Inliners International Forums General Discussion Hi Performance 302 jimmy

Hi guys,
I read the whole thread about the 302 jimmy build for bonneville. Thanks for sharing the info. It mentions using Hudson rods, which sounds better than the quotes I got on billets. I was just offered a 54 Hornet long block( no- I can't have the Hi-compression head or the manifolds). I think the engine would be a 308?? Are these the correct rods?
Any thoughts on using 3 carter sidedrafts???
I also lost that thread. Where is it? thanks Wilbur

Try looking at Building a GMC 302 for LSR, https://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=47701&fpart=1

Thanks Jalopy 45.
That is the thread. I've got it bookmarked this time.
I was a member a while ago, but hot rods had to take a back seat to some health related B.S.
I'm starting my Bonneville project up again.
A big Jimmy in a 49 chev fastback.
The tip on some usable long rods in this thread is gold for me!
Last thing I need is a sidedraft manifold for the big port head.

What are you planning for carburetor?

Wilbur,

I'm the one that built the 302 Jimmy using Hudson rods. I originally tried to buy a set of aluminum rods from Bob Johanson but got screwed out of the $600 I sent him, and finally had to get half the money thru a collection agency.

At that point a friend and I researched rods and came up with the Hudson rod. It's 1.125" longer than the Jimmy which let me run a much shorter, and lighter, piston. I had custom JE pistons made and with the lighter piston, rod and wrist pin each piston/rod/wrist pin assembly was 1.25 POUNDS lighter than the stock Jimmy assembly.

The engine is in a HAMB dragster and we hold the HAGR records for MPH & ET. The engine made 346 HP and 347 TQ on our DTS dyno. We turn the engine 6000 rpm all the time without any problems.

Lots of work, time, money to build a good Jimmy.

Ron

you may want to look into 39-54 pontiac inline 6 rods instead. i purchased a set for my proposed low-deck stroker 248 based build. they are real beefy looking compared to the hudson rods, share the same metallurgy and rifle drilling that the gmc rods have, feature a 7.565 c to c, the same 2.125 rod journal diameter that the hudson rods use (ideal for conversion to the chrysler 251/265 rod bearings that Ron used), and have a 1.5" wide big end that can be easily narrowed to the gmc's 1.25" width. i plan on using off-the-shelf 3.905" bore late model gm ls1 pistons (around 1.38" compression distance) and a 270 crank offset ground for a 4.375" stroke for 314 c.i.

for your 302 based build, you will be able to use off-the-shelf 4" bore or larger ls2 or 6.0 pistons and run a little more stroke to compensate for the higher deck

garagepunkfan -

First, welcome to another Wisconsinite!

Second, tell me more about these rods. If my math is correct, they may be a viable option for a 292...? I'm sure someone will correct me, but that's the first thing that popped into my head.

-Sam.

not much more to tell than what i have stated already as i have not mocked up my combo yet (currently unemployed and just starting as a full time cnc program student with a 14 credit load this semester). the small end bushing will need to be honed from its stock .9369-.9375 dimension for the .945 ls1 pins. i also mis-stated the compression distance, its 1.3385"

The 292 pin is 2.100".
A .030" O/S Pontiac bearing (= 2.095" ID) will need the 292 journal turned down .005" to fit.
10.875" deck with 4.12" stroke and 7.5625" (7-9/16") rod makes the new piston CD 1.2525" @ zero deck - should work.

Sam, the only issues I see with this conversion, is you will have to remove around 1/4" of an inch from each side of the rods, and this might possibly machine in to the bearing tang notches in the rod and cap that locate the bearing shells. The 292 cranks already have little journal overlap, because of their stroke and relatively small journal sizes to start with. So that could cause further crank flexing by reducing the rod journal even more. Not saying it wouldn't or couldn't work, just these things came to mind right off as potential issues for concern.

Not sure about your math.
270 crank offset ground for a 4.375" stroke
The original 270 stroke is 4.00"
The 270 pin is 2.3125" (nominal), reduced @ maximum eccentricity to 2.125" removes .1875", which moves the journal centerline outboard by .09375".
The new stroke is 4.1875", not 4.375".

