Projects Old school cool 248 Gmc

THATS NOT AS GOOD AS YOU THINK IT IS !!
Take the clip off and slip the cam up and clean and lube everything with some light lube heres what we use on a rebuild . Will last for years !!! Might as well know off the drive gear remve th4e shaft and polish and clean / lube everything ....Just remove spring from piston on advance unit ....

 

Hands down Nicsons !!

 

well crap, apart it comes. Use the stock style vac advance?

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crap, all you guys should have told me before! oh well that is an easy change. I will look for a pair.

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When I built my 270 In the 70s I got a set of Viper long tube headers from Clifford. They worked great. I only took them off because a piece poppet out where I had to put a little ding in one pipe to clear the intake manifold. I now have some of the repo Fentons. They work.

 

Don't know on the Nicsons but your choice of the Clifford tube is a very good one way better than Fentons too me. I'm using the Nicsons because I bought them 40 yrs ago for $50 at the Pomona Swap meet. I too would have done the same. Many years ago Clifford had long tube and just the 3/8" thick flanges. I have a set or 2 somewhere in my stash.
Your project is doing great ...keep it up for all of us.

 

I asked about the vipers but they no longer make them. I prefer long tubes but these should be better than the Fenton's.

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I didn't take any pictures today but I got the cam gear installed and was pushing on the crank gear when it appears the rods and cam hit. The crank gear needs to go on about a 1/4 more but I think I may have to grind the cam a bit. It is a bigger cast core and the rods may be slightly different with the 302 rods. We will have to further investigate.

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The rods should be the same. I don't know if the 270-302 cam had notches like the 292 does. There could be a difference in steel and iron cams, I don't remember hearing that but there shouldn't be an issue in a 248. Didn't you put in some after market rod bolts? That's the first thing I'd look at.

 

it actually is a 1941 270 block because the 248 was scrap. The cast cam does have cut outs for the rods, they may need deepened. Pictures to come. Does show stroke is not easy to get in this application.

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I'm not sure I understand your explanation. The crank gear should have been on the crankshaft before it was installed in the block. The cam gear should be on the cam with 2 bolt flange behind and the proper clearance. From memory .003-5". The front plate which may or may not have a mount then goes on with the oil squirter aiming straight down at the crank gear. Then you slide in the the cam lining up the 2 marks. I have the crank gear aiming at where I believe the cam gear will line up. The cam will sit in all the bearings just barely so you can make the cam turn the few teeth so it will finish sliding in lining up the marks. I just did this last month with a large cast cam, not a cut down steel using the same rods.
You them install the 2 flange bolts. The cam should "rattle" slightly on the gears and you should feel the clearance on the cam flange.
I have put the cam in before the rod/pistons and after. It is easier with the pistons out so you can put you hand in easier to help with the cam. J.D.

 

I did it a little wrong, rotator is in, put in the cam and bolted it down then was pushing on the crank gear. Crank started to turn and caught the cam. I will pull the cam tomorrow and then install the crank gear. it was a bit of disorganization but I didn't remember the crank gear wasn't installed yet.

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Now time for the rest of the story. The new rod bolts were indeed hitting the cam. Since the cam was the easier part to remove I pulled it and used one of those fancy hand held belt Sanders to deepen each of the reliefs. You can see where 3&4 were just touching the can. I then recleaned and polished the bearing journals and reinstalled the cam and checked a few of the tight bolts and there is an easy .015 or more clearance in each one.

We then started on the head. All the guides had been previously knurled so the all were replaced this morning. First we cut them out, finish honed the interior, cut down the rest of the old guide, drive in new guides, shortened the new guides to allow enough for .480 cam lift, .200 for positive seal and .015 for some fat. There will also be clearance added when the new 1.94/1.60 seats are cut so the valve sits deeper.

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I understand the cam hitting problem rod bolt/nut after seeing it. I have never replaced them in 45 years of racing and never lost one, just changed the nuts. Glad it worked out.

 

I thought that might be it because that was the only thing you had changed in crank/cam relationship. The head looks nice. It is interesting that the chambers can be cut that much.

