Projects Old school cool 248 Gmc

I have been looking for a motor for my Grandpa's 55.1 GMC for ever. I recently bought a 6x6 302 finally but wouldn't you know engine pops on facebook a few weeks later. Castings show 248 block, 302 head, small port nicson and Fenton exhaust. Reasonable deal so I talk myself into it. looking at the outside I noticed the dist looks funny, ends up being a Mallory dual point. Borescoped the pistons an noticed bump pistons. Everything appeared to show it was a fresh engine that was never started. Today I tore it apart. First thing I discovered is is aluminum retainers and dual valve springs. pulled the head and found some big dome pistons that end up being jahns .020 pistons. Removed the 37 Chevy tranny and clutch and GMC 4 bolt flywheel. Drained 4 gallons of water out of the pan and was scared that the rotating assembly was garbage. pump and delivery tube are rough but rotating assembly looks ok. Crank.is for sure a 248 crank with mains .010 and rods .020. excited about the cam grind. hoping the compression isn't too much for pump gas. more pictures tomorrow. Probably not a great truck engine but it deserves to be used.

Later
John

 

more pictures

 

before tear down pictures

 

I have a 248 in my hoard. The cyl head has been removed. I bet the crank is still good.

 

I have a second 248 in my shed on the farm. planned on using the pan, bellhousing and tin anyway, so i will be robbing lots of parts from it now. Wish I could find a large port 2x1 but will be making adapter rings to use my small port Edmunds soon if I can't find one.

 

wow....thanks for joining the HAMB..........

 

thanks for having a such a cool site!

 

Knowing Jahns I'll bet the domes are solid and can be milled and ash can the aluminum retainers.

 

Very interesting piece of history you have there !
Home made tubular pushrods … can you give us a closeup of the lifter end of them ? Also, it looks like they used the '37 - '40 Chev pass. flywheel as recommended in the '50s "Hop up your Chev - GMC" books. That's not enough clutch for a GMC, let alone a healthy one. The 4 bolt crank flange was a source of trouble when used hard. Most racers tapped the dowel holes for 4 more bolts. That 12 volt starter probably won't mesh with the early ring gear, so check that out …

 

Glad you posted this, I have always been curious about the 248. The 270 and 302 get all the love. Keep us updated.

 

isky track cam, can't make out the other numbers, crank has a couple of dark spots that may not polish out. Mains are .010, rods .030. I will likely use a GMC bell housing, flywheel, pan and stomp starter. planning on drilling the flywheel/crank shaft for 2 more bolts. Probably get a sm420 to start but hopefully a t-5 in the future. wonder how wild the cam is?

 

Do you have a dial indicator and can you get it on the cam lobes??
That would nail down the lift and you could ballpark the duration and over lap.

 

LDB … 1007 Long Duration Bigelow. That's as wild as you could get in a flat tappet shaft without going roller

 

do you have the specs for that cam?

 

Combustion chamber does not look like 302. If intake ports are less that 1-7/8" is a small port 228, 248 or early 270. With that style chamber you can can have around 60cc's on the piston pop up and be near 9.1 comp with a 4" crank. Less with a 3-13/16" 248 stroke. With cleaning everything looks savable but examine the oil pump gears carefully. You can live with some pitting but nor deterioration. I'd find a stock side plate if it were me...Have fun with it..

 

I have another 248 I figured I would get the pump from or order a nos from eBay.

 

I figured with the side cover slotted this much the head must have a big cut.

 

The head has big ports. it matches the big port gasket.

 

The 302 had a "D" shaped chamber. With what I see and your saying it has large intake ports, check the serial number to see if it ends in 417. If it does it's a big port 270 H and to some more valuable than a 302... Harder to make compression but it has unshrouded valves even when oversized valves are installed.

 

new development, I just went by the .020 on the piston, the actual bore is 3.950 so this is actually a 280 now.

 

someone ground the chamber back for I am guessing piston clearance. Head is a 983 casting.

 

and the pistons are cast

 

Thinking about building an intake, I have 3 zenith 11475s and a 14475. Use some 1.5" or 2" sced 10 alum elbows and get some flanges cut to mount the carbs and head flanges. Need to check the truck to see any clearance problems with 3 carbs.

 

In the past there were some crazy cubic inch racing classes.
I have a 1940 270 block bored to 3-15/16” to be under 300”. Rings were a bitch to find in 1974.
I’ve never heard of anyone grind out the chamber of a D head to use earlier open chamber pistons but guys did things because it was all they had. The piston look like they might been for the big chamber and the guy got a 302 head and made it work. The 302 head will rev better with the 248 crank. Good luck, this fun reading and watching.

 

the block has 2 sleeves now, boring it this far surely makes for thin cylinders. We will Sonic check then sleeve if it is too thin. The rings and bearings look new. I am planning to clean and reuse them. The block and head will get cooked and shot peened. Rods resized with better bolts.

 

For years guys have worried about rod bolts. Maybe there are some ARP replacements but up until I bought a set of 8” custom rods I used stock rod bolts turning 6300 threw the gears at the drags and holding full throttle over miles at Bonneville and El Mirage and never lost a rod. I bought new big block nuts and ran the same set over 40 years torqued at 40. My rods had .018” side clearance so I could splash a lot of oil on my flat tappet cam and never lost one of those either. I’ve used the same cam and lifters in 4 different blocks.
My only suggestion is to add a full flow oil filter. EZ now with it apart. Only one hole needs to be drilled in the block and tapped for 3/8” pipe in the line from the oil pump and tap one hole in the oil passage for a plug. The oil line feeding the head needs to have a line tee’d in to spray oil on the distributor gear in my opinion.
Good luck

 

The only water damage to the rotating assembly is the nut and end of the rod bolts on 1 & 6 so I at least need new own replacements.

The cam is the LDB Isky. Specs are a little difficult to find. Though it is referenced in the California Bill reprint's Chevy section.

It had a little hub on the crank that was just a pulley. Would a 248 balancer or 235 be better? I have an ATI in my 427 in my camaro but it might be Overkill with a cruise in engine, that may see an occasional 1/8 mile pass.

 

I think @RichFox might be interested in this. I recall him doing some nice GMC 6 stuff.

 

My earliest Isky catalogs are '64 & '67, and don't list the 1007 any longer. They still list a 1007 for a Stude 170'' 6, but that's probably not what you need.
Triple intake & carb clearance problem only if you have a hydro or 3 speed column shift.
Use a harmonic balancer that matches your water pump & gen. pulleys.

 

I wonder if isky will share this info