. . . and with a 3.905-inch bore and a 4.1875 stroke, one should then have a 300.9 cid engine. If a 248 GMC block can be bored safely to 3.905, then that's a lot of capacity for a 248. . . . but why not start with a 302?

God's Peace to you.

d
Inliner #1450

the bearing tang notches are well inboard of the original width. i forsee no problems there, and its 1/8" material removed per side of each rod

panic, welded stroker then.

Ouch $$$!

I have no idea what the limiting factor for stroke is in the GMC (rod shoulder to cam, to bottom of cylinder wall, etc.), but if you're going to weld it you could use a longer piston, even longer stroke, and ordinary shorter aftermarket rod, like the common 7.25" BBF, BBC, BBM.

BTW: one important clearance factor is that the stovebolt and GMC have the rocker directly above the cam (vertical pushrod), which places the cam in the rod path for longer strokes.
The Gen-3 292 etc. have the entire cam tunnel moved side-ways (angled pushrod) for clearance.

To use it in your GMC application...yeah an 1/8" of each side. To use it in a 292 engine that I was answering samwise's question to, requires a 1/4" of material to be removed from each side, thats what im referencing, the stock 292 rods are just slightly over 1" wide, and to use the same rods as you mentioned will require that much more additionally to be removed.

i would begin to worry about piston skirt speed with a longer stroke and a shorter rod. selecting 1937-38 pontiac rods instead (which i could not find readily) gets you a 1.999 journal diameter which could improve the situation by netting a 0.3135 stroke increase. i purchased 9 new-old-stock federal mogul 1939-54 rods with slight water damage on e-bay for $9.99 (for the whole lot!). take-out ls1 pistons are commonly found inexpensively on late-model performance forums, i purchased new main bearings and new-old-stock cam (to regrind) very reasonably from Memphis Equipment. so far i'm into this cheap. if the welded stroker, cylinder head work, balancing, and what little machine work that i can't do myself and will have to be farmed out is the most expensive part, then i'm ok with that

The mean piston speed tracks stroke length directly: 4.500" is only 7.4% higher than 4.1875".
For 4,000 f/m it's 2,400 ÷ stroke. 4.1875 is 5,731 RPM.
The instant (max) speed does go up with shorter rod, but again only a few percent.
What does change quickly is max piston acceleration, which more or less ~ with RPM^2 (6,000 is not 20% higher than 5,000 but 44%).

3.905" is pretty big for a 248 (std. is 3.719"), almost + 3/16".

Do you make the deck height 11-3/32"?

Garage punk,
Can you post up some pics of the pontiac rods? Tom

panic, Harry Warner stated in one of the Wayne catalogs that max overbore for both the 270 and 248 blocks alike was at 3 15/16. i calculated theoretical deck height at the 248's stock 11 3/32, which puts the piston a couple of thousandths below the deck. good info on piston speed/acceleration.

let's try this for a picture

http://i.ebayimg.com/04/!BlfY11gB2k~$(KGrHqMH-EMEtrLW3Tt9BLb11T,gNw~~_12.JPG

That looks like the rod I've been looking for to make a destroked 302 using a 248 crank. I knew there was a rod some where but old rod specs are hard to come by. Thanks, Beater

With all deference to Warner, these opinions as to maximum bore size were all believed valid at the time, but largely since the wall didn't actually fail immediately and not all these engines developed enough power to hurt themselves. Jahn's etc. sold many 3/16" and 1/4" O/S pistons to people who "never cleared up that overheating, oil consumption, blowby problem".
Since perhaps 1970, the wall thickness in general and in particular at the thrust surfaces is now known to be much greater than was previously acceptable. This does not include allowance for internal corrosion, core shift, inclusions, etc.
Not enough wall permits the shape to distort under combustion pressure, and makes it difficult for the ring to seal. You get a small amount of extra displacement, but the leak-down rate, oil consumption, crankcase pressurization and ring life are worse - on balance, it's a net loss. Jenkins reported less power from a SBC 350 block at .060" than at .040" for these reasons, and recommended "only enough oversize to clean up and remove any starting chamfer".
I'd try a sonic test of the specific block you're going to use before deciding on a bore size.

