302 GMC big six engine buildup for 33 Plymouth

I just purchased a 1955 GMC flatbed truck with a 302 GMC in it for the purpose of taking the engine for my 1933 Plymouth coupe. The engine runs and has a mere 37,000 miles on it, I will soon have the engine out and be building it for my coupe. I have collected some ideas as to how to build the engine from other threads but have not found all the information I need to set forth. My plan currently involves getting the original cam reground, using a 270 crank and chevy small block pistons for the lighter weight, hopefully use the original rods? And for the head I would like to have it ported and polished and use all chevy small block valvetrain, Is there any issues with this plan? What would I have to do to use the stock rods and sbc pistons? and is there anything I could do differently or better? All and any input is greatly appreciated!!

 

the car, or the body at least

 

the truck





The Engine

 

I'm not real familiar with the GMC 6's, but what is the reason for putting a 270 crank in a 302?

 

from what i have read it is approximately 10 pounds lighter while still retaining the same strength

 

I also plan on using Fenton cast iron headers and Howard 5x2 intake

 

SBC pistons will not work,the compression height is all wrong. And the GMC uses larger wrist pins.

 

Sure like those 270 & 302 Jimmies! Neat choice, size-wise...but you're bound to get some flack for putting some GM power into a MOPAR anything. (rules...)

There was a group of guys a few years back that ran a 270 Jimmy with five Stromberg firepots at the 'old time drags', Fremont (Baylands) ...Engine was in a two-tone green '47 Chevy, the racing team was called "Gang Green", out of Arizona...

Nice gents, and they sure went fast. I hung with them for a couple of hours, loved the onlookers' reactions to FIVE pots on a SIX cylinder engine! "How can that work?"
Hah! When the lights came down, they saw 'how it worked'.
Memorable. (that was in 1984...)

 

there is no way to remedy this? such as other rods, or is there other pistons adaptable to the gmc that can be gotten in forged aluminum? the lesser reciprocating m*** is the majority of my concern

 

Seems like I heard of someone using Chev V8 slugs in a Jimmy, not too long ago.
The wrist pin diameter could be an attribute, as you can 'put it where you want it'.
I think it was a 348 or 409 piston, though...
There are a few Jimmy experts on here...I'll be interested to find which pistons they were.

 

I pondered a hemi for a while, but hey, everyone has a v8! how many people have the chance of saying they have a 6 that can kick a v8's ***?

 

those experts are the ones i hope will **** in at one time or another here! nothing counts like experience

 

You might want to post over on inliners.org. They deal with inlines and probably know a lot about your particular engine. Personally I'd shy away from playing with the bottom end, if it truly has low miles on it. Anything down there probably gets expensive quick. And changing one thing for another would probably screw up the balance. a re ground cam, better induction, head work and the dual exhaust sounds like more than enough to begin with. Maybe mill the head when the other work is being done to bump the compression?
Good luck!

 

i have been referred to inliners many a time, but on here i have always seemed to glean better info, and thanks for the thoughts! the 10 pounds seems significant but i to agree i may be better off using the original

 

http://www.patricksantiquecars.com/chevy_parts.html

Patricks can help with performance recommendations and parts.

 

Check that engine! Bet it is a 248 or 270. It's stamped on a pad by the distributor. Wrong truck to be born with a 302. A 302 is a man!

 

its a 302, and from what i know it is just the right truck to be born with one

 

I am no expert however Ross, venolia and arias all build pistons for the jimmy...the 302 is 4"borex4"stroke no need to change that just the pistons, cam...the valves use smallblock chevy .100 long, call Tom Langdon he built me a small cap HEI dist. he can also help with cam lifters and etc for the jimmy...he is a wealth of knowledge...Patrick ****s is a good contact for jimmy stuff as well. PM or email me if I can help in any way.
I am building a 302 now...I have made a intake for fuel inj and fabricated a SS header for turbo setup that I'm putting together...love these old jimmys. Good Luck and reach out if you need anything Fabmancoe

 

and i have already checked out patricks, that is where i want to get the headers and intake from

 

thanks for all that, sources are just what i need!! and wow, sounds like a dream engine to me! what are you putting it in?

 

i have a friend who said they used 327 pistons back in the day?

 

Jimmy is in 40 Ford PU

 

wish i had the car , always love the early mopars

hugs

Jen

 

I am a Jimmy fan as well......have a complete 270 and a 302 block/crank.....I am interested in seeing how you proceed on this build.

