Inliners International Forums General Discussion Hi Performance GMC 302 custom rods question

I've been trying to gather as much information as I can about running custom rods in a GMC 302. I've seen discussion on here about Ron's use of Hudson rods and Gary's use of aluminum Howard rods, which has been very useful.

I'm thinking of having a set of longer rods built so I can use pistons with less compression height. I've seen mention of offset grinding the crank to use a SBC sized 2.100" rod journal. My question is what would be done to deal with the excessive side clearance when doing this? The SBC rods are .940" wide and the GMC's are 1.25" if I understand correctly.

If I go through the expense of having custom rods built, it seems like it would make sense to stick with a common rod bearing size, but the only thing close that I've seen is the Chrysler 251/265 journal size (2.125" journal, 1.085" wide bearings).

What do most people do on the big end when using custom rods with a GMC 270/302? Is it best to just stick with the GMC sized big end and use GMC bearings, or is there something that can be done to make the SBC sized big end rods work?

When having custom rods made, the whole width of the rod will stay the stock GMC dimension, whether you make the journal size a BBC or SBC or some other size. The BBC journal(IMO), would be a better choice to offset the journal with since it is closer to the rod width of the GMC. Some of the other Hudson and older production rods that are longer than the GMC are a good compromise to cost, but quality race bearings aren't going to be available for those rods, and thats why the BBC and SBC rod journal sizes are more common to use in race applications, if thats your intention.

So you just run a standard SBC or BBC width bearing with the GMC width rod? That sounds simple enough.

Some extra side clearance is harmless, but the entire width difference (1.430" - .990" = .440") will allow the rod to wander around too much IMHO.
The width of the journal can be reduced as part of the grinding operation to create a shoulder matching the original GMC area.
The pistons can be constructed with the pin bosses very close to the rod eye width (eye + .010") to guide the rod from overhead.
As long as the tangs, dowels blah in the bearing are secure in the rod as usual it should work.

Yes, you use the corresponding bearing for the journal size you are using(BBC or SBC), and retain the stock GMC width for the rods B/E width.

61-65 Jag 140 rods have a 1.180 big end width. Has anyone tried those? Somewhere out there..........

Beater, how long are the Jag rods?

Quick,

Your going thru the the same problem I had when I built my 302 Jimmy. The Hudson rods and L6 Chrysler bearing combination has worked well. The engine has seen 6000 rpm numerous times with no problems, and the bearings look perfect. You can talk to JE and they can make you a set of pistons like mine with, or without, the dome. If you have a custom set of rods made stay away from Howards Rods. I tried to get a set made by them and finally had to turn it over to a collection agency to get half my money back.

Good luck, Ron

Ron do you have a source for the hudson rods?

Erik, This info is from Panic.
I found some Jaguar 3.8 rod data:
Length 7.75"
Big end bore 2.233" (292 is 2.225"), would need an undersize and a smaller undersize on the crank.
Piston pin .875"
Width 1.180"

Yeah, I read about your troubles. I'm not so sure I want to run an aluminum rod anyway. I'm leaning towards some Crower billets right now as they'll make whatever length I want and I've used them for rods in another vehicle of mine. Having launched rods through the side of the block in the past, I believe in overkill for connecting rods.

Beater,

I have several Hudson rods and have a friend in Minnesota that can get more. Any of them will have to be reworked/narrowed
etc. If you have access to a mill they are easy to narrow but they need lots of attention. Really, just standard machine shop rebuilding.

The nice thing is they are 8.125 long and JE can make the pistons.

Ron

Thanks Ron. I got a lot of irons in the fire right now and the 302 is not one of them. When it's turn comes Hudson rods my be the way I go. Beater

Too much side clearance on the rod big end will allow too much oil volume to be thrown on the cylinder walls, more than the rings can handle, as well as causing a reduction in oil pressure (this is why you shouldnt use the chevy V8 rods in your 194-250 sixes).

