Projects Old school cool 248 Gmc

My Jimmy 302 has Z28 springs and small block s/s valves. They measure 50lbs closed and 180lbs open (0.425 inches) lift.

 

Mine has .474 lift figuring a 1.48 ratio. It will be actually checked soon.

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Checking this morning the ratio is way off because check of number one was .430 lift on the exhaust and .445 on the intake

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Put in last last intake valve (he only had 5 in the box so we had to order one). Put the head on with a couple of bolts to ***ure the pistons didn't hit the head, they don't. Then put in a couple of lifters and checked the valves but we had miles of clearance. The rocker ratio is way off as well. Flipped my rocker shaft over and found a few rockers had tip wear so I pulled the shaft apart to surface the tips. Wow that job ****s. After it was all apart found the shaft had heavy wear and was bent so apart came shaft number 2. I wish I could find an easy way to check rocker ratio to pick the best 12 out of the 23 I have. Also port matched the header flanges. I need to sand blast them and paint them with VHT. The tubular pushrods from the 248 actually are very close to the correct length but we will recheck after the rockers are redone.

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Your valve lift is true on your measurement only. Every cam manufacturer thinks a GMC rocker ratio is 1.5 which we is not true. Your cam card states the cam lift is .320" so to them with no gap it would be .480". So .465" with a gap if the rocker was 1.5.

I can't see your head but back before I made roller rockers I played with different lash caps, the bottom of the rocker arm stands (mine were aluminum) , and push rod lengths to get the most out of my Howard 380 cam...as I remember it was a pain in the a**. But getting the geometry correct did help a few thousands of lift. Did it help? Who knows but what it did do was help with guide wear as originally the rocker was pushing the valve sideways. When I got it right guide wear went way down.

An ez check is at exactly 1/2 your valve lift ...is the rocker arm level. If not, and most aren't mine normally needed a cap. Of course my valve tips weren't high enough so I spent $$$ on 10* keepers and retainers so I could add them.....Does any of this really mean anything; not to most, but racing these makes a guy think or over think everything. Only the first 1 to .200" lift means anything anyway because at full the piston has quit ****ing.

A note on pushrods. I've never used the stocks because of their small diameter. 5/16" from Smith Brothers have never failed. I'm not sure whether they have the big bottoms like the stock ones anymore. A lot of cam manufacturers can sell you adjusters that have different ball tops and longer too.
It's fun watch your work. JD

 

I know it does really matter but I like things to be close. How did you make your rockers? It won't be for this motor but the next one will probably get rods and pistons so it maybe more important (well to me)

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Oh and the rockers are flat when at no lift. I will see if lash caps help, I have room.

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I second the Smith Bros Pushrods recomendation, when I restored the Olds engine for the Orange Crate they custom made me a set of custom length pushrods with the proper large radius ends on the lifter end and a small radius on the rocker end. They did it for me very quickly and reasonably priced. Great communication and service!

 

A real PIA, my design was great but without a good machinist and repeatability which is now common but not then. All the rockers were alike then rights and lefts from the same basic one. Roller tips to one side and push rod adjustments to the other, then the opposite. Isky rollers since they were clipped in, Chrysler adjusters but now ***an’s. Twin Torrington bearings in each with a gap in the center for oiling. Centerless ground the stock shaft to fit the bearings .002” since I didn’t know any better. The shaft broke after 25 years so I made a thicker one. A real challenge for me with right metal, rifle drilling, pre-grinding, oil holes, heat treat, final centerless grind. I did all the legwork driving parts to each shop. Offset the stand .100” calculating 1.7-1 ratio.... missed it ended up 1.63-1.
Arias/Howard made roller rockers and so did Skinner. Any way was all fun 35 years ago....and 10 years ago making the new shaft.
Added 2 photos. You can on a few when the valve is closed the rocker is not near level. Changing out the GMC stock valve may cause this geometry from engineering design.

 

I checked the lift with 0 lash and it indicated pretty low ratio but the bent shaft didn't help. I am thinking I can cut one cylinder width out of my bent shaft and find the rockers with the highest ratio. Probably a waste of time but it would make me feel like I did the best I could in getting the best of what I have.

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Because of the major block deck and the unknown head cut the side cover needs modified. I am just going to make a 2 piece side cover with a flat cover made from engine turned stainless and a outside surround to support the gasket area. I thought about cutting my Initials or something cool but I don't think it would look right. Here is my cad drawing of the surround.

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Struggling hard with what fasteners to use. Though about counter sinking some Philips screws but my holes would have to be pretty exact to work properly. I may just use some ****on head Philips screws.

 

Always a problem when cutting a head or block. Most all my side covers have elongated holes by filing. Personally I like a fastener with a head on it and not a phillips or slot so I have started using stainless shoulder ones from Mc Master-Carr. They are available in different depths too. I like internal star type washers. They hold and have a clean edge.

Stock ignition diameters are ez for a flat side plate. It's the big HEI's that cause a problem. Tom Langdon's is higher and has a small diameter. My sons engine is the first one I've used. So far so good. Nice cad drawing...

 

I was thinking the Philips was a "truer" fastener to a early 60s build. Though the original bolts I have are hex with a Philips. I use McMaster Carr at work and have been looking at all their bolt designs. I know it is a small detail but I want it to look and feel right. I have a Mallory dual point that fit with the original flat cover. I plan to use it but I also have a 2.8 HEI I may use eventually if the dual point becomes a problem.

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Mallory made some small HEIs for these too. An original side plate would look good with the same treatment you gave the valve cover. Coming cover to. It all looks nice together.

 

I am personally quite fond of the old cursive "GMC" logo for the side cover.

 

Cut one out of a spare valve cover and stick it on the side cover.

 

I can't find it in a form I can import to AutoCad

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I was just trying to make something cool while using a local vendor and alliance member. Plus I guess it will truly be my personal addition

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My preference has always been Allen heads with air cooled VW sump screen copper washers. You can always get a tool in the socket, no matter how much stuff is in the way. Screwdrivers slip out at the worst possible time ...

 

Is this better? I will actually lay over the flat engine turned panel to support the gasket.

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This morning I cut off the bent rocker shaft to the length of 2 cylinders, pulled the crank gear and put 2 light springs on one intake and exhaust. This will allow me to spin the cam by hand and check to find the highest ratio rockers out of the 2 sets I have (plus I do have one more shaft I found in the barn if I get crazy). It is probably an effort in futility but it will make me feel better that at least the rockers are closer in ratios. Pictures tomorrow.

 

I also know a guy that can bead roll with skill, wondering if I should just have the cover cut and send it to MI to get rolled instead of doing 2 pieces.

 

Worked on the support piece for my side cover some more. FPM cut me out a s**** piece to check my hole placement, needs some adjusting.
Used my engine to make a rocker test fixture. "rights" are all very good with 6 over .470" of lift. "Lefts" are consistent but avg about .424 lift. Yes I checked the lobe lift and it was dead on .320". Laying a right and left on a shaft shows them to be visibly different. Not sure if anything can be improved by trying to resurface the tip farther out.

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This looks a bit better

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Little more artish

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I like the very first one, but I'm a simple guy.

 

with no GMC?

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The first one with GMC.
The next time I clean mine up I'm going to cut the whole oval logo from a valve cover, tack two screws to the back, bolt it to the stock side cover and buff it with Scotch-Brite again.

 

Me too First GMC one.