Projects Old school cool 248 Gmc

my machinest has welded a few rods but it isn't ideal. I localized the heat as well as added it spot by spot. I thought about normalizing the rods but decided they really didn't get hotter than when you heat for a pressed rod pin. The rods most definitely are from sets all over the earth.

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if I had known I’d send you a set. They fit in a postal box. Contact if you want to do it.

 

I sent you a PM. Truthfully my OCD is getting to me on the rod deal.

 

I spent 20 years as a Automotive Machine shop forman and over 50 years building hot rod and race engines....that connecting rod deal makes me real nervous.

 

Had to go to work at 1am until 10:30am to fix the lube pump on our big recep compressor. Who says Engineers can't spin wrenches Because of work just made it a hours work on the GMC. Put the crank in to check that it spun and then put in the rear main seal. Trying the felpro seal first. If it leaks on the run stand it will get something else. Also tapped the front main for the bolts to come in from the timing cover side.

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Massive cranks in those old Jimmys

 

I've been told it's only fifty pounds lighter than an Allison V12 crank.

 

I recieved my set of rods from Jimmy Six and got to work. Cooked and shot blasted them. Then drove out the old bushings and had the local blacksmith/machinest make me some adapter bushings from the big pin small end. Cut .002 off all the caps and rods and cleaned them in the solvent tank. I will be putting bolts in and torquing and start honing both ends tomorrow hopefully.

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Those look nice. I'm still impressed with how clean you are able to get this old stuff.

 

The oven to burn off the oil and grime then shot blasting makes things look brand new.

 

I've bought new parts that weren't that clean. What a great process, sure as hell beats diesel and a brush.
Are the rods the same for the two sizes of pins and just the bushings different. There was at some point a change in the size of the tab on the big end insert.

 

Very early ones had a bigger tab...I've got a set of those bearings I'll probably throw away some day. I think I might send Neb the next set of rods when I build another engine just to clean them.

 

It cleans up aluminum intakes great and go back and look at the block difference.

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I compared the small end forging and it is bigger on the late 302 rod. I put one on the crank rod journal to see if the "narrow" or "wide" rod bearing came into play but they fit the journal fine. I will open the side clearance a bit as Jimmy Six recommended to splash a little more oil on the cam.

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Got the big and small end of the rods resized then balanced the rods within a .5 of a gram (without welding). Assembled the rods and pistons. Drove in the cam bearings and made sure the cam turned.

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They went to a good home.

 

Cleaned up the rod bearings and put the pistons in today. Cam gear should be here today so the short block should be close this week.

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I hope your heads not cut too much.

 

To make the compression too high? I cc'ed the chamber and they average 130cc. It is a 302 head someone ground into an open chamber. Not the greatest work ever but should work. Compression should be right at 9.75.

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I’m sure if you put one of my heads with .100”+ milled off those pistons would hit. I thought those were for the earlier head but opening up a 302 style looks like a good idea. What was the cc of the piston top. The ones I had Ross make for my sons engine are 61cc’s and the head was an uncut “H”. Fun watching your build. JD

 

They cced at 60 once I coated the mold in rtv to prevent the alcohol from soaking in. I don't know that opening the 302 head was great, but 270H heads would be a difficult find.

This thread is no where as cool as a landspeed thread I was reading today about headers where you have some awesome stainless headers on your GMC.

 

Thanks for the compliment. They were designed by Burns Stainless for my engine. They suggested a builder/welder which I used. I did not race for a year; my year budget was the cost for them. And yes they worked.
As for the H head, they had the same chamber as the earlier small port.
A few years ago I was working with a shop building a GMC. I convince him to use a 270 in his 51 Chevrolet Bel-Aire and the latest small port head with the smallest chamber instead of a 302 to save $$$. I had his engine cylinder head machinist use an end mill at the intake entrance cut a 45* taper to a large port intake manifold. This causes a venturii when the intake opened and gave his engine unbelievable low end torque. You could not believe how happy this guy was with build.

 

I have a 477 and a 819 but I still think I will go with the 302 head. Still should be years ahead of the stock 248 and honestly it ran pretty good.

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Honestly I have pretty high hopes for this engine. I have been thinking about hot GMCs on and off for 30+ years. I keep saying 200hp but I hope it is a bit hotter. The Nicson 2x1 and the Bxov-2s and the Fentons will likely be the limit. I need to get it running first before worrying too much about the intake and exhaust. I think my goal is to take it to the Colorado dirt drags next year.



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I have a chance to sell my Fentons to a guy building a 235. I did have them welded but they are a vintage set not a Chinese recast, any idea of value? What manifolds should I get? New Nicsons or Clifford's tube shorty headers?

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Sold my Fentons for $200 and bought Clifford streetrod headers. Maintenance tech I work with at work has a kid that may test positive for Covid 19 so I am stuck at home cleaning my garage until the test comes back. Got one side clean but the other will take a few days. Finding lots of tools and stuff I forgot I bought, but also lots of brown recluse spiders so it is a bit of clean and spray action.

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Fentons are great on a Chevy, but they don't belong on a GMC IMHO.Your tube headers will probably flow better than Nicsons, bit I wonder if that effects lower end torque? I'm cleaning my workshop out as well. I guess a lot of us are doing that.

 

I believe these are what belong on GMC’s on the street

 

well I already bought the Clifford tube headers. I guess if they don't work I can change them for the Nicsons. Does Buffalo still sell new copies or do I have to go on the hunt?

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The maintenance guys boy tested negative. Work said for all of the no hourly people to stay home so I will only be going to the machine shop in the morning and stay home the rest of the time. Lucky thing about living in BFE is most of normal life is distant

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