I do believe there is at least one Inliner running tractor rods but I can't remember which ones. It's not the source, it's the specs. With the Jimmy rod journal at 2.311 offset grinding the crank with a minimum bore gets you the extra cubes and a block that can be used again. I wonder what the wall thickness is on a Jimmy 6 VS a SBC? My thought is, how far can you bore a Jimmy before it has the same cylinder wall thickness as a STOCK SBC. Just to make an apples to apples comparison. A 302 starts at a 4 inch bore.

yes, agreed; a sonic check is definitely in order for this level of overbore. jimmys do seem to share the robust cylinder-wall construction typical of the higher-quaility gm engines of the era, i.e. 320 buicks, 303 oldsmobiles, and the aforementioned 239 pontiacs. 302 gmc's routinely take a 1/8" overbore without sonic check and there are a quite a few currently running, driving examples of 303 oldsmobile builds over on the HAMB that sport a whopping 1/4" overbore and are driven pretty hard. an educated guess tells me that gm probably shared the same core patterns between 248 and 270 blocks and simply bored the 270 a bit larger and machined the decks to different heights. 270's are still routinely overbored to the 3 15/16" size with no ill effect.

I'm pretty sure a GMC has considerably more wall thickness than an SBC:
1. it was normal practice at the time
2. it's a sound accounting decision (fewer blocks scrapped due to core shift, etc.)
3. extra weight is harmless in a truck chassis
4. more possible re-builds = longer service life

However, no "modern" V8 seems to work as well after about .060".

However, IMHO a comparo to a Chrysler 440 is closer (despite the bore size difference), because the long unsupported wall inside the water jacket is roughly proportionate to the deck height (SBC is 9.025").
The low-deck 9.98" B is preferred to the high-deck 10.725" RB for just this reason: wall stability.

IMO the large overbore trend of the 1950s is entirely due to a few factors now irrelevant:
1. "my Ford V8 accepted a big overbore without frequent failure, therefore I'm going to bore my Cadillac 3/16" as well".
2. "stroker crank? What's that?" Biggest motor wins.
3. bad ring seal, high case pressure, etc. were normal for not only rebuilds, but even some stock engines

An "ill effect" is subjective to the application: a street rebuild with 200 hp will tolerate a thinner wall than a 500 hp blower motor.

I realize you're in a tight spot - budget concerns suggest a specific bore size to avoid a cu$tom piston, so it's a compromise like everything else in motor building.

garagepunkfan -
Not knowing what the application of your GMC will be, I share the concerns of Panic and some of the others. You need to take a hard look at the rod to cam clearance. Even with a 4" stroke, the clearance is very tight. I assume that with the smaller 2.125 rod journal, the big end of the rod will be smaller than a stock GMC, but it may not be small enough for a 3/8" stroker.

Another concern would be the amount of overbore. If this is a street engine, I would be worried about overheating due to the thin cylinders. I have run 302's with up to .210 overbore in competition engines with the water jackets filled, but I wouldn't try it in a street engine.

The last thing I would question is if the tangs on the Chrysler bearings will be compatible with the Pontiac rods (I have no idea about this). Let us know how your project is progressing.

Bob

We have some creative thinking here, and i admire it. We also have a lot of machine work to get to 300.9 cid -- that is, if a 248 block will take a 3.905-inch bore, which some of us seriously doubt. If the 248 does take that overbore, it will -- like the 4.125 in my 302 -- be the last one.

A stock-displacement 302 could be built with far less machime work and a greater tolerance for rpm than a bored and stroked 248 that does not quite match the 302 in displacement or, i daresay, in power. In the long run the 302 will be far less expensive to build and maintain, and infinitely more durable.

God's Peace to you.

d
Inliner #1450

i want to apologize to wilbur46 because i really had not intended to hijack his thread. i hope all of this is some use to him as well. thanks to all of you for your replies and your insight. i have been thinking about this concept for well over a year now, and it's still a ways from reality. i'm taking panic's advice to heart a bit. i will reduce my proposed stroke to 4.1875, deck the block to 11.004 and build a 300 incher to avoid having to weld the crankpins, but i will stand by my concept that a long-rod, short-piston-skirt longblock with somewhat reduced rod bearing surface area (less friction), a favorable rod/stroke ratio (1.81), and a modern lightweight piston with a low-friction ring package can only stand to improve durability over a stock 302 bottom end. we all know the tall, heavy piston dillemma. as far as cam clearance goes, the big end is smaller than the gmc and with deciding to limit stroke to 4.1875, its centerline will be moved only 0.09375 closer to the cam. it has to be close to a wash between the two. only a mockup will prove it out.

