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Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
Lots of work on Brad's part ... pure enjoyment (and references) for you!
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| | | | Joined: Oct 2016 Posts: 9 'Bolter | | 'Bolter Joined: Oct 2016 Posts: 9 | Hello Everyone, been awhile since I've posted anything of any sorts. So I'll cut straight to the chase, building a bare GMC 302 cylinder head from scratch as I obtained a short block that has Venolia Pop Up Pistons with a 3/4 cam. I'm going to upgrade intake and exhaust valves to Stainless SBC combo (+.100 1.94/1.60). I've read and heard of some individuals (on other sites) and searched to no avail on this site as well that upgrading the valve springs to Cummins valve springs is a better fit than the z28 version people tend to go with. There's a vendor on Ebay who sells these 302 heads pre built with such upgrades but is pretty much silent when messaged the same question, I get it, he's trying to run a business. Does anyone have any info that could clarify which Cummins valve springs are being used? Are they the 12 valve or 24 valve springs ? Any insight and direction would be appreciated. | | | | | Joined: Oct 2016 Posts: 9 'Bolter | | 'Bolter Joined: Oct 2016 Posts: 9 | Thanks @Fibonachu - I read that one as well. I'll see If the machinist knows the specs off hand. | | | | | Joined: Feb 2004 Posts: 29,832 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,832 | Fortunately, there's a pretty wide span of "acceptable" valve spring specifications. As long as the spring coils don't "stack solid" at full valve opening, a few pounds of difference of "full open" spring pressure or "seat pressure" won't damage a camshaft or affect the performance of the engine. Very few of these old engines get run the way they were designed to do when they were new- - - -wide open throttle for hours on end, at or near maximum horsepower RPM. The way most owners these days drive 'em, the engine is likely to die of boredom, not overwork!
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
| | | | | Joined: Oct 2016 Posts: 9 'Bolter | | 'Bolter Joined: Oct 2016 Posts: 9 | You make a good Point @Hotrod Lincoln......thanks | | | | | Joined: Aug 2000 Posts: 248 'Bolter | | 'Bolter Joined: Aug 2000 Posts: 248 | Hi Jerry
On the 261 with 6.5 axles and a 50% reduction in 4x4 low range, it is quite easy to reach and stay at 3000rpm in second and third gear while bouncing around relatively slowly....... but the 261 seems to take it in stride..... down shifting with double clutch going up steep hill is a challenge and I missed quite a few...dead stop and start in 1st gear until you reach the peak.....can't brake going down hill as the wheels will lock and the trucks keeps rolling on the washed out stones...it is like ice and you have to pump the brakes. Fortunately you can adjust your brakes with two 3/4 inch wrenches lying underneath without having to jack it up..... adjust the twin cams on each backing plate and back on the road in 20 minutes
Bob C.
CMP Heaven
The Hammond Barn
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