The Stovebolt Site Forums The Shop Area The HiPo Shop MNSmith's GMC 302 build

I have a lathe and a Sunnen hone, ya know, and I do that kind of custom stuff all the time! I'd be glad to adapt that set of spare rods to any piston/wrist pin combination you decide to use.
Jerry

Have you considered running that engine on a fuel other than gasoline? Higher compression figures work well with alcohol, plus various "oxidizer" additives such as Nitromethane or Hydrogen Peroxide. Just hang a trailer on the back to haul the fuel tank around. Dad's 300 cubic inch flathead V8 in his 34 Ford 3 window coupe dirt tracker running on an alcohol/peroxide mix got 8 laps per gallon on a 1/4 mile oval.
Jerry

Great outta the box thinking Jerry. Perhaps E85 or CNG.

RonR

That had me laughing, Jerry. I'd like to keep it streetable without the extra expense of special fuels. What are they up to? $2000-2500 for a barrel of nitro? Pump gas is the target.

A nitro 302 though. I wonder how long it would last before sending the head in to orbit!

CARB would give you 30 days in the electric chair if they caught you running a rig like that on the street, too! Some of the Bonneville guys experimented with O2 gas injection on 302s back in the dark ages- - - - -not sure how long the pistons and valves lasted. Ron, I never considered E85, but that opens up a whole new can of worms to play with, compression-wise! Just break out the pin vise and some carburetor jet drills!
LOL!
Jerry

Jerry takes reaming you a new one very, very seriously.

I'm glad I didn't have a mouthful of coffee when I read that one- - - - -I'm sure I'd be needing a new keyboard!
Jerry

Doing a little back and forth with a piston manufacturer.

When there's not much to do, you paint body parts and make plug wires.

You might get a piston maker to fit a GMC pin into a piston with the compression height you need, and/or get them to alter the compression height on a piston they already make.
Jerry

The .990" pin isn't an issue. I've got choices to make. .468" below deck gets me 8.5:1 compression, .435" below gets me 9:1. I corrected some numbers that I had given him, so I think I'm waiting to hear back about that. He says that flat top would be the best design. Otherwise, I think we're at the point of agreeing on piston numbers, total cost and how much of a deposit he would need to get started. Email has been quiet today so I figure they've shut down for the week. Race season is kinda over for the year. I'll call on Monday if I don't hear anything by Wednesday.

I've always found it to be simpler ( and less expensive) to alter existing parts rather than allow a custom fabricator to put a vacuum cleaner into my pocket. Those parts don't know, or care what the original application might have been. I just paid a king's ransom ($45.00) for six brand new pistons that will be going into my 300 cubic inch stroker 261. They think they're supposed to be in a Buick 231 V6. Another project coming down the pipeline is going to be figuring out how to put a 24 valve DOHC Atlas cross flow head on a stovebolt block.
Jerry

We'll see how enthusiastic I am if a case of sticker shock gets me. I do have a set of the Jahn's that came out of it, but I doubt they are the correct diameter anymore. I should measure and verify. Someone also, at one time, handed me a brand new set of 265 pistons. Two more in the box than I need for either the straight or the V. Definitely too small of a diameter. I can always put them on the pile of things to sell so I can buy parts for this project.

But, yes. If sticker shock is an issue, alternatives will be looked at. This engine will run, one way or another. It is not destined to be a boat anchor.

There's such a wide variety of basic 4" diameter pistons of all brands and OEM applications that I would have a hard time imagining there's not something close to what you need, considering either/or compression height and wrist pin diameter. Instead of limiting your search to forged pistons, look at what's available as hypereutectic cast. Those things have virtually zero thermal expansion, and they're tough as woodpecker lips. When I machined the crown on a set of Speed-Pro hypereutectics I was going to use on al altered-stroke Chevy 305 V8, I ended up having to use a carbide tipped lathe cutter bit- - - - -a tool steel bit would get dull after a couple of passes.
Jerry

Sticker shock wasn't so bad. Pistons ordered. 6-8 weeks wait time. So, I work on little things. Like oil lines for the full flow oil filter, fixing a broken sprinkler, and hanging Christmas lights.

Inpatients preclude me from doing what a lot of you guys and gals do. By the way no skills either. Keep on doing what you do so I can pretend I know what you are talking about. I will only do this.

Found a T-5, S-10 version. I can start the process of fitting that to the bellhousing while I'm waiting for the piston tree to grow my pistons.

How much power are you expecting to make? Is an S10 T5 going to stand up to it or do you need to build that too?

Not even part of the thought process right now. Wanted something in place so I could work on sheet metal and such. Dog house, trans tunnel, where to hang the computer, will steering clear where I want to put the oil filter and fuel system, etc. That little dance one does when they're trying to put together a jigsaw puzzle, but aren't sure if the pieces will fit or if it is even the correct pieces in the box!