the cost of offset grinding the crank would be cheaper for me than to purchase a $700-$900 302 core which i do not have.


i'm shooting for around 12.25 compression with flat-top pistons by filling the chambers to 64cc's, mostly on the sparkplug side, and using extended tip plugs to reduce the cartridge-fire issue. i will try to emulate the early pontiac v8 as a model for the combustion chamber. i'm planning on building a dual-quad intake for rochester 4gc carbs, and plan to run e-85 for fuel

cam specs will be: flat tappet, 242/252 duration at .053", .316"/.316" lift at lobe (.445"/.445" calculated with a 1.41 rocker ratio), intake opens 21 deg ATDC, closes 41 deg ABDC, exhaust opens 48 deg BBDC, closes 24 deg BTDC, 100/102 installed centerlines, lash .018"/.018" i plan on using EMPI 31mm vw type 1 mushroom tappets with bronze sleeves pressed into the lifter bores, Manton 3/8 pushrods, second-hand sbc titanium retainers (they show up on e-bay frequently and pretty cheap) with buick 455 valvesprings, pontiac 350 intake valves cut down to 1.94 and pontiac 455 exhaust valves used as-is. i will make my own rockershafts and girdles for them. ignition will be a stock delco dizzy converted to dual point with a dyna-flyte kit, mallory voltmaster coil, and an msd-6

i also have an aluminum flywheel and will use the new BHJ 235/261 balancer

I agree on the shorter piston - the tensile (fatal breaking) load on the rod is pretty much a function of the recip weight, and reducing it by 1/3 or more is a big step forward.
The rod ratio is much better, but it's preferable to the strokers using the 292 rod (which is worse, and with a smaller pin).
Trying to do trig mentally is ^%#${@!!, but... since the rod angles at every point are almost the same (rod ratio determines the angle), and the pin is only bumped over by 3/32", I agree that the cam clearance should only be slightly less.
On the cam: I know that some exhaust bias was effective back in the day and (JMO) probably still true, but the flow data posted recently on the 235 cause me to question all of it.
The split between the 848 as-is and well done show much more intake improvement than exhaust. From this I conclude that a highly modified engine wants even more exhaust bias than a hot-rodded stock head.
I'd leave the event split for now, until you can have the head flowed. You may out to be on the money (can't be too far off), but the results might be better.
Is this specifically a VW Type I grind? I.e., requires a 31mm tappet?
The stock GMC rockers don't impress me, especially with higher speed and more spring, but I'm not sure what will work. The similar Slant 6 (laminated) rockers have been stiffened back-in-the-day by simply spot-welding plates to both sides across the bridge above the shaft, drill through the plates first for rosette penetration.
AFAIK the GMC is about 3-3/8" long O/A, which is huge vs. automotive stuff (except Chrysler hemi exhaust). There's probably something for a tractor or Waukesha that's close but I haven't found anything. One good thing is that the pushrod and valve are parallel, which certainly isn't common in modern engines.
BHJ: I'd ask them if your longer crank length and stroke length changes the frequency much (stock 235 estimated at 216 Hz). They may be willing to slightly mod the rubber to adjust without $$$.

the grind is a variation on the crane fireball 316-2 grind for harley evos with the lobe centers tightened up a bit. the cam calculator software i was using indicated that as i moved lobe center closer to 100 degrees on the intake side the dynamic c/r rose slightly (to 11.26:1) and the effective stroke (3.80) moved closer to the actual dimensional figure of 4.1875.

i don't claim to be a cam expert so any advice is appreciated, and i will admit that i don't have a good sense of how well a grind like this would work with the mushroom tappets, only that they would effectively exaggerate the camshaft events to some degree, but also to reduce valvetrain mass in a very cost effective way. there seems to be very little info out there on what to look for in a mushroom tappet grind, or what diameters work well. that's really why i put all this out there. Delta's really reasonable on regrinds, so a mildly experimental grind just to see what works is not out of the question

i may end up circumventing the entire intake and exhaust porting altogether on the head and raise both significantly (milling the entire upper port side of the head away and brazing in a big chunk of billet steel with new ports - similar to what they did with cleveland fords in pro-stock in the '70's . i have several small port closed chamber heads to play with.

i am not in love with the gmc rockertrain either and the stiffeners are a great idea.