There was an article on the Egge machine site that dealt with stroking a 261 crank by offset grinding and using stock chevy 292 rods. I think in some 12 Port News issue some one had a chevy ll with a bored 261 stroked this way to 292 inches.

On the EGGE site they had two senarios, the last one was more expensive and would solve the excessive rod side clearance problem of using the narrow 292 rod. They welded a bead all around the journal on both sides to form a "cheek" and machined it to suit using the 292's side play dimension.

On the EGGE site, they surmised that the 292 rod was suitable strength-wise over the chevy "pinch bolted" rod-I think GMCs have a more common wrist pin system, but there maybe more easily available aftermarket 292 rods around than those of Hudsons and Allis-Chambers, etc.

Dont know if GMCs and Chevys have the same deck to crank centerline dimension like the chevy 235/261 and 292 hve but this might work.

For a strip car you might be able to live with excessive rod side clearance-but certainly not in a street car(you'd need to carry a case or spark plugs with you).

The only “leak”, or outlet path, for oil pressure in the rod’s journal to bearing clearance space is at the sides of the rods. This annular clearance area is easily calculated. The journal OD is 2.311”; with a bearing clearance is .003” (generous, but possible), the leak is the bearing ID area minus the journal OD area. Here, it’s 4.205 in2 ((2.311+.003)^2 × Π ÷ 4) minus 4.195 in2 (2.311^2 × Π ÷ 4). The leakage from each side of the rod is .0109 in2. The radial clearance between the rod and journal cheek is the surface area of a horizontal cylinder, which is 2.314” OD times the width (W) of the side clearance, or 2.314” × Π × W.
For the side clearance to control oil escape volume, the side clearance area must be less than the annular clearance or less than .0109”.
If the side clearance area is equal to the known leak at .0109”, W is .0015”, or 1½ thousands. Any value of W higher than .0015” (the rod is .003” narrower than the journal’s cheek-to-cheek width) has no effect on oil pressure.
Since the side clearance is always much larger than this, any more clearance is, of course, harmless.

sorry 'bout the above rod width lecture, I mis-read panics's entry (it now does seem right). The wider rod "overhang" would act as a "hood" to prevent too much oil sling to the cyl walls.

How strong would the Jag rods be? Some of the smaller mowog motors (MG,Austin-H,etc) have a weird 45 degree cap placement. Jags are a long stroke motor but not with very large bores (thus maybe not seeing as much piston loading as American iron).

I would have to guess the 292 rods may not be the answer for a shorter compression height. Which is desired -why?- to lesson the rocking of the piston?, or to raise the ring set further up for better sealing?

only comment to the oil clearance/oil pressure exercise is the possibitity of the fillet changing some of these dimensions and thus some of the calculated results-possibly a moot point.

There was a set of Jag rods on eBay a while back for a good price but my PayPal account was pretty high so I didn't get them. They looked pretty beefy, not unlike 292s. I think I was actually looking at them for a 292 application. I should have gotten them so we could take a close look. HERE is a set.

Panic, I'm glad there are guys like you who can think these things through. I'd just hook 'em up and run them hard till they stampeded through the pan!

I don't like the idea of the rods sort of wandering around, so reasonable side clearance is a good idea.
I'm not sure how the oil is flung off, and tight clearance may cause an oil stream to be aimed somewhere else, but the volume shouldn't change.
This whole topic began (AFAIK) a few years ago with use of aftermarket rods on reground Chrysler RB cranks.
Eagle etc. advertised the rods as Chrysler, but they're .990" wide (made from BBC cores) and there was a lot of freaking out because of the extra side clearance, but the math was sorted out slowly.
The Jag rod is probably well made (QC, metallurgy) since the car was expensive when brought out (1948?), but how many times can it be rebuilt before it gives up.
That's also an engine famous for exploding 500 RPM above the redline, but IDK what part gives up.

Thank you.