The Stovebolt Site Forums The Shop Area Engine Shop 248 GMC Engine

Have the original 248 engine that I would like to put back into the 54 GMC truck. Was locked up, gotten the pistons out. What is the best way to look at the block to see if there are any cracks or damages as well as seeing if it can be rebuilt? I have to drive over 100 miles or so to get to a machine shop that "might" be able to work on it.
Then the issue of finding parts, etc.
Ideas?

Truck now has a 54 Chevy 235 engine which I got running briefly, like to rebuild it as well. But rather put the 248 back into it as it looks kind of funny with Chevy engine in a GMC truck.

Hone the cylinders to clean up any surface rust, then use dye penetrant to check for cracks. It's usually available in a 3 spray can system- - - - - penetrant, solvent, and a powdery "developer" that makes the penetrating fluid show up when it's sprayed on. Another crack finder is called "magnaflux" where a strong magnet is applied to a cast iron or steel part and magnetic powder is applied with an atomizer spray to outline cracks. Either system should be available on Ebay.
Jerry

Cheap "fix" is to pretend your truck was built in Canada. Canadian GMCs of your vintage were equipped with Chevy 6 cylinder engines.

Or supply a 302 to give your truck a HP boost!

Ed

Never knew you could get the dye, thought had to go to machine shop to have it magnafluxed. When you say hone the cylinder, what type of hone to use? Stone or flex?
Then there is mic the cylinders, need to know what to look for after I dye the block. Is there any links or books to help work on the 6 cyl engine? I have rebuild v-8 engines before but used a machine shop to do the work and I assembled the engine.
Trying to make a plan on what to do after the body is off and order parts.

Going to check with a customer or two on magnet check as they use for well-heads and pipes.

What is the best way to clean the block? Have a hot water spray cabinet parts washer, pressure washers. Don't know anyone with a hot tank unless I drive few hundred miles.
Would like to do this on both blocks the 248 and 235.

I know, nothing is close in North Dakota. I have a boy in the air force stationed in Minot and he tells me the same thing when it comes to finding a machine shop. So the question is, what is it that you are trying to do here? Rebuild a 248? If your goal is to rebuild a 248 then why mess around trying to determine for yourself whether you have a usable block, particularily if you lack the training and equipment to make such a determination. If could be a very expensive mistake on your part.

You want to know up front, without question, whether you have a block that is usable. And for that, you want someone who knows what they are doing to to give you that info. Mony spent investigating whether the block is a good canadate or not is money well spent. If you try this on your own, and make a mistake, miss a crack, whatever, then spend the money to clean and machine the block, only to find out you missed a big crack in a critical place, that is just a waste of time and money. So even if you have to drive a distance, find someone decent. Not just a machine shop that "might" do the work. Find one that "will" do the work, even if you have to strap the blocks to a pallet and ship them to someone.

So, if you really want to find out whether you have salvagable blocks, get them to a machine shop and have them do what is required to sort that out. That usually means a dip in the hot tank for an initial look at the cylinders and a visual inspection of the entire block. The machinist will run the dial bore guage down each cylinder and determine if the block has been previously bored out and if so, by how much, and if the cylinders are tapered from wear to the point they need an over bore, and if so, by how much. That initial inspection tells you a lot, like if the block will need sleaves or just a basic bore and hone. Now you have a real basis for making your decision whether to go any further with this block.

If the cylinders pass a manual inspection and are determined to be usable, and the block itself passes a visual inspection, the block next spends a bit of time in the shot blast machine, particularily if its really crusty, and they all are these days, lol. Once the outside and inside are nice and clean, magnaflux. If the block passes the crack test, now you have a real basis for rebuilding.

Just my opinion and yes, I know it costs a few dollars, but everything does these days. But you have not lived until you have spent all kinds of time and money on a rebuild, without doing a proper investigation, and then find out it has a fatal uncorrectable flaw, your heart sinks. Time, effort and money spent now will reap rewards down the line.

The spray cabinet is, or at least used to be the preferred method for block prep. You need to strip the block down to bare bones- - - -even remove oil gallery plugs, freeze plugs, etc. There's no such thing as "clean enough", particularly where oil passages are concerned. We cleaned up John's 261 engine parts with a vat of boiling water with lye and Dawn dishwashing liquid in it, heated over a wood fire, followed by a good scrubbing and a boiling water rinse. I use rifle and shotgun brushes and cleaning rods to scour all the crud out of passageways to remove anything the pressure washer loosened up- - - -otherwise any leftovers end up in the bearings as soon as the oil pressure in the freshly assembled engine builds up. There's a "no-micrometer" way to check cylinder taper using nothing but an old piston ring, a feeler gauge, and a calculator. Send me a PM for some details on that procedure. It's easier to explain by email or a phone call.

Anybody with a few brain cells to rub together and some common sense cam build engines. "machinists" don't really walk on water- - - - -they just want everybody to think they do! Some of the oldtimers I learned from in the mid-1950's were barely able to read and write, and they had no concept of CNC machining, mainly because those machines didn't exist, but they were expert engine builders. People who insist it takes hundreds of thousands of dollars of specialty equipment to do a good engine job should come talk to my garden next spring- - - - -I'll save a fortune on fertilizer!
Jerry

When I am in doubt about an old engine, I pull the head and fill the water jacket with Diesel and let it sit for a few days. If there is a crack, the Diesel will find it. Same goes for the head. Turn it upside down and fill the water jacket with Diesel. If there is a crack in the combustion chamber, Diesel will find it. Thats how we did it 65 years ago when these vehicles were worked and driven hard. I was 14 when I built an engine to put in a 1950 GMC to drive to school. I had no formal training, but I had watched some old mechanics. I still own a dozen trucks and supervise maintenance. But I still play with GMC and Y-Blocks. They were simple machines can be fixed with simple fixes. And before you bore an engine, make sure you can find pistons. They are very difficult to find.

The last GMC six I built was a 256. (270 bore- - - -248 stroke) Made in Canada for military use. We honed the cylinders and knurled the piston skirts to get the right piston fit.
Jerry

RP14 Listen to crewelge !! Easy cheap tests,use a ball type glaze breaker to clean up the bores. If you cut the ridge out of the cylinders could use a simple feeler gauge with a clean piston inserted from the deck ( in other words backwards hold onto the rod) to get an idea on wear. If you can get .005 or .006 feeler gauge in there probably need bore work. You have to consider wear on the piston too,that is all stuff you can do fairly cheap at home,might even borrow a glaze breaker from a local mechanic if you have one or get him to help you !! There is a lot you can do under the old shade tree !! Another thing ,look at your main bearing bores should see machine marks with no sign of movement (shiny wear area,s or blackened area) if the shaft is still in it should turn free ,then inspect on the way apart looking for signs of trouble.

crenwelge Good advice to RP14 have shade tree'd my whole life on this stuff,when I went to work in the aircraft magnaflux shop I found out what real magnaflux is ! Some guy with a magnet and some dust may find a crack that you can see across the shop but on aircraft you have to do better. Then you sign your life away !! Next thing is getting powerful magnetic field out of radial engine parts,so you learn, that was 52 years ago guess it worked never lost one. We could check those 6-banger heads caught several too !!