Ray

 

surfin, Hope you dont mind I am sharing our PM...thought the other guys may be interested??
Quote: You had asked if you could get 3-400hp with a $3000 budget
<TABLE border=0 cellSpacing=0 cellPadding=6 width="100%"><TBODY><TR><TD style="BORDER-BOTTOM: 1px inset; BORDER-LEFT: 1px inset; BORDER-TOP: 1px inset; BORDER-RIGHT: 1px inset" cl***=alt2>Originally Posted by surfin
my dream would be to have 3-400 hp with a budget around 3000, if i were to leave the bottom end as-is, is that just crazy talk or actually feasible in your opinion?
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Crank and rods are good no need to change...you will want 9.5-1 compression a 3/4 cam with good ignition.. sbc .100 long valve and matching cam lifters and springs. a decent header and carb set-up...you will easily make 300+ HP but you will be short with $3000 It should be balanced and Buffalo Ent can get you a new water pump...front harmonic balancer and ****** adapter for accepting gm ****** and sbc starter all this ****t adds up $$ thats why everybody goes sbc...it costs money too be different!! I still think its worth it...you just have to work harder to come up with the stuff...Hope this helps...Steffon
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http://hoppelfab.com/

 

no problem! this build will hopefully progress with expedience, im just a 16 year old kid that needs a car by when i can get my license in a few months, so i have motivation and i am on a schedule! haha, no stopping til its dooone

 

I'm just a hobbyist builder not a pro.But the 302 I built nearly 60,000 miles ago is still running good in a Hamber's 37 Chevy truck
What's done on race 302's making lots of power for short durations may have little in common with a daily driven street engine. The compression height on a SBC piston is nearly a half inch less than the GMC.I suppose a guy might have found a freak SBC piston . There's talk of using longer Hudson rods if you want to spend money.
Venolia or Ross pistons will run you about 500-600 bucks for the set.later 302's use a large pin,earlier engines use a .990 BBC type pin. I did the SBC valve conversion on a 302 head.1.94 intake,1.60 exhaust.Get .100 longer valves if possible,use SBC springs with Manley spring seats and aftermarket retainers.I had to fiddle with the rocker arms to get the geometry correct.The valve stem contact pad on the sheet metal rocker can be driven out slightly then spot welded in place to get things where they belong.Low cost Sealed Power tubular pushrods are fine for less than 5000 rpm.You might want to keep the 302 crank ,on a street engine the 10 pounds won't mean much.Whatever crank you use try to find one that needs less than .010 clean up machining.The crank is hardened and too much cut removes the heat treating and on a street engine that's run hard ,cracks may develop.This info comes from many crank grinders after a guy I know had a under size crank journal crack.
Be prepared to spend 3000 bucks if you buy carefully and do your own ***embly for a fully machined 240 hp streetable engine.To a Flathead V-8 guy ,that's a dream

 

As Truckedup stated, the engine he built is still going strong. It is my daily driver and has been more reliable than my '91 Jeep Renegade.

 

The early 302 was developed for military 2 1/2 and 5 ton trucks with a dual range Hydromatic. Your engine is a later civilian version, and likely will have the lighter crank all ready. If not dont worry about it. The best bet on pistons are the forged Venolia set from Patrick's at about $470. These will give you the proper dome shape, pin size, and compression height with out juggleing parts or doing expensive machine work. These are very high quality parts too. The 5 carb set up is a pain in the *** to set up and keep tuned, makes no more horse power than a single 4 barrel, but sure looks cool. The "middle" cam that Patrick sells works about perfect in a light vehicle. 240 hp may not seem impressive, but 240 hp at 3300 rpm with proper gearing damn sure feels impressive.
Good luck with your project.

 

I used Venolia in the 302 shown above.The machining on the ring grooves was not the best for 500 bucks.It had to be "corrected".These were ordered directly from Venolia and although they have engineering prints they wanted a plaster cast of the head combustion chamber for the compression ratio I wanted.
On my recent 261 build I got pistons from Ross.A bit more expensive but the machine work was excellent and what you get for 600 bucks.
You also need to specify the compression height ,and have the piston machined for it. As a fits all off the shelf piston may not give you the quench required.And the block gets decked if necessary to match the quench during an engine mock up.
Piston pins can be an issue on the later 1.060 pin rods. This requires somewhat expensive custom pins. I chose .990 inexpensive BBC pins and custom bushings.The rods differed in length by nearly .007. This was corrected by offset boring the custom rod bushing.Cost 225 bucks....I don't know if it's worth it,but it can't be wrong.