Inliners International Forums General Discussion Engines GMC Old School Build

let me start by saying I was going to post here first but I couldn't figure out how to post pictures without using a 3rd party and photobucket crashed I am hesitant to use another sight.

I have a thread on the HAMB that is a longer verson of below with pictures, but I wanted you guys to be included and offer opinions if you want. Hope the link is OK to add. https://www.jalopyjournal.com/forum/threads/old-school-cool-248-gmc.1178420/

As early as I can remember I have loved by grandpas old 55.1 GMC and after learning the racing history of GMCs there could be no other power plant. The original 248 had cracked the head on top under the rockers and filled the crank case with antifreeze. My plan always was to find a 302 but though we should be tripping over them here in farm country the larger 302-270s are pretty elusive. I recently bought a military 6X6 302 and wouldn't you know a week later a hopped but GMC shows up on facebook market place. The only thing I knew for sure was it was a 248 block, a 302 983 head, a small gmc Nicson Dual with 2 Rochester Bs, Fenton exhaust, mallory dual point, a pan modified for a car and it was stuck.

Long story short it had been assembled but never started. The internal surprises were a ISKY LDB cam, 3.960" cast Jahns open chamber pistons (55cc), 302 head combustion chamber ground open like a 270H, tubular pushrods, aluminum retainers, double springs and the bad, 2 pinned sleeves and 4 gallons of water in the pan that rusted off the oil pump pickup, oil pump pressure line and corroded the pump but everything else looked good.

After finding the pinned studs the block was scrapped but I wanted to use as much as I could of the old engine.

I found a horribly stuck 270 (1941?) short block in a snow drift that my awesome machinist and I spent a Saturday beating the pistons out of. We saved its crank and block. I bored the block an additional .120 to 3.960 on 5 cylinders and have to sleeve number one (broke the cylinder removing the pistons). The block sonic checked with an average of .220 before the bore with the thinnest cylinder of .198. The block will be short filled to reinforce the lower end of the cylinders. Then we will mock it up with the 248 rods and the Jahns pistons to determine the amount the deck needs to be removed to get it to 9-9.5 to one compression.

The Isky LBD has a lot less duration and lift than would be common now so I need to pull the forged steel cam from my spare 248 at the farm for a core to regrind at Bullet racing cams. I also bought a Nicson Dual Large GMC, 2 Stromberg 1bbl 99-25 (duals off an American Lefrance fire truck) and dual Chrysler air cleaners from an inliners member. Carbking doesn't like the Strombergs or the 11475 Zeniths I have for a dual application but I may try them because it is what I have. Should be a 295 when I am done. The plan is just a hot street engine.

Thanks,

John

John, That is an ambitious build. I can't get my head around all of it yet. I'll have to read it a time or two. What are you filling to reenforce the bottom of the cylinders and what will you use to do it? I know some fill the water jackets bot that is only for race use, drag race.

Tom

Moroso Block filler, a short fill (to the bottom of the freeze plugs) doesn't cause problems because most of the heat is from combustion at the top of the cylinder and head. Very little is at the bottom of the cylinders.

OK, 270 block and crank, 248 pistons and rods. Rods is rods, sorta'. So the issue would be difference in compression height od 270 and 248 pistons and you hope to fix that by decking the block? When you did the sonic check on the block did you check the deck thickness? It's been too long I don't remember how that made the 302 & 270 blocks taller for the 4" stroke but the blocks are taller. You can build a bad ass 248 street motor.

we didn't check the deck thickness yet but I still can. It is a undecked 270 (11.3667) but the 248 shows 11.11 deck height vs the 11.25. A 248 would be super thin at 3.960. The old block had .089 left in one spot. A .125 cut of the deck should be acceptable per my 55 Wayne catalog. Still need to cc the 302 head with the chamber ground open.

