Inliners International Forums General Discussion Hi Performance Were there any of the GMC 302 with 7 mains.

I am working on an engine program where we are going for maximum effect with the GMC 302 platform. One engine will use the Howard head and one will use a stock casting. I was considering adding the mains and doing a billet crank.
Which head casting is best to start with.

The engine will use mechanical fuel injection per rules, and a good ignition system.

Seven mains came with the later series sixes. I don't know if there was ever a custom block or a girdle setup.

There were 4 different GMC heads. They will all fit all blocks. Two had large intake ports and are the choice of most racers. The 270H head has a domed chamber and works with pop up pistons, The 302 head has a D shaped chamber and uses a flat top piston. Lots of work has been done on each type. You'll have to figure which fits your build.

"adding the mains"

Where would they go?

I'm thinking about a girdle where the 3 extra mains would be part of the girdle and then bolt this assembly to the block and be the main caps for the original 4. At the very least I will build billet main caps
With that billet crank then We can really turn the rpm level up.
I am having some M13 tool steel cam cores spun up to fix cam flex.

I have not got to put my hands on a block yet, off hand what is the bore spacing and cam journal diameter

My point was that there is no place on the block to act as the upper half of the extra main bearing supports. Seven main L6 engines have a bulkhead in between each pair of cylinders.

I assume the new crank will have an extra journal bisecting the existing counterweights where needed?

The bore pitch is staggered (common practice at the time), and larger between cylinders #2-3 and 4-5 than between cylinders #1-2, 3-4 and 5-6, perhaps 4.375 and 4.5625?

It will be interesting to see where you go with this. There is very little post 50s information on these engines even though much has been learned and new tech is available.

The Howard is a big step forward compared to any original, but the piston dome is going to be a puzzle. It contains the entire chamber volume, so something like a modern (heart shape etc. sunk into a full size dome (bore diameter). The surrounding surface area can be quench, or only part. Choose your own static CR. Some high end piston manufacturers may have already made shapes like this since the "Heron" (plank) chamber has been used in other engines.
The intake ports are 45 degree downdraft, all valves are vertical and parallel.
Howard intended stock rockers to be used, so the top surface has that bolt pattern, but I wouldn't use them. Not easy to find better, they're really long (about 3.41" adjuster to pallet), perhaps individual or paired pedestals?

Yes, the crank would be a custom one-off billet. with 7 main journals.
I still have not got look close to one of these blocks. We have a bunch of complete engines, bare blocks, and cranks, I just have not driven the 120 miles to where all of this is stored.
Thanks for the Bore Spacing numbers. What is the traditional maximum bore on these? I was under the understanding that 4.250 bores were possible.
I need to either source drawings or draw them up for all surfaces on this engine. We have CNC and water Jet capabilities and I will make any drawings we do public
I need these surface
1. The rear of block to build adapters for transmission.
2. Deck for head gasket and torque/ honing plate
3. pan to build custom oil pan and dry-sump
4. front of motor for motor plates

4.25" may be possible in a 302, but 4.125" is the largest I've heard of, and even this may compromise wall rigidity.

Here's the "12 Port News" section on the Howard head:
http://www.inliners.org/12_Port_Story/pt_six.html

The OEM rocker shafts are .791" like the 235/261, but longer.
Crower makes roller tappets (66259-12).

270 and smaller GMC motors have the crankshaft centered under the cylinder bores and the pin offset in the piston (as is normally done). The 302 crank is offset and the pins are centered.

A cam vendor gives the rocker ratio as 1.65:1, but since the G.M.C. accepts 235 rocker (1.477:1) this may be a typo.

Thanks, I will be using Jesel keyway lifters. Hope there is enough meat in there for a .947 lifter, there are buckets of used one leftover from Pro Stock. The rockers will be Jesel as well. I am thinking about going to used 8 mm hollow titanium valves and cutting them down and regrooving them. I know that RPM is limited to around 6500 but I would like to get aggressive on cam profile.

The stock tappet is .990", no room for a bushing, no idea how much wall thickness there is to install one. What would be your guess at minimum bushing wall thickness?
The tappet bore is slotted for the key? How deep?

The rocker dims are limited by needing to be both directly over the stem tips and the pushrod reliefs (the fulcrum/shaft/trunion falls wherever). I asked Jesel a while back for rockers at 1.7 or 1.8:1 with point-to-point of about 3.40" (levers: 2.19" and 1.21", etc.), no reply. The Howard may have larger pushrod relief holes, so the length is only approx.
I wrote an .xls to calculate rocker ratios from distance from pushrod to stem tip, PM for a free copy.

The intake looks like a rectangle (w/rounded corners) 1.69" tall by 1.88" long, approx. area (slightly less than) a 2.00" injector throttle.

Just a thought: drill an exploratory hole horizontally into the pushrod relief to gauge the wall thickness. If you're lucky, it's big. When done, tap & seal it with a long screw.
This would mean that enlarging the hole (toward the fulcrum, not as a circle) will allow greater latitude in where the rocker's pushrod end must locate.
Yes, it will introduce a slight angle to the tappet cup, harmless (and normal in many engines).

The .937 lifter bushing is 1.062 the lifter bore needs to go 1.060 for a .002 fit. Do you think the lifter bore will go .070 bigger? . I know this lifter system is extreme overkill, but when you can put your hands on these lifter systems so inexpensive it's a no-brainer. These lifters are all over eBay as well, you just have to have the ability to install lifters.

on rockers we can make what ever ratio desired by moving the fulcrum point. here is a example of R&D on a set of LS heads , the ratio here is 2.1 to 1. The intended lift is 1.025 the stands are 3D printed plastic for proof of concept

That's the kind of new tech I'm talking about. Wayne Horning & Bill Fisher didn't have 3-D printers to to prove a concept. We street guys always benefit from the crazy racer fringe.

