Connecting Rod Options

[Allen W.]

I am in the process of building a “numbers matching” engine for my 1966 L79 coupe and have a question regarding connecting rod choices and would like some additional opinions on the matter; should I use a set of aftermarket rods or prepared stock rods? The vehicle will be primarily used for periodic short to medium distance road trips with some spirited driving. The engine will be virtually stock with the exception of a slightly more aggressive cam (224 int. and exh. duration with 110 degrees of overlap with 0.470 lift for both int. and exh.), light pocket porting, and gasket matching.

I am currently of the opinion of using the stock “first design” connecting rods after prepping them by smoothing the beam castings to reduce stress risers, then shot pining them to increase the surface hardness and to add a residual compressive stress which should increase their durability. I would also want to replacing the 11/32’’ stock rod bolts with high quality aftermarket replacement, as well as reboring/rebushing the small end and adding a oil mist lubrication hole in the end of the rod as necessitated for a floating wrist pin (read below). Alternatively, I am considering using aftermarket connecting rods produced by Crower, Oliver, or the like. (I need to do my homework first to see if they even offer small journal rods.) Will stock connecting roads prepared in the manner described be suitable for my application or should I invest in a quality aftermarket connecting rod?

Also, has anyone noticed a shortage in the availability of reproduction pistons? I ordered 0.030’’ over 302 CI, 11:1, fully floating pistons about 4 months ago from a local Federal Mogul (ala TRW in this instance) parts supplier. After being pacified several times with tentative manufacturing dates they evidently have still not been made as I checked on the progress of the order today with no new progress. I have attempted to order them online from gmpartsdirect but they were also unavailable there as well. What are some alternatives sources from which I can order TRW pistons? I realize that the full floating setup is unnecessary for a street application; however I am doing so strictly for personal reasons. At this point I am also considering the stock 327CI/350Hp pressed setup.

Thank you for your comments and input.

Matthew Wiles

[William C.]

Re: Connecting Rod Options

I can only say from personal experience that the OEM rods with good bolts perform fine for "spirited" driving at least up to the oem redline of the SHP engines. I am confused with the question about the 302 pistons, as they are not directly interchangable with the 327 pistons due to a different dimension from the centerline of the wristpin to the deck of the piston to adjust for the difference in stroke length between the 302 and 327 engines. The 302 piston will stick out of the block if used with a 327 crank, and small journal 302 cranks were, I believe only used in 1967, making them rare indeed, although a 283 crank can be used instead, but why give up the torque of the 327 in a car you intend to drive?

[Gerry Proctor]

Re: Connecting Rod Options

I too am confused about the 302 pistons and suppose you'll clarify that issue. But regardless of that, there are other custom piston manufacturers like Ross, JE, and Jahns that will probably be a bit quicker on filling an order. Although you can expect to pay twice as much over an "off the shelf" TRW piston but you also get a lighter, higher quality piston. I suppose it depends on how badly you need the slugs.

The rod work you propose are standard durability modifications that are used in performance build ups. To say they are necessary may be in question (although the factory "pink" rods used in the LT-1 were magged and shot peened) but beyond the smoothing the beams and shot peening, the most substantial improvement is in the rod bolt. The stock 1010 forged rod is a pretty durable piece. It wouldn't be a bad idea to look at having the crank re-balanced if you're wanting to run upstairs with the engine especially if you go to a lighter piston.

[Wayne K.]

Re: Connecting Rod Options

Allen,

Just a thought on the piston compression ratio. When my L79 was rebuilt the builder put in 10:1 flat top pistons as he advised it would be much happier on the 91/92 octane gas that is available in our area. I have had no problems with engine detonation running premium unleaded pump gas.

[Clem Z.]

Re: Connecting Rod Options

before i would not go to all that trouble with stock rods i would buy after market "sportsman" type rods. small journal(2.00) should be available because that is a "trick" used by engine builders to get more power from late model engines with large journal(2.100). make sure your pistons use "spiralock" pin retainers or at least "double snap rings" to make sure that you do not have any trouble with them pounding out. you will also need to have all parts balanced.

[Duke W.]

Re: Connecting Rod Options

My first question is whether you have the "first design" small journal rods or the second design. The second design has little hump of metal on each side of the rod adjacent to the bolt lands. I don't think this design went into production until 1967. The first design is definitely weak. My number 7 (first design) had a bad crack at the above stress point. Who knows how long this would have lasted.

If you have the first design, I would recommend replacement with new second design rods. They are not very expensive. As a minimum I would magnaflux the second design rods whether they are new or used and replace the bolts with the highest strength 11/32" bolts you can find, and this will require that you resize them.

Beyond this, you can do all the standard rod prep and have them shotpeened. It is my opinion that shotpeening a used crack free rod will set the fatigue clock back to zero. Most fatigue failures originate at surface flaws, and the combination of surface smoothing and shotpeening will completely alter the surface and eliminate microscropic flaws. Of course you will also have to have them weight matched during the balancing process.

All of these operations are costly, and if you run the numbers it will probably be cheaper to buy a set of off-the-shelf rods such as Crower Sportsmans. Whatever you do to the OEM second design rods, the Sportsmans will be better and make for a bullet proof bottom end and long as the bearings don't get oil-starved They are sold in weight matched sets and are ready to install as is, but your balancer shop may want to tweak them a bit during the balancing process.

BTW, IMO your cam has too much overlap for a street engine running OEM manifolds, and it's going to kill a lot of low end torque. The original 151 cam is the way to go with its 114 degree LCA; a 110 LCA with 224 degrees duration at .050" will result in too much overlap. It might work okay with headers, but not with manifolds. If you pocket port and port match the heads/manifolds and do a three angle valve job with .040" inlet seats and .)60" exhaust seats, you will achieve about seven percent more top end power without harming low end torque and driveability, and the engine will pull strongly all the way to the 6200 RPM redline or until the lifters pump up.

Aftermarket cams are typically developed with headers and the OEM cams with manifolds. The higher overlap of aftermarket cams can effectively utilize the wave dynamics generated by headers to improve top end, but they will still loose out on low end torque. OEM exhaust systems with manifolds don't generate any useful wave dynamics that will improve volumetric efficiency with lots of overlap, so the end result is lower torque across the rev range, especially at the low end.

The way to develop a high performance street engine is to cam it for 80 percent peak torque at about 2000 revs and spend up to your budget limits to improve the inlet and exhaust systems, and in the case of a small block Corvette the best payback is working on the heads.

Duke

[Gene M.]

your cam choice...........?

Duke is giving good advice. The Comp Cam 270 grind will not pass NCRS ops tech.... too much over lap and too lopy at idle. You may even have trouble with power brakes if you have them.

[John H.]

Re: Connecting Rod Options

Stock 2nd design rods (prepped, with premium bolts - like ARP) will work just fine, and you'd be hard-pressed to find a better all-around cam than the stock "151" L-79, which also uses all proven stock GM valvetrain components and standard springs. Like Duke said, the Crower Sportsman rods will simplify the project, and probably cost less than prepping/re-sizing the stock rods.

I'm assuming you want the 302 pistons for a Z/28, not your L-79, as they won't work in a 327. Custom pistons are one alternative, the other is to buy a set of GM 5.94" high-performance PM rods (GM#12495072), which will let you use any of the hundreds of 350 pistons out there; these rods make up for the different compression height of the 350 pistons (vs. the 302 pistons) and will give you the correct deck height with a 3.0" 302 crank.