What is the width of a GMC connecting rod bottom end? Other than the size of the small end,are 228-302 rods about the same strength? Thanks

Strength??? 50+ year old castings??? There are two different pin sizes depending on the year CID.... the ones I have for a 270 block are 2.455...

Humbley yours

They are old forgings not castings .I'm talking about the thickness of the big end,it's more than one inch and less than an inch and a half.

1.430.....

Tony,
Are you looking for another rod to use in the GMC? The stock rods are strong but the crank is the weak place.

Ron

Ron

Doesn't look easy to improve, either.

By using 1952 Hudson L6 rods, Pontiac pins and JE custom pistons I saved more than 1.25 POUNDS per cylinder and the engine lives at 6000 rpm. I would have cut the crank counterweights if I could have found out how much to cut.

Ron

Ron and others,this is a 261 Chevy project.I trying to figure the best way to get the piston near zero deck clearance at TDC.The GMC rod is .19 inches longer than a Chevy rod and I was trying to find a combination of an inexpensive flat top piston from another engine that might work with the GMC rod.Doesn't seem to be any that will get the piston up there.And from what i see the so called Caddy 261 leaves the piston down the hole with no proper quench.A set of 500 buck custom pistons is not out of the question so I thought using GMC rods will at least move the piston pin up higher and make use of the full floating small end bushing in a .990 size.
I read you can machine a GMC rod thickness to fit the Chevy (1.240) If the GMC rod is 1.4 then that's a bit to machine down and will likely mean I have to trim the sides of the GMC connecting rod bearing to fit properly.Or maybe the Chevy crank jounal can be machined slightly wider.I haven't seen a GMC rod in awhile so I can't remember if the bearing shell is a lot narrower than the rod.I could also recut the locating tabs if necessary to fit a 261 rod bearing if the outer diameter is the same as GMC.The bearing crush will hold the insert in place.
One more thing,stock replacement oversize 261 flat top pistons sit .060-.070 below the block deck at TDC.I built a 261 15 years ago and had the block decked .050 to at least make an attempt to get the quench reasonable.But with a typical .050 compressed head gasket somethine near zero deck might be better.I'm not sure that decking .060 is a good idea on a block with at least a .060 overbore.
Obiviously,clearance notches for the intake valve is probably necessary

Tony,
Could you give me the 261 Chevy dimensions for the following and I'll see what I can figure out:

Rod journal
Rod width
Rod length
Deck height
Bore
ETC

Ron

I don't think .060 decking is out of the question... we do .040 on Dart Blocks and then re-deck when we take NA blocks and convert them to EFI. Zero decks are hard to fit. We mock them up several times before we get it right. You always want to leave .010-.020 for expansion. Don't forget your valves and lifters. Depending on the lift you are trying to generate you could run into trouble. The jobs not done until your cam has been dialed in. Nothing worse than your Piston getting a little too cozy with your valves!!! You are going to have an awesome compression ratio!! Hope you take video of the break-in... I love hearing tight motors.... nothing like them!

Humble Newby

For what it's worth here is a LINK to pictures of 3 rods. They are: GMC (the big one), 292, Ford flathead (the skinny one). There is also a SBC (the broken one).

Ron,the 261 rod journal diameter is the same as GMC,261 rod is 6.812 inches long center to center,the rod big end is 1.245 wide.261 stroke is 3.937,stock piston compression height is 2.062.#61 has a 3.75 bore,mine likely will need at least .060 to clean up.Supposedly you can bore them safely to .110 over. I have no way to measure the deck height ( crank centerline to block deck) accurately.But with the demensions I gave above the stock type piston sits .070 below deck.And I pretty sure this block has not been milled in the past
Canepa Twin,the 261 block deck is about .500 thick.I measured a #010 350 block and it was about .550.I know it's two different engines and the 261 has 18 head bolts with blind thred holes in the block.I just don't know if cutting the block .060 is the best way out,but it's the easiest.Also the only available head gaskets for the 216 are .005 compressed and the sometimes available .038.And by zero deck I do mean actually about .005 below deck.I'm no pro builder but I have assembled a few engines needing mock ups to check clearances and then do more machine work if necessary.
And with all this zero decking business the final compression ratio will be 9-1 with a .060 bore and flat top pistons.

