliners International.

The problem with any LSR is that the OEM head casting engines in XO have a 325" displacement limit.

Panic, I am not up much on acronyms, what does LSR refer to?
Al

Sorry, "Land Speed Record".

Thanks for the LSR definition. I knew what to expect with the term "SLR" (single lens reflex) type camera but that does not relate to this subject at all. I would like to build an LSR type car but I would be using a straight 8 Pierce-Arrow engine, I have stashed out back, for that project, not a Chevrolet 6. However, the reality is' it will probably not happen. I have just to many projects and to little time. Maybe for someone else..... On the subject of the 261 Chev. engine. Is the 261 the only engine that came out with the two grooves running down the length of the valve cover?
Al

I think the 235 and 261 engines that did not have a PVC system did have the same valve cover (in the same year).

I do it know if the PCV systems valve covers were the same in the same year.

The Pierce sidevalve 366" (3.500" × 4.750") L8 is (AFAIK) legal for XXO, which has a 375" displacement limit (or sleeve the 385" engine down?).
I suspect that, in addition to the obvious age and wear problems (where to get spares? although pistons in that size are very common), the cranks are fragile due to length, 9 main bearings, and should be examined closely for cracks. A current design harmonic damper is strongly recommended.
If you decide to modify the engine, pretty much the same things that apply to the Chrysler 265, Ply-Do 230, Pontiac 239, etc. SV L6 engines (reground cam, bigger intake valves, careful seat preparation and block relieving, stronger valve springs, minimal quench distance) are sources of information. The intake and exhaust designs will be fairly similar to Buick OHV L8 engines as to grouping, firing order, etc.
The exhaust ports are individual, with some adjacent (but not siamese): 1, 2 adjacent to 3, 4 adjacent to 5, 6 adjacent to 7, 8
The intake looks like 4 siamese ports: 1-2 3-4 5-6 7-8
Cam lobe placement: Ex In Ex Ex In Ex Ex In Ex Ex In Ex

Great pictures: http://www.claytonrestorations.com/posts/bwg_gallery/1932-pierce-arrow-engine/#