G'day I have a GMC 302 however the Head has no Rocker Assembly. There is a GMC 248 Rocker Assembly on the dreaded Ebay at the moment and my question is would this be OK for my 302 head. I am new to these engines and have limited knowledge so will probably have a lot more questions. Can somebody lend me a hand. Karl
Yes, it will be ok. What are you doing with the engine? Street or racing. Most parts interchange between all the GMC inlines.
I think a 302 was the only GMC that had bronze bushings on the rockers but other than that they are the same.
There are 2 types of stands where the assembly mount to yhe head. A pressed steel type and an aluminum type. I prefer the aluminum ones. If you are building any other than a stock engine I advise you have larger than stock diameter push rods made. Smiths in Oregon and Manton in Az. have done them for me. 5/16" diameter is sufficient..good luck.
That's interesting as I had an aluminum stand brake. After that I stuck with the stamped steel stands. Different strokes.
Every old time hop up guide for GMCs tell you to go with the steel stands & stamped rockers on anything even mildly modified. I've never had any wear or breakage problems with either. The stamped type should be easier to find. Tube pushrods are a must.
Yeah. Well. Tone has put a whole lot of passes on his GMC. If he says aluminum stands work for him, you can take that to the bank. Use the change to buy a book. Are you having new pistons made for the 248? I know I could not use 270 OEM pistons with a 302 head.
3 7/8" Arias pistons were made with domes for 9 : 1 using the '54 302 army truck head. Didn't mean any trouble - you guys have the racing experience, mine is truck & hiway use.
Listen to Jimmy, he has forgotton more about 6's than most will ever learn. BTW, Thanks for your help Mr Tone! I hope we get a chance to talk and race again this year.
No trouble. My experience with aluminum stands was poor. JD seems to have had better luck with them. My point was that what you read in a book can not be compared to the many years of running GMC engines that Jim has.
Thanks Rich and Tman I appreciate your words a lot. I have never heard of a problem with either and I have always used studs to secure them to the heads. On one set I have, the rocker top edge and bottom edge are tig welded for strength and I would definatly recommend that for roller cam spring pressure as they are only spot welded. When I built my own roller rocker system full size aluminum blocks were the way to go. As for those who don't know Rich is one of the guys I have always admired for his innovation and machinist skills. If you can think it up, Rich can make it. When I first saw Rich's installation of a Ford Y-block head on a vintage flat head 4 cylinder Dodge engine I knew anything was possible.
Guys thanks for all the replies. These appear to be stamped steel stands and I think stamped steel rockers. They appear to be a complete assembly with shaft, springs, rockers, rocker cover, pushrods and even the bolts. It's "buy it now" for US$150.00 - I don't know if this is a good price or not.Because I didn't have any of this assembly I was originally thinking about machining rockers out of 7075 Aluminium with Bronze Bushes, Shaft from 4340 Steel, Stands from 6061 or 7075 Aluminium, get some springs made etc. - I love drawing and machining my own parts - however I have nothing to get the dimensions off so it's probably best if I just buy a complete set up and use that. One question I have now - is it possible to machine the rockers to accept bronze bushes like the 302 rockers as this sounds like a good thing. This motor is for a street driven 47 Chevy coupe that I would like to have triple carbs, reasonable cam, cast iron headers etc. on. If I can get someone out here in Aussie to help me with some head work I will otherwise it will remain pretty stock. The car won't be raced unless it's at nostalgia type drags for a bit of fun. I will have a lot of other questions and will probably have to chase up a lot of other parts to get this to happen. Again thanks a lot Karl
http://www.jalopyjournal.com/forum/threads/rockerarms-unlimited.738230/ http://www.rockerarms.com/pages/gm.html Warren
akk... You can make them in many ways: bushed, bearings, or nothing. The set on my current engine I made 20 yrs ago and used 2 narrow Torrington bearings (small gap inthe center for oiling) on the stock shaft after .003" was ground off on a centerless grinder. It did break near the end after 18 years of hard use. I made a new shaft with a 3/8" hole in the center (rifle drilled, ground, and heat treated) . The photo of the one on the trunk is for an Arias/Howard head and is rocker to shaft. No bushing or bearing and they work great. 7075 material and a hardened shaft. A friend of mine is using the Arias rockers on a chromed shaft on his stock head 302. The one for my Skinner 12Port is also Torrington bearing.