It will be a very interesting build. Please keep us up on it. Do you have pictures on the HAMB?

yes, lots

I went there and saw them. Those are the cleanest GMC blocks I have ever seen. That is a good cleaning system. I've never seen pistons like those. The head puzzles me. I would not think you could grind the 302 chamber to that shape yet there it is. I wonder if there was welding involved? They did a lit of fill and mill on the Chevy heads.

My 270 has small ports and even the least desirable of the 4 GMC heads. It works just fine. So much of the lore is about race motors. I talked to Bill Fisher on the phone once and he told me just what he said in his book that the small ports are actually better for the street, even for a hot street motor. He made quite a deal offing honest of what the engine will TRULY be expected to do.

You'd be days trying to get those pictures on here.

not sure about the head bit the only alternative I can afford is a 477.

Baking and blasting turns junk to gold faster than an alchemist.

I hope big port isn't too soggy but we will see

The pictures are for others and honestly for me to remember. If inliners stored images I would have shared this thread to HAMB

That looks like a BXOV-25.
B: newer version of AA
X: rotated flange pattern
O: 1/8" oversize throttle
V: vacuum operated power system
2: SAE #2 flange pattern (2-11/16" bolt centers) and 1-7/16" barrel
5: electric automatic choke

Hi Hillbilly . . . nice find. To bad about the pond in the oil pan :-(.

I see from the thread over on HAMB that you're looking for specs on that Isky regrind. I have the 1956 catalog. Chevy and GMC cams are found on page 59 and 60:



Unfortunately, back then there was nothing but advertised (valve event) durations - so there is no telling what the .020/.050 specs are for modern comparisons.

Another thing interesting about these old catalogs is the breath taking prices. $59 was a ton of money in '56 - adjusted for inflation that is well north a $500 in today's money!

regards,
stock49

That price thing is so true. When I think of some of the theo speed equipment I bought back then, starting in '59, I wonder how I did it? I was making $1.25-$150 an hour. In 1967 I was clearing $47.76 a week.

These are BXOV-2 and have manual chokes, not sure if 2 is enough but they do have the 1-7/32 barrels.

This cam has a split duration after running it through the cam master.

Dual pattern cams were standard equipment in the stove-bolt Chevrolet engines - they play well with the physical separation between the intake and exhaust valves in the stove-bolt head. They are unusual in a head where the intake and exhaust valves are situated side-by-side. How much difference in duration have you found?

IMHO Two is plenty. Biggest benefit of dual carbs is balancing mixture - same with port dedicated triples (like the early Corvette or Harper). But in an open plenum manifold more then two throttles is hard to manage. There is only so much CFM demanded by the underlying engine - with each throttle/venturi servicing a fraction.

I still need to flow these carbs to determine how much these flow. The 228 zenith's I have flow 120cfm dry at 1.5" of Hg. I know they flow more at 3" but that is a bit light for a 5000-5500 295.

295 × 90% VE X 5,500 = 422 CFM @ Hg 1.5", 597 @ Hg 3".

Yes, they're too small - but you knew that.

I need to find some large 228 zenith's off a 270 GMC.

Those Zeniths have changeable venturies. I think there are 3 sizes available. Some have adjustable main jets. Sizing is not the same as for carbs made for V8s.
http://www.zenithfuelsystems.com/downdraft_228.htm

An example of "what (in addition to venturi size) will be different in a 200" vs. a 300" engine": the position and size of the idle & transition holes.

not sure if the fit a 228v11, wonder the cost.

Get ahold of them. I see them on eBay from time to time and was able to but complete rebuild kits. Some are over priced and some are cheap just depends on who is selling them. I'm going to run a pair on my now 175" Chevy 4 cylinder. I was told by a carb expert that they are good in a single application but hard to tune in multiple setups. I wish he'd told us that in the '60s. They would never have been our carb of choice. They were on a lot of trucks, GMC and International for sure. Lots of farm equipment. You should have no trouble turning up a few rebuildable ones in your area. Try the equipment wrecking yards. Anyone with a lathe and a hunk of aluminum could make a venturi of what ever size they want. I'll be rebuilding a pair soon.