The rocker geo is going to be fairly straight-forward as both the pushrods and valves are parallel.
I've seen several references to GMC and 235 intake rockers interchanging (despite the stem angles differing: zero for the GMC and -3°26’ for the Chevrolet) but it suggests that the working lengths (stem tip to pushrod) are very close: about 3.41" (the Chevrolet exhaust rockers' angle is way off). The GMC appear to be skewed left and right as well (as are the Chevrolet at 8 degrees), but how close? IDK.
Using a high ratio offers an advantage in that a smaller lobe can be used and still clear the tightest cam bearing.

You may need a dual pattern cam favoring either side depending on how well the ports flow. Based on the 1-3/4" exhaust valves, the intake lobe may need more duration.

Already been done. Would send pic's if the site would let me.

What's "already been done"?

Girdle with added mains

Seeing is believing. Send me the pics and I'll post them.
As I said before: assume the new added caps are part of the girdle.
In the original block, the four upper main bearing halves seat in the “saddles” (semi-circular recesses that accept them) machined into the block’s four bulkhead webs.
Are there new webs, or where do the saddles go?

Several Bonneville guys have done 7 main GMC's before for the cranks and cams. It requires machining and welding new structures into the block. Just have LSM make you a billet block. It'll end up costing about the same in the end as converting a 4 main to a 7 main.

Your bore size will be limited to core shift and other dynamics relative to the build, such as compression and cylinder wall loading from rod length. You may have to sonic check 10 blocks to find one that can be bored that large. You should also prep 2 or 3 blocks to have something to fall back on when you have an unexpected engine failure for some reason. You'll also need to make the cam 7 mains as well. We regularly broke roller cams in our Comp Eliminator engines with 4 journals. Even making the journals larger so you can make the base circle and cam barrel larger wont last if you still retain just 4 bearing journals, regardless of the material the billet is made from.

Site won't let me post photos

You have to use an outside photo hosting site to load pics here. Which one are you using?

My computer

Ok, most forums, including this one don't have the ability to allow you to directly upload pics from your computer. You have to search online for a picture hosting website to upload your pics to, and then transfer the link into the box for uploading images. Many are out there, make sure you get one that's free and doesn't require membership or dues to be paid.

Try to find one that will stay around and not start charging after you have lots of pictures on it or all the pictures you posted to online sites will have their logo across it or just disappear.

There is some kind of presentation about this site scheduled at the Inliner Convention next month.

Hi Duff . . .

I have verified your Club membership status. I have added permissions for the Photo Library to your account. You can upload photos over there and use the {img} tagged links that the utility creates to reference photos over here in a discussion forum.

regards,
stock49

BTW There is a write up on how to use the Photo Library over in the General Forum Information area Inliners post circa Sept 19th

Here's the girdle



It looks like the 3 extra main saddles are not attached to the block at all, but bolt to the girdle above the extra caps.

that looks Awesome.

Yes it does!That is some nice work.

What does th oil path through the mains look like?

That main girdle looks great. I have a question about using aluminum the expansion rate vs cast iron in something this long. The head floats on a head gasket but the mains need to be rock solid. Is there another of those main girdles floating around? I would be happy with a print.
I have opened our engine up and it's to say the least interesting. I would guess 40 maybe even 60-year-old thought process

the rockers are decent, I will be building a higher ratio set. I want something in the 1.9 to 2.0 range.

Never seen a dome anything like that. Are those elevations quench height?

Those types of girdles are very common on inlines and Flathead Fords for Bonneville racing, and the dissimilar aluminum vs. iron material is a non-issue as much as Rodeck and Donovan aluminum blocks with steel caps are a non-issue. Guys in Brazil have also been using girdles like that in aluminum for several decades and it's simply not a problem.

The steel caps are only inches long the expansion is minimal, but when two metals are the length of the block the difference in expansion rates can cause problems. I'm not trying to do or repeat what they have done, We have a fairly good depth of professional race engine development. I'm not wanting to run down the hill to one cow, We are walking and going to have a go at them all. Records that is. From looking at this piston, man I hope that.
I wanted to see what's out there. We are considering doing a clean sheet head for one of these engines. But to utilize the flow numbers of a good head, the engine needs to turn 10,000 rpm.

The cam journal diameters are 1.8407"/1.8417" on all 4 journals. The housing bore for the cam bearings is 1.9715"/1.9735" for all four of them as well.

The bore spacing is not consistent from bore to bore because of the 4 main design causing a wider gap between cylinders #2 and #3, and cylinders #4 and #5. The spacing are approx. based on the measurements of some OEM head gaskets. 4.450" between #1 and #2, 4.520" between number #2 and #3, 4.450" between #3 and #4, 4.520" for #4 and #5 and 4.450" between #5 and #6.

Another fact you might not be aware of is the GMC block has main housing bore ID's that are (4) different sizes.

Also, a Comp Eliminator racer a few years ago built a 4.250" bore 6 cylinder. The stock bore spacing does not allow this, so he had to stretch the block length to accommodate that by using an LSM billet block. So you may be limited to a 4.125" bore.

In the girdle photograph, where are the fasteners for the new caps? Is this the finished product?

Panic, I'm going to guess and say the fasteners for the 3 new support mains are 180 degrees from the one that thread into the block. They probably bolt the girdle to the crank with the 3 new mains first, and then set the crank and girdle into the block as an assembly and install and torque the original remaining 4 mains.

Ah, that makes sens, thanks

Talked to our engineer and the girdle needs to be made in steel to match the expansion rate of two large pieces.

Next on the list is cam drive. Has anyone ever made a gear drive, or even better a belt drive. Working on cam cores now