The Stovebolt Site Forums The Shop Area Engine Shop 1951 gmc 248 engine

I am having trouble trying to give the machine shop what he needs to do my motor, he needs the rod jouranals size, bore size, crank main journal size and ther is a "A" prefix in front of the engine serial number. I neeed help. Thank You, Moonshine Mike

Some GMC 248 information - from Rob English's website

The 'A' usually indicates it has been rebuilt before.

It costs just as much to rebuild a 270 or 302, plus pistons are available, if that is an option for anyone else.

Ed

I'd be leary of any machine shop that has to ask those questions...
I believe a shop manual carries that info doesn't it?

As shown at the link in my earlier message:

The A preface means:
+.005 cylinder bore
-.010 main journal
-.010 rod journal

I doubt if the GMC Maintenance Manual shows that engine code information, but it might and it should have the needed specification information.

I'd buy a GMC Maintenance Manual to match my year engine.
http://www.classicpartsusa.com/product/SM47G_1951/1951_Truck_Parts_Literature
http://www.faxonautoliterature.com/...k-Repair-Shop-Manual-Reprint-P25248.aspx

X2!!!

This information readily available in MANY locations!

Think I would consider looking for a different shop!

I personally would be embarrassed if someone brought me a carburetor from a 1951 G.M.C. and I had to ask the customer for specifications!

Good luck.

Jon.

Your "machinist" sounds like a doofus who shouldn't be trusted with anything remotely related to automotive machining! According to my Federal-Mogul shop specifications manual, all of the GMC inline sixes have the same rod and main bearing sizes and use the same bearing part numbers:

Rod bearigngs : Federal Mogul 1905CP
TRW CB427P

Main bearings: F/M 679M
TRW M929P

Connecting rod journal standard diameter: 2.3110"-2.3120"

Main bearing standard diameter:
#1: 2.6835-2.6845
#2: 2.7145-2.7155
#3: 2.7455-2.7465
#4: 2.7765-2.7775

228 Std. bore: 3.563"
228 stroke: 3.813"

I'd strongly recommend finding another shop- - - -if he screws up the block or the crankshaft by grinding or boring them incorrectly, you'll have a hard time finding a replacement!
Jerry

One other comment on this subject:

In original literature from another GM manufacturer, occasionally an engine block might have a defect that would prevent using standard parts. These engines would be assembled with slightly oversize bore, or slightly undersize bearings, or both. A mark would be made in the engine number, and the vehicle would be sold to the original purchaser at a discount (often, to fleet sales, such as taxis). I do NOT have sufficient GMC literature to know if GMC also utilized this practice.

While historically interesting, it becomes a moot point at the machine shop. What is important at the machine shop would be if the block and crank would clean at clearances acceptable to published standards, and pistons, rings, and bearings would be available.

Using the dimensions from the "A" build would be like reading the numbers on the jets in a carburetor and expecting to know the size. This is why measuring tools are made.

Jon.

Buick had a habit of installing a few V8 engines in the mid-1950's with .005" oversize cylinders, and using Std. diameter rings in them. They were almost always oil-burners. Then they decided service departments should install .010" oversize piston rings and file-fit the end gaps.- - - -the oil burning got worse! The only fix was a .010" rebore, new pistons, and rings that fit the cylinder diameter!
Jerry

Thank All of you, I am going to get my motor in the A.M. and carry it to a rel machine shop.

The bore on this motor is 3.810 and looks like it has never been bored, but the engine number shows a 248 stamped on it, is it possible this block is not really a 248.

Quick glance..........Looks it might have been bored .080" in its prior life. Top of the piston maybe marked with an over size.

Or it was bored or "fixed" on line .005" bored at the factory which mean nothing now and prior it was .080" bore with .010" of wear. to get you close to where you are now ?

Stock 248 is 3.718" 270 would be 3.781"

A 3.810" bore would be a 256 engine or a 270, bored .030" oversize. I believe you said the pistons had "30" on top? A 256 has the same stroke as a 248. Rod and main bearings will be the same as the 248. A 270 also has a 3.781 standard bore, with a 4" stroke. You MIGHT have a 270 that has been bored 30 over and re-stamped with the original engine number. Check and post the casting/forging number on the crankshaft, please. I have a list of crank numbers to reference.
Jerry

As another data point........ my 228 is an "A" 228 engine number.

Has original factory pistons currently measure 3.557 stock bore is 3.563. Would appear this particular "A" engine did not get an over size bore at the factory?
Factory piston with selective fit ID numbers, were knurled once in its life. Knurling is pretty much worn flat now.
When it was recently rebuilt it was bored .060 over to clean cylinder rust problems from sitting.

GM-018 is the marking on the crank.

GM-018 is the marking,Jerry I really appricate this.

There should be a 6 or 7-digit number on the crankshaft somewhere, for instance:

3.813 strokes

2144958
2191127
2195570
6107430
6107432

4.00" strokes

2090911
2135416
2136119
2192411
2194012
2195958
2326229
2342204
234224
2342664
2404929
2491012

There might be other forging numbers, but that's what Federal-Mogul lists. To get an accurate measurement on the stroke, the crankshaft and at least one piston would need to be in the block with the bearings in place.
Jerry

I will get it in the morning, Thank You , Moonshine Mike

The number on the crank is 2191127. there is a stamped number on the top of the middle journal 531592 but somebody has stamped that number on there. Tks. Mike

OK, you've got a 256 engine bored .030" oversize. That's basically a 270 block with a 248 crankshaft. You can run .030 over 270 piston rings, which might be easier to find in a parts catalog than a listing for the relatively unknown 256 engine.
Jerry

Will the Rod and Main bearnings be the same as a 248 or a 270.

Check out my first response to your post. "Reading Is Fundamental"!
Jerry

been so fustered lately, with this This is like going back to school.......... But I am learning just bear with me i will get it right with the help of my friends, MERRYCHRISTMAS to everybody and thank you for you,lls HELP. MOONSHINE MIKE,,,,,,,,,,,,,,,