Inliners International Forums General Discussion Engines GMC 302 Performance Pushrods

Five out of twelve of my My 302 GMC rocker arm adjusting screws have damaged threads and two of my tubular pushrods have come apart. I replaced the screws and want to order new tubular pushrods. I need performance pushrods since I run higher rated valve springs.

I need help with pushrod ordering:
1. Do I use total length from ball to top of cup? i.e. 10 19/32 in.
2. do I use effective length = total length - cup depth i.e. 10 3/8 in.
3. do my measurements look correct?
4. Is there a recommended performance pushrod make and model for GMCs?

Contact smith brothers push rods in Oregon. They supplied a set of push rods, adjusters and nuts for my 261 that were machined and polished to match together. The difference was night and day between the trw push rods and stock adjusters. Get both as your stock adjusters may not be perfectly matched.

The reason I say this is that In 2015 I installed a set of Clifford chrome moly push rods that I had bought from jack Clifford when he was still alive (and squirreled them away for 25 years or so.). Installed them and installed a nos sealed power rocker set. I never thought to blue up the push rod cups and run them in on the adjuster balls to make sure they were fully seated. Turns out they were not fully seated in that the adjuster balls were ever so slightly larger then the push rod cups so that the balls were sitting on the edge of the push rod cup. Drove car about 250 miles, and about 6 runs down the dragstrip at the 2015 and 2016 trifive nationals. On a little local car show run on thanksgiving morning 2016 I Had one of the push rod cups fracture into two pieces that fell into the lifter area and the broken push rod jammed an exhaust valve rocker open. When I pulled it all apart, two other cups were on the verge of splitting and all of them showed that they were sitting only on the rim of the cup.

I sent smith brothers a push rod of the length I wanted and an adjuster, they sorted out the parts. When I got the smith brothers parts in the mail, I could not believe the quality and how smoothly they worked together. Uses a hex adjuster so you can use the more traditional adjuster tool (z shaped with the box wrench on one end and a hex t handle running through it. Makes adjusting way easier) I don’t have the ability to post photos here (I don’t host my photos and you can’t post directly from an I pad) but if you message me an email address I can shoot you a couple of photos. If you can post them for the benefit of the others that would be great. They were a godsend and I now have several hundred miles on the motor and about 20 more runs down the dragstrip at the 2017 trifive and not one single issue (and I took it all apart just to be sure).

Mike

IDK (but I suppect) if the GMC adjuster ball & pushrod cup are the same as stovebolt 235/261, but those are 11/32" - not a common size.
A 3/8" ball will not fully seat in the cup, and burn the edges, then collapse it (as described). A 5/16" ball will bottom in the cup, but make only partial contact, wear and overheat very quickly.
I like to lap them in myself, but if the sizes are correct and quality is good it may not be necessary (the factory didn't). If you do, clean-clean-clean to remove all compound!

Smith is great, the 2 other premiere pushrod mfg. are Trend and Manton.

E-mail photos to me at jdiamond@optonline.net and I'll put them on my site.

Thanks for the quick replies. I'll gather up bad screws and the pushrod with separated cup/tube and take pictures. The the bad screws were all on the exhaust valve rockers. The Maker of the pushrods is unknown. I'll measure the key dimensions more accurately.

My 40 Chevy has been running with the GMC engine for 24 years, but only going 1,000 miles per year along with a drag race or 2 anually. So, I can't complain too much about the engine. I don't have a record of the type of the pushrod maker or size. It is tubular with a ball on one end and a cup on the other. I'll measure the key dimensions.

My valve train consists of an Isky E2 (.298/.448 cam/valve lift, 264/228 deg duration, .018 lash) cam, mushroom solid lifters, tubular pushrods, GMC fabricated rocker arms, 1.94/1.60 In/Ex Valves, 470 # springs.

One thing I wonder about is the length of the pushrods. It looks like the adjusting screws are way down in the rocker arm threaded socket leaving only a few threads above the lock nuts. Seems like the pushrods may be too short by 1/8 inch or so. I compared them with the stock pushrods and they are identical in length.

That motor ran for 24 years. I am not so sure there is anything wrong with the length. My motor is a 59 261 with a Schneider cam, .493 lift, 280 duration, uses stock length push rods. So really all you need is one good push rod and one good adjuster and you can send them off to smith brothers and then follow up with a call to discuss and. You should be on your way.

Too much adjuster exposed slightly changes the rocket geo and may reduce lift slightly, but nothing that would interfere with normal use - as long as you can get a good adjustment.