Which cylinder head casting are you going to be using for your engine combo. I just got through porting a 983 casting for a guy, and that is probably the best casting # for using on a performance build-up such as what you have planned. Also, what valve sizes are you going to be running.

Sorry, no sale on the cam.
That lobe is a roller tappet only, designed for a very big roller tip (.9375" OD, not made for automotive apps TIKO). It might not actually be damaged by operating against a mushroom, but the curve area will be way down, and the accel/vel rates will be all over the map - not a good idea.
If Delta has any VW Type I (aircooled flat 4, 1600cc etc.) profiles, these are all suitable if the timing figures are bumped around. I'm not sure if their GMC profiles for the original tappets are actually based on the .990" tappet, or just a borrowed SBC .842" profile (very common), in which the VW tappet would be a lot of work for no change in valve action.
Remember to tell them your rocker ratio, since VW cams are normally made for the original (much smaller) ratio.

There's no easy fix to replace the rockers: find something, probably from a large engine.
I wonder what the later GMC 305 V6 rockers look like? Too short.
Ford 534, etc. also possible.
The 2 factors that have to be very close:
1. length almost the same (pushrod hole center to stem tip)
2. ratio > 1.5, which means the shaft has to move toward the pushrods

For inserting a port, a large-radius piece of Schedule 40 weld-ell in 2" ID allows you to make the insert at an angle and have either a side-draft or down-draft port without fabricating the curve itself.

However: Toyota F (Land Cruiser) engine has some commonality with GMC, and these parts are easy to find. However: there are several chamber types as to valve angles, etc.

panic

ok, lets try this one on for size...a Schneider 289F vw Type 1 grind, installed 6 degrees advanced...

240/264 duration at .050", .380"/.404" lift at lobe (.536"/.570" calculated with a 1.41 rocker ratio, .518"/.552 at valve, factoring .018/.018 lash), intake opens 20 deg BTDC , closes 40 deg ABDC, exhaust opens 64 deg BBDC, closes 20 deg ATDC, overlap 75 degrees, 106/106 lobe centerlines

RPM Range:
2800-6600



Static compression ratio of 12.22:1.
Effective stroke is 3.82 inches.
dynamic compression ratio is 11.06:1 .
dynamic cranking pressure is 240.44 PSI.
dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 11.06 :1.
V/P (Volume to Pressure Index) is 198

CNC-dude, the heads that i have are the 2194819 small-port, closed chamber variety.

i saw the modified 235 head on e-bay with the downdraft intake ports and i'm aware that the Rajo's were modified somewhat similarly and i had thought about that approach as well, but it doesn't solve the crappy exhaust port issue. it's possible that the gmc heads flow worse on the exhaust side than 235 heads do. the head bolt bosses could all be counterbored to use shorty allen head capscrews and one could raise all of the ports as panic described, plunging into the head at a 30 degree angle on a horizontal mill at all port locations and brazing in the weld-ell's. a new flange would also have to be brazed on.

Should work, but I'll bet it's intended for lower static CR.
Delta or ? will have to get the last word on whether your existing lobe will take it - big cut off the base circle.
Your cylinders are also 60% larger than the subject engine (2 liter), so any cam will be much milder.
Remember to think 400 V8, not 300 L6 for the characteristics of the cam, valve size, CFM. etc. I always forget this!
Can you tolerate such high DCR?

The BHJ may not need anything at all done - I notice that the same SEMA damper is used for all Slant 6, despite 1" difference in stroke.

i think the e85 fuel will make the engine very forgiving of the high DCR (94 to 96 octane). any suggestions about ignition timing?

Nope!
Part of that is based on quench, some on rod ratio, how compact the chamber is @ TDC (surface to volume ratio), what the dome looks like, fuel burning rate.
Just a guess: 30° is probably safe as a test (i.e., shouldn't knock with E85), and more conservative will raise the temp.