Sometimes you want to make stuff, sometimes you want to have stuff made for you. Fitting the T5 to the bellhousing. Will probably go with a McLeod 10" clutch and hydraulic throw out. I don't believe I can fit a 11" or multi disc in there even though the flywheel is drilled for either.

Keep me in mind when those pistons need a home. I can use them on one of the Frankenstein 261s. Bore size is not an issue, as at least one version of that engine is going to get all six holes sleeved, the one that's getting an Atlas twin cam 24 valve head. Some preliminary measurements today confirmed that a pretty fancy adapter plate can probably marry up the block and the head.
Jerry

I'll know next time I crack open the V6. It's only 200 cubic inches from the factory.

I have an oversize bellhousing from a big truck 302" GMC that takes an 11 1/2" clutch. Cheap!
No selling on this forum. PM me if you are interested.

Well? These showed up. Guess I better get back to work on that engine.

We need a "bated breath" emoji for this thread.

Getting the gap right.

You may have the gap right, but #6 cylinder is going to have a much higher compression ratio than #5.

#6 is .400 or so in the hole. #5 is .070 in the hole.

I only have two hands, so one cylinder at at time. #5 is next.

I think that Bill is referring to the two different piston crowns. 4 and 5 have dished pistons, while #6 if flat.
At least it sure looks that way.

That's the way it was at that point in time. But, even if I left it that way, #6 would still make less compression than #4 or #5. With the 12 port head, #4 and #5 would make 13+ compression. #6 will make 8.5-9 compression. You might be assuming that the dished piston and the flat piston end up in the same place in the cylinder at each cylinder TDC. But they don't.

#6 is at TDC and almost 1/2" below deck in the previous picture. The pic below shows the dished piston at TDC. It is .070 below the deck.

At this point in time, #4,5 and 6 will make 8.5 - 9 compression. And if I put in another few hours, #1,2 and 3 will match #4, 5 and 6.

Thanks for the explanation Mark.

Better.

Makes sense. #6 was a new piston, and others were older ones that you're replacing. Wasn't clear back then with that pic, but sure much clearer now.

Was chatting with the previous piston maker. He looked up the job and the pistons were ordered in 2015, for this engine. Here's where things went a little upside down and now things make more sense to me. Piston maker had a crank to deck number of 11.360". I measured it at 11.1875". With the proper deck height, the Arias pistons would have been a 9.5:1 compression ratio.

Makes me feel better about the cam that's in there. Though I still want to call the cam grinder and see if I can get the card for it.

Once I get a little further along on putting the Atlas 24 valve DOHC cylinder head on the 261 Chevy block, it's going to be interesting to see how the compression calculates. With the bore and stroke I'm planning to use, the displacement will be just a fuzz less than 300 cubic inches. There will be a 3/8" thick steel adapter plate between the head and the block, with the pistons and cylinder sleeves sticking up into the adapter plate about 1/8" or a little more above the top of the original 261 deck height.
Jerry

So, somewhere around a 1/4" cylinder chamber? Keeping the rings down in the block?

Question for the general population. First I've seen. The rod bolts were twice tacked to the rods, each of them for 4 tacks per rod. Is that some kind of old trick? Also, a what would you do question. Reuse the ARP washer flange nuts, torqued to 40-45 ft lbs? Or use new, which I do have some grade 8 fine thread, washer face stover nuts somewhere around here. If I do reuse the ARP, I'm going to put them on different rod bolts than what they came off of, so at the very least, they wouldn't be locking right back where they had started at.

Since all the cylinders will be sleeved, the ring position won't matter. The sleeves will project up into the adapter plate about 1/4", making the surface smooth all the way to the intersection with the adapter. There will be a copper sealing ring between the top of the sleeve and the plate, similar to how the cylinder on an air cooled Volkswagen engine mates to the head. All the Atlas head gasket will do is seal compression and coolant at the top of the adapter plate. Coolant will be routed from the block to the head via external fittings- - - -no coolant flow through the adapter plate.
Jerry

I'm adapting stovebolt connecting rods to use ARP bolts salvaged from NASCAR rods after a race or three, by installing 3/8-24 Helicoils in the rods and inserting the bolts from the bottom. There's approximately twice the thread engagement than using nuts, and the bolts don't stick down as far to interfere with the camshaft as the stroker crankshaft turns. The Helicoil 'I'm using is also stainless steel.

Those bolts are rated for 9K+ RPM, on an engine that produces several times the HP any stovebolt engine could make, regardless of how it's modified. Overkill? Sure, but why not? The ARP bolts we used on round track engines back in the 1970s had a "stretch" specification, not a torque value, and we usually had to torque them to somewhere in the 65-70 foot pound range to get the proper stretch.
Jerry

I forgot all about the sleeves.

Had some time to put the pan on. So, the bottom end is closed up.