I thought some of the inline guys would like this.....Here is a few details and pictures of my 302 jimmy stroker build, keep in mind this is a street engine, The idea was to get the most cubes and use all off the shelf parts to keep the cost down, I started with a really bad block because i couldnt find a good one, the block needed 3 sleeves, we bored the block to 4.125, 1/8 over stock, the motor is using nos chrysler 251 spitfire rods, 7.880 long, with the crank offset ground .100, for a 4.100 stroke, Its a little less than 329 cube, but I call it 327 to rattle people, The pistons I used are off the shelf KB #127, there 400 sbc pistons for use with a 5.7 rod, 1.433 compression height, .100 dome. The whole deal worked out without decking the block, the deck height worked out to be 0, Although I would have liked to take a little off the block just to square it up, It ends up with a light short skirted piston compared to the big slug the jimmy used, saving alot of weight, I would have weighed one of the jimmy pistons but not one came out in 1 piece, Im using the 302 head with 194-160 valves and sbc hardware, with some port and polish work on only the exhaust side. I took .060 off the head, Using the dome pistons and .060 off the head, Ive got just under 9-1 compression, which should be good for pump gas. The chrysler rods are pretty beefy, I knocked the stock small end bushing out and machined the end for stock sbc press fit pin. The big end is about .050 narrower than the jimmy rod, but ive run my stovebolt stroker with success like this, The chrysler uses a 2.125 big end journal, about .187 smaller than the jimmy, so with the smaller journal and the longer stroke you end up having to do some time consuming relieving to the camshaft for clearance,I must have had the camshaft in and out of the motor 50 times. Other than that it was a pretty easy deal, A couple weeks and we will be firing it up,
I envy you, I can't even find a bad block. 302 is right, I should have said. I would like to own a hoped up 302 one day, but I hear they are getting harder to come by.
It's very interesting and you must be doing this because it's fun. But it seems like a lot of work for 9 cubic inches. More power to you anyway.
I recently had a 270 crank crack clear through the #5 rod journal - when I took the crank to a reputable shop he said that the crank had been ground too far (it was 0.030 under) and that the surface hardening wasn't deep enough from the factory. His explanation of the crack (and we found another starting across journal 6) was that with the loss of surface hardening, the surface was no longer in compression and fatigue failure was the ultimate outcome resulting in cracks. I am curious if you have any concerns about the large offset grind with your crankshaft? I eventually found a low mile std dimension 0.000 crank in good condition and had it reground to 0.010 with surface hardness checked good on all journals. The first crankshaft lasted about 15,000 miles with excursions to 5000 rpm and just under 200 HP at the crank. Has anyone else had a problem with a GMC crankshaft cracking?
I love sixes but don't know a hell of lot aboutem . Are they that hard to find? Exactly what year are we talkin about?
I cant wait to see how this comes together. I would love to have something like that in my fleetline. I second waldo. I belive he said it came in a 329 cubes which is 27 more cubes. It looks like a lot of work but really not that bad and it should run like a scalded dog. The use of off the shelf parts really helps the easy part. He did his homework very well on this one.
aNY IDEA ON WHAT DISTRIBUTOR IGNITION YOU ARE GOING TO USE ??? I HAVE SOME GMC IGNITION STUFF LAYING AROUND INCLUDING A COUPLE OF THE MILITARY SHIELDED UNITS CONVERTED TO ELECTRONIC ETC. i OWN A 322 GMC IN A 29 MODEL A WITH FOUR SPEED AND QUICK CHANGE... JIM
No. It's a 320 with the +.125 bore. The stroke and rods and cam clearancing nets him 9 more inches. Do the math. I send my cranks for nitriding after grinding to restore the surface hardening. Pretty common
It's always fun to see guys still trying to use off the shelf parts to get more out of these engines. Some of those parts are not easy to find anymore. I am planning to destroke a 302 with a 248 crank. I'm looking for a set of 8" Hudson rods. The destroke is to allow more rpms with less stress so I can get more from the McCulloch that will be on it. Bill Fisher built some short stroke street engines.