Jon told me that too. I Will use a pair of wide bands or build an intake with 3 97s or 94s

I had three on a 261 for a while that ran fine. I guess I just didn't know they were hard to tune. I only took them off because I found a '54 Corvette 3 side draft setup. I've run 2 on other engines. I had a lot more trouble with 3 and 4 97s. After doing that for years I found a 4bbl manifold for the Flathead in my '51 Ford woody. It was the sweetest running flathead I ever owned.

Of course, what was I thinking.

I have talked to more than a few guys that say progressive on a inline doesn't work. Better to run 3 small carbs. I am going to do a IR with a pulsation tube tieing all the runners together.

The Harper 3x2 setups worked. There is a thread here about them. They used 3 97s with the throttle shafts split. One side on all three carbs was the primary circuit. The other side was the secondaries and they were progressive. The linkage was complicated but Jalopy45 here has successfully made some for one of his. The manifold itself is three 90 dredge elbows with a small balance tube between. They are one of the coolest, most mechanical, Rube Goldberg looking pieces of inline speed equipment ever built and they work.
Search Harper on this site if that doesn't work go to Jalopy45's page and search his posts.

3 small carburetors can work very well in IR config.
All that's missing is... power.

Ordered a Schneider cam
264 adv. 224 @ .050 intake
274 adv. 230 @ .050 Exhaust
108 LSA
.480 lift with 1.5 rockers ( I know they are less but not sure how much)

Not sure what to do about the lifters. I have a nice set from the 248 but they came off a forged steel cam and the new one is a cast iron cam. I have a rough but usable set from a cast cam but it has 4 of the lifters with holes and 8 with the hollow lifters. Not sure if if there is a material difference between the 2. I can get the 302 lifters ground but my ADHD is killing me with the lifters not being the same. Not sure I want to pay $250 plus for new lifters make in who knows where.

The CFM calculation (CFM = D × RPM × VE ÷ 3,456) is not divided by the number of carburetors for IR. 375 CFM total is not 125 CFM for each of three.
For reasons too lengthy to explain, three individual carburetors for an L6 can, and should, be much larger than three carburetors on a common manifold.
Except for the fabrication, the Weber DGV 32/36 is an excellent choice for IR. Other choices include Rochester 2G, SU HD6, Stromberg 175CD.

I was going to use the 11475s I have or Holley 94s but I will certainly look at those others. Honestly If the need to be much more I will be looking at 3 350 holleys. I have lots more time tuning them.

Ordered lifters from Amazon. Maybe not the typical place but they had hylift Johnson's for a decent price.

To the my understanding It Is not a good idea to put used lifters on a different cam it can wear the lobes out faster due to the wear pattern it ran on a different cam or lobe. It is always best to keep the same lifter on the same lobe.

just my cheap 2cents.
any other input is welcome on this.

If you can't get new lifters or they are super expensive, you can get your old ones resurfaced to use again. These lifters are $19-30 a piece.

If it was your standard BBC SBC 292 etcc I wouldn't even consider surfacing lifters but in this case the lifters are expensive. Still I found a set for $211 so I bit the bullet and bought them.

any other input is welcome on this

I agree, known to cause problems.

Schneider & Bullet cams recommended resurfacing the lifters because of the limited supply.

I think is was in California Bill Fisher's book that he advocated using used lifters because the bottoms had become hardened from use. I think '39 Chevy was supposed to be best.

I am curious to hear your feedback when your shipment arrives and the lifters are in hand. I spent some time back in '15 looking into this topic:
Inliner's Engines Posting
I found the new production lifters extremely heavy compared to stock and NOS after-market. The added weight seems counter productive to controlling the valves as RPM increases.

Personally, I would go for NOS after-market. They still show up regularly:
Ebay Sealed Power AT-887 lifters.

Indeed, but California Bill was talking about steel camshafts and chilled iron lifters.