I'm going to buy a cam for street and road racing, don't plan to SERIOUSLY race but have a strong running short stroke big port with 3 H-8 SU's, tuned in and out. Roughness is OK but should be drivable in a <2000# car how does your 280 degrees fit these?
cheers;

I am not sure what you are asking but the pistons I have are venolia pop up forged with a relief for the .493 lift. It sits at near 0 deck so I needed the relief. I ran a chet Herbert regrind .480/280 in a flat top 235 without any valve relief but that was with stock clearances and the piston sat in the hole I want to say .030-.040 maybe more I just forget now.

For the schneider, I called and discussed the project , 57 chey, 261, street, once in a while a run down the strip, 4;11 gears, four speed and that I did not mind adjusting valves once in a while and wanted a pretty good loopy sound. Told them what RPM I thought I wanted to work in. They made recommendations as to lift and duration, center line and over lap and that is what they ground for me. Its a great cam as far as I am concerned. You might consider giving them a try, I think it is way better then trying to pick something out of a catalog. And, because they grind cams for a living, they get to tweek and adjust and alter their grinds to get you what you need and want. Its a pretty good resource for us 6 guys.

This is getting off the Push Rod subject but I an't resist camshaft discussions as I may make a change for more high end performance.

I am satisfied with my Isky E-2 cam which has lasted over 24 years in my 3400 pound 40 Chevy coupe w/ a mildly modified GMC 302. Mods include: Areas 9:1 pistons, large valves, ported and polished big port head, HEI , 3 Weber 32/38 carbs, dual headers and 1 3/4 exhaust w/cutouts, A/C, a T-5 5 speed and Ford 9 in. 4:11 rear. The E-2 cam specs are shown above in my earlier post. For on the street driving the E-2 works great. It idles smoothly but is noisy due to the .o18 tappet lash and will pull the car along back roads at 1000 rpm and 27 mph in 5th. It cruises well on the freeway too going 65 at 2400 rpm. Drag Strip Performance has varied.My average is 14 .2 seconds and 85 mph. My best with a good tail wind was 15.86 and 88.9 mph. If my mild engine were in a <2000 lb car, 1/4 mile averages would be 15 sec. and mid 90 mph ranges. The only reason I might change cams is I would like to break into the 90 mph and 15 sec range consistently. But I am now 76 and have Parkinson's disease, so I might have to be happy with what I have.

if you are not rebuilding the motor or anything I think you are probably on the sensible side of reasoning, lol. If you are rebuilding the motor, may I encourage you to re think that position? I think we can be straight with each other here. When the window closes as to a physical and/or mental ability to keep playing with our cars, it generally does not open again. So no time like the present to play a little longer.

Thanks mdonohue05 for the good advice on enjoying what I have a little longer. Speaking of that, I hope to attend the convention in Flagstaff in July and bring along my 40 Chevy. The trip takes about 8 hours from Sam Diego. I may drive or tow the Chevy.

Thanks to you and panic for the performance pushrod tips and
adjusting screws. I plan to send out samples to Smith and Manton soon and get quotes on a set of 12. I think Manton is very close to home.

Would love to make a convention one of these years but I have the trifive nationals a couple of weeks later in Kentucky so I have to pick one and the nats are it.

The trifive nationals allows any of the participants to make as many fun runs down the strip as they can squeeze in over the two day show as part of your entry fee. Yep, you do not have to be a race car, or register to be in the racing competition. Everything can make a run, or a lot of runs, from stock restored to gasser, no competition, heads up go down the track and giggle like a little girl when you are done (I think between my son and I we ended up going down the track 20 times or so). On the last day (Saturday), after the awards are given out, they open the track for a couple of hours for more runs. It is as much fun as you can have with your clothes on. This is my son driving my actual high school car from the 70s. It was his first time driving it down a drag strip so dad asked him to take it easy.

I was the only 6 cylinder powered car making runs the entire weekend.

https://www.youtube.com/watch?v=lMVz66tyrKQ

Mike

I have had similar problems trying to get a Holley 395 to run on my 302 GMC. The thing would not quit bogging down on takeoff. I fooled around for months and bought 10 power valves. It would go great once the bog cleared. Seemed like it could not adjust to low rpm and air speed. I gave up.

I have been running 3 Weber 32/36 carbs on my GMC for 24 years. They work fine but you really need a manometer to balance them accurately. They run well at all speeds especially in the transition from low to mid range and then high speed - no bogging with the Webbers.

Agreed. I still have my Unisyn that I used for years on my dual and triple 1 bbl intakes. There was a guy on eBay that was selling a rubber adapter for the Weber’s to use that tool for getting them dialed in at idle.

I started building a triple 32/36 intake before I got the Clifford. The Clifford runs so good that I stopped building the Triple.