Gear, just a friendly suggestion here. Looking at the deck of that block, I'd want to take a skim cut too. You do know you can deck domed pistons right? .010" off the deck, and .010"-.015" off the domes. Unless you checked and corrected center to center on those stock rods, you might have a few slugs out of the hole. I can see this is not your first rodeo, just friendly suggestions. Anyone with a mill, piston vice and a hour can deck those pistons for you, and this way you can sleep pretty with the blocks deck. Not only to true it up, but for the correct finish to work with the head gasket material also. Nice project, thanks for posting, TR
I have two blocks with .150 over and no water. One block ran in a vintage stock car for numerous years.....
The 302 only came in the 2 1/2 ton GMC truck I believe, why there never was much for speed parts, the motors in no cars nor pick-ups. Just Bonneville, really. Mid fifties production vintage, somebody help me here...
I wasn't really looking for cubes, although i'll take all I can get, I wanted the longer rod and the shorter, lighter piston. It makes a engine designed too move 2 1/2 ton truck into a car engine. It may seem like alot of work, if your calculations are right its easy, the cam is a little tricky, look at a stock 292 chevy camshaft, Its relieved about the same. As far as the crank, My 261 stroker using the so called caddy pistons, is stroked if I remember right .125 using a 292 rod hasn;t given me any trouble so far, but its not a race motor, either is this one so I think I'm okay, besides these motors look tough as nails to me. As far as the rods, they were a nos set I found, they measured within a few thousands of each other ctr to ctr, they were good as is. I wouldnt have minded the piston out of the hole .005 or so for compression purposes, another note on bore, We bored this block 4.243 for sleeves and never saw water, we ended up with roughly .060 thick sleeve, It was a questionable block, so nothing was lost trying it, I was surprised the machine shop saved it.
GMC bubba, Im using a wico xv magneto I found for a ford six, I gutted and machined the gmc dizzy and a sleeve to mate the two, The original advance is still in the base of the distributer, I rebuilt the mag, we will see how it works out
sorry bob, I dont think theres any help for you, your too far gone, hehe lots of jimmys ended up in hot rods, street cars, 30's 40's 50's chevys, capable of more power than a stovebolt six........
The 302 came in military and civilian versions between about 1952-59 . The military version has no fuel pump opening,different oil pan and a few more casting lumps.Speed parts were available,what fits a 228-270 fits the 302 other than only the intake ports on a 270 are smaller except for the limited production 270 H head.. A well mannered GMC 302 street engine might make 230 hp.Short duration racing engines make silly hp with very expensive parts.
I heard a rumor that this mill is going to get fired soon....can't wait to hear it run! And just wait till you see the car
What balancer are you guys running on the 302 engines? My experience has shown that these motors like a larger balancer. I have used a zero balanced BBC piece with very good results. By that I mean better bearing life as well as longer crank life. One other benefit is a few more RPM.
What did did you use for - or did you use - a harmonic ballancer??? The lack of a ballancer - or an inadequate or improper one could be the cause of your crank woes. Mart3406 ===========
Motion Guru is busy but until he shows up here's what I know.His 270 engine was built with care,forged pistons,balanced etc.The damper was a dedicated 270 piece rebuilt by Dales Damper service. Is his engines's damper adequate? Who Knows really because there's so little long term durability information available. I built a 302 in 2005,the usual stuff,more compression with forged pistons,hotter cam,balanced and so on.I assembled the engine with the usual hobbyist skills.The machine shop didn't want to turn the crankshaft because of harness as Motion Guru mentioned, but had to go .010 to get proper clearances.The stock forged crank is hardened to about .040 but can vary widely.The stock 302 damper was also rebuilt by Dale's Damper.The engine now has about 60,000 miles on it and still runs just fine. I believe,maybe wrong,that is you avoid excessive high rpm's ,a good stock damper will be ok on a street engine.
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