I'm confident that there are 7.00 Rods for the 302. If that will get you closer I will have to look, but I may have a set. After Friday I will have some time to open a few boxes and look. I don't have any babbit pounders ot splash oilers so I couldn't be of any help there. The 261 has a decent size combustion chamber. At a .050 (deck plus gasket) you could be over 9:1. How awesome is that!!! Like Ron said, the crank is the weak link here. If you have access to Mag I'd be sure to run it. (as Im sure you already have) 9:1 is a healthy bang. No doubt you are competent, not insinuating you aren't. Just mentioning things worth looking at.

Someone should write a book. A while back there was a discussion about GMC rods in a 261. All the GMC rods are the same except for a bearing change made in the mid 50s I think. They are 7" and a lot wider.

The GMC rod swap into the 235/261 engines is a relative easy job. The GMC rod is much better, since it does away with the pinch bolt design of the Stovebolts, and allows a floating pin to be used as well as a longer length.

I just found this. It is interesting. ROD SPECS

Thanks to Ron for the piston catalog info.
My math is a little fuzzy here and there...I said the stock 261 piston compression height is 2.062. My bad,that's 235 compression height,I just measured a NOS 261 piston as 2.030 which is close enough to the Silvolite catalog's 2.029. I haven't measured a 261 rod length,but Pat Smith lists it as 6.812.

Here's one to look over

http://www.oldgmctrucks.com/page0311.html


GMC rods come in a few sizes, biggest difference is the heft and piston pin size. You want .990 pin and out of a 270 or 302. I can't remember off the top but I believe the last 3 of the casting numbers is 419, Pat Smith sold me a set just after French Revolution.

A .990 pin is nice cause it's Big Block Chevy size, inexpensive.The 302 I built had the larger pins,1.090 I believe.I had the rods rebushed down to .990 and that little job cost me over 200 bucks to have custom bushings made.
I made an error;the 261 piston compression height is 2.029 not the 235 height of 2.062.I did measure a NOS 261 piston.

Armond, Thanks. That has a lot of info.

Sealed Power makes a bushing for about $20 bucks a set that will work for that, try that next time. It is used to put a BBC wrist pin in a 440 Chrysler which uses a 1.094" wrist pin.

The shop made a big deal about those bushings,they actually farmed it out to local non automotive machine shop.I don't take my business there any longer.
The machine shop I'm using now is leary about milling .060 off the block deck.They recomend the tighter quench but done with a rod piston swap if possible.Or just order custom pistons to fit the 261 rods and be done with it.
Armond sent me a Silvolite piston catalog.Using a GMC rod and a 3.1 Buick piston gives a lighter piston with a more modern ring package than the heavy 261 piston with fat rings.But the piston position at TDC is still about .060 below deck.I believe the Buick also uses a pressed pin so goota deal with that.The 225 Buick V-6 piston has a better compression height but only comes as a dished piston as an inexpensive cast piston.
In the long run my ideas butt up against the reality of spending 500 plus dollers for a small degree of improvement.

When I did this, the deck took a .070 cut with no problems, street or racing. Aircraft Spruce and Specialty sells chrome molly tubing in many sizes. I simply found one that was very close and the local machine shop charged 50 bucks to make them fit. I just used them as a pressed pin and it was fine, though I did drill a small hole from the oil ring land to the top of the piston pin.

http://www.aircraftspruce.com/catalog/mepages/4130tubing_un1.php

Keep in mind that Ross Pistons offers Inliner discounts, and has a 235/261 forging for these engines. There discount is very generous, so you might get a good savings compared to other options.

CNC Dude,yeah,that's what the machine shop says.By the time I get some GMC rods,machine them,play with pistons,cut the block and all that,be about the same cost.The GMC rods will give a better piston pin location.....but this is a street engine that won't see over 4500 rpm so.............it's all a learning experience ans I do appeciate the thoughts by everyone.