Projects Old school cool 248 Gmc

Only a 1940 270 block will go out to 4". I lost a later year 270 trying to get to 3-7/8" so I could use stock 283 rings. Now I only bore to available ring size..Good luck

 

Hot Tank, magnaflux and sonic test the block first.

 

It will be cooked, blasted, then sonic checked (depending on the cylinder condition for ultra sound) and magnetic particle inspected. Sonic checking will allow us to bias a cylinder if it helps keep the thrust side thick. If we have too I will sleeve a thin cylinder. Probably still do a short block fill. I was told this was a engine left on a chassis with no body so I guess it is possible I will be saving it (or sealing its fate). It is a bit of a gamble but I want to use the Jahns pistons and head if possible. If all else fails, I will sell the parts and build the military 302 I was saving.

 

Stopped by the machine shop on the way home. The news is the yard pulled the engine and took off the head and pan to be sure there was no bad surprises. Has had water in some cylinders but does not have split cylinders and a bent rod like the 302 I bought from them. They are 2 hours away so I told them if they came up to Fairbury drop it off or I will get it next week. They race circle track and have been coming to this shop a lot lately. Good machine shops are running thin around here. Lucky to have this shop local and to be such good friends with them.

 

I bought a nicson dual 270x, 2) bxov-2 1-7/32 99-25 and 2) 30s Chrysler air cleaners. Hoping it will be enough for a "road job". Does anyone know the proper color for a nicson?

 

Some were metallic blue, some metallic burgundy ...

 

My 3 single 270 is metallic burgundy.

 

both of mine are metallic burgundy. Thinking it should be again.

 

Well, I just sold a 2000 6.5 Diesel I had for another project to the machine shop for an engine they are overhauling that is cracked. It will just cover the purchase price of the 270 so at least my pickle jar cash won't get any lower for the purchase. Hoping to get the 270 late next week.

 

Since I am saving this from the iron pile, max overbore shouldn't be a sin.

 

spent all day trying to get this 270 apart. Hammered the pistons, lifters, and cam out. Split one cylinder but is was horrible pitted anyway. shaking and baking now.

 

cleaned up nice and showed no cracks on the exterior. Pistons said .050 (seems like a odd size).

 

crank, and front mount cleaned up good, timing cover rusted through.

 

Well I’m subbed and enjoying all of this...
Regards, Randy


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Well I am glad it isn't just for my own entertainment. Not everyone thinks these old GMCs are cool.

Beating those pistons out with my buddy holding a semi king pin while I hit it with a sledge hammer made me feel like I was beat to death the next day. Still feels good to be saving some parts that would be sent to the iron pile by most people.

 

So though it is heavily pitted the crank is currently standard, so at least there is material to grind. The rods will be going .030 because I can reuse the bearings I have. The mains I will have to buy after grinding. I will try and slip down to the shop each day to tap holes, convert to full pressure oiling and clean more on the block. Then Sonic check and bore. Then assemble to measure deck height, then disassemble and deck.

 

I'd usually rather be the guy swinging the hammer than the one holding the bar. I was born in Fairbury, NE about 70 years ago. And I remember going to an old machine shop there about 60 years ago. I had a Fairbanks Morse crankshaft I needed changed to fit a Cushman clutch. Can still remember the guy telling me he was going to "cut" the shaft, not "grind" it. Sorry to digress. I guess some of us been doing this stuff a long time.

 

My buddy had a chain vice grip on the king pin and was holding on the end so avoid bad swings, but we didn't have any.
I would say "cutting" uses an insert and a lathe, "Grinding" uses a grinding wheel, but you never know.
My family has been in and around Fairbury since the 1850s so you may know some of my kin.

 

Cleaned and tapped holes today. Then sonic checked the block. Thinnest spot was .198 and everywhere else .220 + need to bore it another .120 so .060 per side. Plenty thick for a street motor, esp with a short block fill. 270 deck measures 11.367 and the old 248 11.11 so 1.63 difference after the stroke difference. According to my 1955 Wayne catalog 248 deck is 11.25 so there is lots decked off (and looking in the water holes it shows it). We are going to bore it, mock it, measure the piston height, then 0 deck the block. We will cc the heads before and try to get the piston cc. Just hoping for 9 to 1 or so. Priced lifters today $20+ a piece has me strongly considering having the tulip lifters I have faced and getting a forged steel core for regrinding. Looking at Bullet as a cam grinder.

 

After reading your thread I’ve decided that your name does not fit. Lol. Here are some options:

Neb “Don’t call me a” Hillbilly.

Neb “I ain’t no” Hillbilly

Neb “Get out of my way” Hillbilly

Neb “I get things done quick” Hillbilly

Just my thoughts! Keep rockin’!

Regards, Randy


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well it is Neb as in Nebraska but otherwise I agree. I didn't pick "Hillbilly" it was a name I recieved because while others build LS engines, I build Propane turbo 454 K30s, or 1000hp camaros with old irrigation 454s and grain truck parts.

 

Sorry I messed up the Neb versus Ned. I’ve been staring at a computer screen all day and everything is a little fuzzy now. Sucks getting old. Anyway, enjoy your progress. Randy


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I am a little worried about compression. Does anyone know the dome volume of a stock 270 wedge piston? I know these are bigger but then I might be able to swag these a little.

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wire wheeled and flat filled all the gaskets surfaces tonight. Started flapping the lifter bores to polish them a bit. Crank is setting by the crank grinder so it should be next.

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Best way to find out is to take one of the pistons, wrap it with masking tape, spray in some light lube and fill with plaster of Paris. I bought a cheap plastic buret off the internet lined off up to 100cc’s. I’ve done every engine since.

 

Got a few things done. Built a mold for my pistons and found them to have a 55cc dome. Bored the 5 good cylinders. Pulled the cam from a old 302 that I have but it turned out to be a cast cam. Thinking a 248 down on the farm has a forged cam. Can't get there today. Min maintenance road and it is 52 deg out so it will be impassable with the frost coming out and snow melt..

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After measuring the piston dome at 55cc I was doing some mathens and see that with a a heavy deck I could push the compression to above 10. Anyone have knowledge of what a reasonable compression limit is with these engines on 91? The cam will likely be in the 224ish duration at .050 range with a 108lsa. Not sure what a reasonable rpm is for this motor but I figure 5000 will be good for a streeter.



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I'm not understanding yet what happens with the one cylinder?

 

Sorry, when we were beating the piston it cocked a split the cylinder. It was heavily pitted anyway and I am not sure it would have cleaned with a .120 bore. Here is a picture of five cylinders bored. Not sure if the block is a 51 or a 41. It doesn't have a boss for the dist oiling and had hollow lifters instead of the tulips.

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I'd keep it below 9:1 ... 5 grand is plenty for the 4 main engines. Unless you intend to go all out racing, that's all you'll ever need … that heavy crank tends to be rough on main bearings even when in perfect balance, and it gets even worse when twisted too high.
Does the block have a 1/8'' pipe outlet for the gauge line at the rear of the oil galley ? '41 block wasn't tapped, & '48 was from what I can recall.