Jimmy That's kind of what I was thinking would need to be machined. The problem I have is that I don't have any parts to reference for sizes and if I buy a set for US $150 I may as well use them. I would rather make my own as I would like the project and what you have done looks really nice. I like the idea of using the Torringtons as I feel this is the best way to reduce friction. Are the rockers on the trunk hard anodised. Thanks Karl
Yes they are. So are the ones for the Skinner head I have also. The ones on the engine are not. I never thought of doing that when I made them. The reason the Arias/Howard do not have any bearings or bushings is (I believe) that the top fuel/alcohol drag engines don't use them. If I would have thought that would work I may have done it to. Well it does work and sure would have been easier and cheaper. Good luck.
Is that a HEI dist in your engine JD. Is it from a newer GM engine or adated from HEI parts thanks for any info Bakersfield Boy Ron
JD, with your carb 'sideways', doesn't that cause a fuel slosh problem? As most carbs are setup have the fuel bowls in line with motors and direction. I would think that at 175+ mph the fuel is tilting with fuel heavy to one side?
2 questions: #1The HEI was new stock 194/292 unit I sent to Performance Distributors in Memphis Tn. 20 years ago. When I got it back I machined off the lip and made a clamp. The other late model distributors I have I just turn the gear upside down and repin it . The gear then lines up with the cam. I use an external oil pump so I don't care about the oil pump tang. This one however will reach the stock pump. #2. I have no clue what is happening with the gasoline in the fuel bowls. Remember there is no launch, we push off at both El Mirage & Bonneville. The adapter is as recieved from Clifford Research which only mounts in that direction for the carb. Also how many dual quads you've seen mounted sideways. I'm pretty sure all the Pro Stockers were that way. With all that, I've never seen a problem with the way it is now. Thanks for your questions. Most don't care much other than I've been pretty fast with an HEI and four barrel.
I run a smaller single feed Holley 4 bbl on a Clifford on a mild 302 that runs the mile and a half at Loring, Maine. Ours is "normal", the linkage was easy that way. I see the mixture pattern on the spark plugs as two sets of three. the first cylinder to fire in a pair is a little darker than the second. It looks like this right down the engine. I don't think it matters if the carb is straight or turned when dealing with WOT. I also run the later 6 HEI. The modification is the same as if you were putting this dist in a 235 Chevy. I machine the body and use a shaft collar as a hold down stop and adjust the oil pump drive tang lenght to fit. Running a stock oil pump. Frank
The GMC along with other engines will never perform evenly with the stock cylinder head. Carburetors work better then anything else. If you compare the porting to the firing order only numbers 3 and 4 have the same pulses between them which is two. 1 and 2 , 5 and 6 do not. 2 have 1 and 2 have 3. So in the outer ports one is rich and one is lean. When you get 3 and 4 plugs to read right that's the best you can do. With mechanical fuel injection this is really relevant. Sequential injection would be the best..but......Good luck
In my opinion at the end of the day this statement makes the most sense. The rockers are like trees stuck on top of the plateau. they are static. who cares if they are aluminum, steel or?? they wont make any difference at the end of the day. To the OP question, all the GMC rocker assemblies interchange. For the street, pick one and use it. Case closed. The limitation is the head. This engine started life in 1929 with primitive facilities for the working man to purchase. it needs a complete overhaul in the cylinder head and ignition to make it work right. and with its limitations its never going to be comparable to todays stuff. If I ever get enough money Id like to make an economical cylinder head the way it should have been done and get 1 or 1.25 HP per cubic inch. Wouldnt a 300 HP 235 or a 325 HP 261 on pump gas that anyone can build be cool?
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