The Stovebolt Site Forums The Shop Area Engine Shop 248 vs. 261

Gentlemen:
I can get a 248 and a 261. Which is better performer if mildly built up? Or is it a case of "Six of one or a half dozen of the other."? Couldn't wait to say that!

What kind of performance are you looking for? For speed, the 261. For work, the 248.

The 261 being the big brother of the 235 would be my choice, lots of stock and performance parts around. The most popular GMCs for building were the 270 and 302s, now getting tough to find parts for. The only GMC I owned had a cracked head and all the replacement heads I found were also cracked and that was 20 years ago.

Have some local Inliners building the GMCs, they also have collected many parts motors over many years.

I found a 261 for the best price you can get"FREE"but with a cracked head,went searchin for one aroun here in my neck of the woods and came up with noth'n,any other heads inter change,like the sm blks do?anyway good luck with the six hop up!!

A head off of a 235 will fit the 261 just fine, but you should have the steam holes drilled. The 235 head will also give you more compression as well.

The 248 will cost you a lot more to rebuild than a 261 and the GMC parts are harder to come by compared to the Chevy parts.

261 it is! I hope..block # is 3769825...is it?

Thanks for the info.

Tom

3769825 doesn't show up in any listings I've seen, but 3769925 would be a 261 of approximately '60-'63 vintage.

Yeah Curt, that's what it is. I took all the gunk off and read it with my glasss on.

The head has a # of B51 848 ???, Does thsi mean it is a 235 head????

Thanks,
Tom

Yes it does, and the 848 is THE BEST head to have!

235 and 261 heads interchange if steam holes are drilled in the 235 head. Most 261's have been rebuilt before and war heads that came from 235's.

8's, 9's and 3's are hard to distinguish on the castings! To guarantee it is a 261, look for a pair of bars right above the starter. Check this link out for more details and pictures: http://www.oldchevytrucks.com/cgi/htmlos.exe/003035.5.2723214209400006236

Be careful which number you are looking at. My '62 261 has a head that says B51 848 over 850, and the real casting number says 3837850. You want the casting number on the top next to the temperature sending unit that starts with 383. The only differance between the 848 and the 850 heads is the compression and the steam holes, they flow exactly the same and use the same size valves and rocker arms. The combustion chamber and ports are the same shape.

The steam holes are necessary because the 261's have siamesed cylinders and need more water flow to the head to avoid hot spots. It is a good idea to run the bypass thermostat or provide circulation back the the water pump via the heater core when the thermostat is closed to avoid hot spots in the head and block when the water isn't up to temp.

4ontheFloor:
Yep, yer right. I wuz lookin' at the wrong #. I've got a 235 head and now this 261. Does the 235 head make a significant gain? Actually, the 235 head was rebuilt. So, I'll drill the steam holes and call it good. (I'm talking to myself).

50GMC253:
My 54 235 has those "Captian bars" above the starter as well...and so does this 261.

Thanks..fellas!

If you change the head to a 235 on a 261, you get about 0.5 more compression, so you get a small increase in torque and slightly better fuel economy. It is not very noticable though, but if you have a good 235 head go ahead and use it.

Everything Four on the Floor said.From my experience,all replacement stock style 261 oversize pistons place the piston around .040 below the deck at TDC.The original GM std size pistons are .020 below the deck.Fitting whatever flat top pistons close to deck height,lets say .010 below the deck by milling the block,using a 848 head with a clean up milling job,.010-.020,will yield about 9-1 actual compression,about all the compression ya can run on available pump gas with this head design.
Back to the original question,the GMC has a better breathing stock head than any 235/261,the 302 type head being the best.The head difference is worth maybe 20 percent more power on a modified engine,all else being equal.Another advantage of a GMC is larger displacement,up to 302 cubes stock,on the street,bigger is better.