Project Blown 305

[Cuttyman9]

I picked up a gmc 305 V6 that I plan to hot rod out a bit.

I was lucky enough to find another in the junkyard to snag some parts from.

My goal: punch out the 305 with aftermarket/custom internals (ideally offset grind down to BBC or SBC rods and use an off the shelf piston for a BBC, if not able to bore it that far use off the shelf BBC Pistons and aftermarket rods with as much stroke as possible)
-Upsize the hole in the rocker arm shaft pedestals, use (BBM BBB or FE full rollers dependent on which is closest to fitting) the shaft size isn't far off.
-build larger shafts for this
-if necessary build new pedestals
-have cam redone for performance and potentially a roller setup(if only defining characteristic is needing to be flat vs banked to work with a roller lifter)
-machine next to lifter bores for link bar
-bush the bores to the correct size for a common engine lifter
-laser cut flanges for the intake to fabricate a steel blower manifold to mount an 8v71 supercharger off an old diesel generator(regardless of if it adds power, it needs to look super mad max-like)
-modify the pan for a deep sump
-build headers using laser cut flanges from thunder v12 (hoping to find magnum heads in the meantime)
-few other ideas but they aren't coming to mind.


My questions:
Does anyone know how thick the bores are? Did they cast one block and bore to desired displacement?
Or did they cast multiple blocks with bore thickness dependent on end displacement?

I ask because if I can I'd like to punch this out as far as I can go (5.125" if possible) and want someone who's had theirs apart and at the machinists testing the cylinder wall thicknesses chime in if possible.

[TJ's GMC]

These engine blocks are All water jacket. From what I have read, .060 over would be tops. There's plenty of bore to begin with anyhow so I'd be looking at stroking as much as possible. If not, find a bigger v6. There's an old story of a guy who hot rodded a 478 v6....made tons of power...more than enough to take on spiced out 454's. And it could handle 5000 rpm. The 478's have a 5.125 bore.

Also...in for the build. Be sure neat for sure.

[Cuttyman9]

Ah, I wasn't sure. With some engines they leave them thick stock (older Chevy 283s and 352 FE fords) so you could overbore to the larger setup.

I would one to snag a 478 block/crank. Does anyone in northern Cali have one sitting around?

My goal is to put down as much torque as I can under 4500rpm. The Piston speed with the max stroke (ideally 5.18" with a 2.0" rod journal) 4500rpm is max.
It would produce a 644ci if I can use a 5.125" bore and 5.18" stroke.

I want to put a smack down on a lot of the big motors. This has the biggest architecture I've ever seen and 6 bolts per cylinder should allow for a good chunk of boost.

Maybe we can get these motors on the map!
Even hot rod didn't mention these on their forgotten motors article.


I wonder if we could locate that guy who built a modified one

[1972RedNeck]

Originally Posted by Cuttyman9

Ah, I wasn't sure. With some engines they leave them thick stock (older Chevy 283s and 352 FE fords) so you could overbore to the larger setup.

I would one to snag a 478 block/crank. Does anyone in northern Cali have one sitting around?

My goal is to put down as much torque as I can under 4500rpm. The Piston speed with the max stroke (ideally 5.18" with a 2.0" rod journal) 4500rpm is max.
It would produce a 644ci if I can use a 5.125" bore and 5.18" stroke.

I want to put a smack down on a lot of the big motors. This has the biggest architecture I've ever seen and 6 bolts per cylinder should allow for a good chunk of boost.

Maybe we can get these motors on the map!
Even hot rod didn't mention these on their forgotten motors article.

I wonder if we could locate that guy who built a modified one

I like your thinking. I have a plan for a similar build (naturally aspirated) in the near future. My plan is to "only" grind the crank down to BB Mopar rod size (2.375) and then build from there with custom pistons, cam, manifolds, induction, and the like.

Let me know when you start having valve train parts made, I would be willing to double your order which should get us a little better price (maybe).

Actually, if we could work together through the whole thing with identical setups (different cam grind and piston CR would be the only difference), we may be able to get a better price overall, and I wouldn't mind having more expertise than just I can provide.

[Cuttyman9]

Awesome!
Yeah I may be limited in what I can do with this thing but I'd like to know he maximum.

When I sort the rocker system I will definitely post updates.
I need to get a FE/BBB/BBM rockers to see what's similar enough.

Those 3 are relatively cheap to buy a set either eBay for Chinese ones or off summit and you can buy individuals of them which is nice.

Ideally I'd like to just upsize the shaft just enough to open the mounts up and move along with that as it's only .05" larger (.79 to .84 with I believe a mopar rocker)

The lifters are a bit trickier because the spots they sit are pretty filled in so it will take milling the spot between them down and pressing a bushing to match the diameter and make a link bar.

It may be easier to bush the hole, run some inexpensive standard mechanical lifters and a big cam

[Cuttyman9]

With those rods and offset grinding getting to 383 from a v6 would confuse so many lol
Need to have a big v6 badge and 383 badges lol

[1972RedNeck]

Originally Posted by Cuttyman9

With those rods and offset grinding getting to 383 from a v6 would confuse so many lol
Need to have a big v6 badge and 383 badges lol

LOL that would be good - of course a 351M is a big time headsratcher as it is.

Yeah I'm thinking my 478M with offset ground journals (2.811 down to 2.375 plus 3.86 stock stroke = 4.3" stroke) will net me a 538 when bored .030" over. While it might be able to go bigger, the fact that we are shoving all the air through 6 smallish ports (compared to 8 large ports on most big blocks) will just mean the bigger we go, the more low end we have with out much top end benefit.

I'm thinking that 450 to 500 HP and 700+ ft lbs of torque won't take too much to get out of a properly setup 538.

[Cuttyman9]

I am hoping to have some weird controversial CI to paint on the hood.

I want to use one of the 8v71 style blowers cuz they have a mad max style.

[Cuttyman9]

I agree, that would be awesome. My goal is double the torque output, I was hoping to find some magnum heads to use just cuz even with the 305 and a blower it's gonna require as much flow as I can get.

I really need to track down a 478 out here, that would open up my options for cubic inches so it's not all stroke if the bore cannot be taken to a much greater size.
I guess I'll need to pull everything off the block and take it in. Also cool is all of these are old motors so they are prolly high nickel content which is supposed to be good for this kinda stuff and hard to scuff.


This whole thing is going in my 66 C20 I'm converting to 4x4, I'm in California so they have have those stupid "clean air vehicle" stickers id like to copy and change the wording on and put on the back with the cubic inches next to the blower protruding old nascar style. I need to chase down the GMC V6 emblems for the hood too.

[TJ's GMC]

The 305's are super common down there....so I'd grab a couple blocks and experiment on one with how far you can overbore. I'm speaking from what I have seen more than anything...when I had mine apart with the casting plugs pulled out of the block...it was nothing but huge water jackets between the bore spacing. Thereby somewhat confirming a Huge over bore would either cause premature cylinder failure or you'd hit water real fast. .060 over seems to be the common safe overbore size on most old engines. These things produced pretty decent torque as it is....so a bottom end lightening...more cam....lighter pistons....and some boost will wake them up big time anyhow. If I can't find a 478, eventually I'm planning on rebuilding the 305 somewhat like you are.