I had Schneider regrind a stock GMC Cam into the following Cam profile: Duration = 286; Duration at .050 = 240; lobe separation = 109; cam lift = .300; valve lift = .450. Intake and Exhaust have same profile. I am getting ready to install the reground cam and was wondering about changes in the way the rocker arms sit on the head. The cam has had it's base circle reduced in diameter quite a bit in order to get the earlier valve lift action. It seems like I will need to adjust the rocker arm screws in or down to compensate for the lower cam base circle. Will this adjustment work? Or, should buy some longer push rods to make up for the lower base circle while keeping the rockers in their current position? Are there other options? Should I be concerned about this at all?
Thanks, Frank Hainey
Do you know the original lift?
Has the assembled height of the valve stems changed?
Just set up one pair of rockers and push rods and see what you have with specified lash.
Unless you know, or have measured the entire history of the valve gear, trying it out is certainly going to be helpful.
Changes to the component lengths and positions occur in both directions, and are somewhat self-cancelling.
I think you are right. I will not really know how things changed until the cam and every part of the valve train is in position. I may be worrying about something too early.
Also, I just re-read the Schneider info and they recommend shorter valve lifters (they look like hydraulic lifters but are mechanical) and longer push rods. Do you think these changes are necessary? Changes due to material removed by regrinding the cam are small compared to the two above.
Not familiar with the Schneider recco, can you post the exact language?
The newer style short lifters will work just fine with the longer push rods. The new lifters do not hold oil inside like the old milkcan style do.
Thanks for your inputs. I talked to Schneider today. I guess I misread their add on new style mechanical valve lifters. Schneider sells them for the Chevy 235/261 with 0.848 diameter. The GMC lifters are 0.990 diameter. The old milk can shaped lifter is all that is available.
I have looked on line for GMC mechanical lifters with 0.990 diameter with no success. They look like hydraulic lifters.
Schneider sells them for the Chevy 235/261 with 0.848 diameter
Typo, mistake, or missing "requires bushed lifter bores" message: 235 lifters are also .990".
Panic - you are correct. I found them. Howard Cams sells a mechanical lifter set 91125 for 56 - 62 Chevy 235/261 sixes with 0.989 dia. There is no reference to GMC but I think they are interchangeable since diameter is 0.989.
I ran a Schneider cam with the later stock style lifters, no problems, 3 years of taking the kids to school and still did 126MPH at Bonneville. Just use the Howards lifters and call it good.
Armond - Thanks for the relaying your experience using these lifters. Do you remember what push rods you used? They will have to be longer since the new Howard lifters are shorter than the old milk can shaped lifters. Will the 56 - 62 Chevy push rods work?
I originally bought my lifters and pushrods through NAPA, back then they had longer tubular pushrods too. Howards may well have them too as their lifters would be useless without them.
Unless the manufacturer says otherwise, the usual rule of "how many exposed threads" for best pushrod length is no more than 1-1/2 turns out.
Reason: the longer exposed adjuster thread changes the angle of the pushrod side lever and reduces the effective rocker ratio (plus minimal change to the rocker geometry).
Are you using the original GMC rockers? If so which ones? I've seen some stamped (early) rockers on running race engines that were beyond their "sale" date (twisted, loose adjuster) but no idea what RPM, cam or valve spring did that.
The only great GMC rockers are custom made at $$$, no modest cost aftermarket ever AFAIK. The problem: they're TOO LONG between PR and stem.
I have been running the original rockers that came with my military 302 GMC. They are stamped and welded. I had some trouble last year with the rocker arm adjusting screws and lock nuts Replaced about 8 screws and nuts along with one rocker arm due to stripped threads. Also replaced one push rod because the base assembly separated from the tube.
I've been running the car since 1993 and racing it off and on in 10 Antique National Drag Races at Fontana, Ca. Head is a ported 302 with dual springs and big valves. Cam is an Isky E2 which is fairly mild: .298 cam lift, 264 deg duration, 228 dur at .050. I am planning to install a Schneider 286 F cam soon which will stress the rockers even more.