Hoping someone here can give me some insight on obtaining the correct preload range for 1950 Oldsmobile 303 lifters. Saw an old post yesterday here, that I now cannot find, saying 0.06 preload is reasonable. Found other on the web data saying 0.03 - 0.05 is the range. Doing a full engine rebuild and just want to do the right thing and avoid any headaches. Car will be pure stock, a cheese burger cruise night car. During machine shop work, block was decked and heads milled. Unknown exact amounts. Doing a head trial fit for preload check. I torqued the two middle head bolt rows(10 bolts total) to 40lbs, with the old head gasket in place. Cleaned it all up first(didn't want to crush and reuse my new gasket). All new lifters, new 1/4" pushrods and new rockers. Just put 2 push rods in place with lifters on the cam for #3 intake and #5 exhaust(cam base circle, valves closed). Crank at #1 TDC. Finger tightened the rocker rack bolts just til the push rods resisted turning. Very crude measurement, but rocker rack mount points were 0.075 gap. Questions: 1) Do I bother to try and fix this, or can I "get away" with 0.075? If I'm fixing it, how? A) Smith Brothers can't do 1/4" rods. Do I go custom 5/16" rods with the correct ball on each end? Get an adjustable rod to determine correct length, custom make 5/16" rods? B) How much of a hack is it to put paper 0.03" or so gaskets under the rocker rack mounts? How badly does that hack up the rocker geometry? Seems pretty ugly. But certainly a cheap "solution." C) I cannot find any off the shelf spacer head gaskets. Is this a possible solution? Where to obtain? And finally, should I be concerned about any valve clearance issues? How close was the valve to piston clearance on these engines stock(stock cam) and have I possibly put that in jeopardy with what looks like around a 0.03" deck and mill combined loss?
I would tighten the shaft bolts. Then see if there is any rocker arm clearance at the valve by compressing lifter, if you have around .050 I'd say you are golden.
Tightening the shaft bolt only results in the lifter compressing as the gap shown is taken up. There is no gap on the rocker at the valve when the photo was taken(all valve train lash is zero), nor is there any gap(lash) anywhere after tightening the shaft bolts. Tightening simply compresses the lifter. From what I understand, the valve lash trying to be achieved is a negative valve lash. That is, not a gap in the valve train, but a measured compression of the lifter after all positive valve lash is zero.
You have shaved the deck and the heads, shouldn't the easy fix be to order custom length pushrods that are shorter by the combined amount taken off the components?
Is the crush thickness for your new head gasket the same as the pre-crushed thickness of your old head gasket? This could account for a significant variance.
I thought on theses engines you had to custom grind the valve stem length after decking the head/block?
Won't the lifter plunger travel compensate? Grinding the valve stems should be done on a valve grinding machine to prevent the possibility of off square valve ends and premature wear. If the valves were ground it also adds to the equation. If you go with shims, use the required thickness of brass shim stock.
what head gaskets are you using? I pulled a couple new gaskets out and measured pre-crush NOS Felpro copper measure .085 new Best composite measure .052
Glenn, yes, the lifter plunger travel is what is suppose to compensate and take up any lash in the valve train. Although there is a prescribed amount of that travel that is "allowed" in the initial setup. That amount being the 0.03-0.05 figure. Found that number on the web, and here there is a thread saying it is ok even for 0.06. Concern is taking up too much of the travel in the lifter than is the prescribed minimum.
vtwhead, well easiest is the question. Smith Brothers does not make custom 1/4" rods. 1/4" is what is in the engine stock. That is something I am considering, but I would need to go to 5/16" rods. Just don't know if that is ok, or leads to other issues that I'm not aware of.
anothercarguy, yeah, old gasket looks to be a metal original OEM. No idea what the crush thickness differences are. Not really ready for to permanently mount heads yet. I guess I could sacrifice a set of head gaskets....
I believe a typical GM hydraulic lifter gets 1/4-3/4 turn of the rocker ball that equates to around .035". I can see your concern if your mockup shows .075". If I were you, I would delay any drastic actions until you install and torque the heads and get the actual number. Then either order the correct length pushrods or shim the rocker stands the indicated amount.
Wow, guys thanks for all of the input. For now, I think I will go with what bchctybob said, and realize I may be jumping the gun a bit. Wait til I have head permanently in place, then measure, and decide to get custom rods, or shim the rocker stands. I'll post update when I get to that.
Steve, what lifters were used for replacements? It is common to use SBC pieces which are taller. If you are not certain, you could pull one out and measure the length.
I would go with hardened shims under the rocker shaft. The new composite head gasket is likely thicker than the original, so it may add some height from the milling. Check the rocker shaft bolts for # threads per inch. If 20 tpi, then 1 turn of the bolt (1''/20tpi=.050) will compress the lifter .050''. After installing the heads with new gaskets, bring pushrods to zero lash by hand, carefully bring up to torque counting the amount of turn of the bolts. That will give you your lifter preload. Be mindful not to bottom out the lifter plunger. If I reached .075 and not torqued, I would add a shim until I reached torque and .050 or so of preload. Hope that's kind of clear. It's cheaper than new pushrods.
I only did a check one time with the help of my father and we were using the original cam and lifters. He had me take each lifter apart, not mixing anything up, and clean all of the parts spotless. Then I assembled each one and tested the total movement of the hydraulic parts in the lifter using a drill press. I believe they have quite a bit. As I remember .030-.050” under the keeper spring was good.
Embarrassed to say I do not know the manufacturer of the lifters. Supplied by the machine shop. Although I am aware of the potential height difference issue. I put one of the old lifters in at a different location simultaneously and the results were the same.
Totally clear. An option I'm considering. Thanks for the preload measurement method. Hadn't thought of that. Issue on this method is that the oil is supplied up through one of the rocker stands, so basic shims won't work, need actual gasket of some sort so as to not lose the oil supply up through the stand. Doable certainly.
I vote assemble the engine sans intake, get a checker pushrod. Come up with the correct length, order and install.
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