 

I hope the head doesn't end up bitting me on the butt later. If it does, then I guess I will have to buy a 270H head.

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Any early small port head will work with those pistons. With a taper to a big port manifold and blending causes a venturii effect when the valve first opens you will have instant torque up to the rpm where a street engine normally runs. If it wasn’t that I had them, both H and D’s I would never have worried about a small port.
Have you sonic tested the side you removed metal to clear the piston? Doug Robinson of the BMR LSR blown GMC’s told me the H had more metal but I have never cut one to see.
Good luck and thanks for sharing.

 

The head was already ground that way when I pulled the original 248 apart. Really, I have now built a whole new engine based on a set of antique Jahn's pistons, a questionable head and a Mallory dist. Didn't end up using nearly anything else, but I have a few spare for the barn. Most of the stuff had ever been run in the 248 so the pistons/rings are more or less new. Kinda scared to thickness check the ground portion, but I may in the morning. I have 2 small port 2 x 1 intakes and a 477 head but I guess I am dumb enough to want the big port just because. If I do use a small head I will have to dig into my "fluids" book to find the proper dimensions to cut a true venturi shape for the best transition. The stupid drag racer in me is having a hard time with "street" even though I know this just an old truck that will never be quick, I seem to always push it a bit.

So here is this evenings question. Where do I get the stud seals for the valve cover? Nothing in my Felpro gasket set and nothing on Napa online.

 

Y-Block Ford rocker and valley plate cover....J. D.

 

Took the day off in a pandemic to work on the head. First I checked the cut back part of the chamber. Thinnest spot was .250" but most were over .400". Should be plenty thick. Took a couple pictures how does not heat the chamber cutting back was. Cut the intake seat, bowl hogged under the valve and cut back the chamber. Then cut the exhaust seat. I did some mild porting, squared the exhaust floor, blended the short side radius as much as possible. Flowed the head and it flows 195 cfm on the intake and 135 on the exhaust at .550". This is very similar to a stock small valve small block Chevy head. Certainly not great but not horrible. Head checks straight so no need the mill it. They are finish grinding the seats and Friday morning we will look at springs.

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You certainly build a fine looking GMC, cleanest machine work I've ever seen ….
The rocker cover nuts were tapered to self seal on the 228-302 ...

 

I see the brass water guides are still your head, these are very important an if you look into them you can see the water driving up the block to the head directs the cooler water to the exhaust valves...
302 GMC is correct if you have them. They not only taper they have a portion that's straight and go into the cover to help center it but has no gasket. The Ford Y-Block rubber seals have a taper and with a flat washer makes a good oil seal. Just another option.

 

My buddies at the machine shop work on Y blocks regularly so I will see if they have a set. I can't afford a aluminum valve cover so I was going to put chrome acorns on my powder coated stock valve cover.

Thanks for all the support guys. I am trying to do everything to the best of my ability and pocket book.

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Do you have the cover with the embossed GMC logo? Coolest valve cover there is for a GMC, IMHO.

 

Yes this one. A Nicson or Wayne would also be cool.

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The newer straight letter one is cool too. Funny the GMC is the only one I can think of where the stock cover is cooler than any of the after market covers. Cadillac and Olds Rocket come close
I tried to cut one down for my 4 cylinder bur it didn't work. They are not as straight as they look and very thin.

 

That one has the hole for a copper tube and no vents. I believe it’s military. But I’ve been wrong on a lot..

 

This one is actually from a 248 that came out of a bus, well that is what I was told. It has a early PCV system. I will take a picture of my military 6x6 302 when I steal the manifolds for break in of this engine. It is likely very similar or the same. My 55 GMC 150 248 has a vented top and just a road draft tube. I plan to run a PCV system but not sure what it will be made up of yet. I also have the valve cover that came on the 248 I bought with 2 breathers attached. They are welded on crooked so idk if I could stand to use them.

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These came today

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Worked on assembling the head today. Going to use "Z28" SBC springs for 100lbs of seat pressure. Also using positive seals on the guides. Pics tomorrow

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