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Update On Push Rod Return From CARS and Need Some Info.


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Ok, so apparently CARS processed my return and sent me more push rods. I wasn't notified, they just kind of showed up in the mail but anyway. I can't tell if these are going to work. They look completely different than the originals. Pretty much the only thing that is the same is the length(10 3/8"). The top part is the only thing that worries me. The originals have a shroud of sorts on them that cover a small hole drilled in the side of the rod that lets excess air and oil pressure out of the rod to prevent a build up of excess oil pressure. Here's what the service manual has to say about it,

"The push rod upper end is counterbored to form a shroud around a bleed hole drilled in the side of push rod tube. The bleed hole permits air and surplus oil to escape from the push rod, thus eliminating air locking and preventing excessive build up of oil pressure which would result in an oversupply of oil to the valve stem."

These new rods are not made that way. I don't know exactly what "air locking" is but it does not sound good! I have seen the original rods in action as the car runs and these shrouds/bleedholes do indeed come into play. Excess oil runs out of the shroud and down the outside of the push rod. What should I do here? I don't want to just slap these on and hope they don't destroy my motor due to excess pressure.

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Edited by shadetree77 (see edit history)
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I don't think I'd worry about it...from what I remember (it was 4 years ago), the pushrods I got from Kanter looked similar to your new ones.

I checked out the listing for these at the Kanter website while I was waiting on my return from CARS and there is no picture available. Kanter is the vendor I was going to try next if these didn't work out. I wonder if they would be able to send me a picture? Aaron, you wouldn't happen to have a picture lying around from when you replaced these would you? Maybe a shot of the motor with the valve cover off or something? Like Danny said, I'd really like to make sure before I put them on and tear something up.

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It looks like (from your manual quote) that the rod is hollow and is the oil supply to lubricate the rockers. If true, the hole is a bleed hole to allow air to escape as the oil rises in the tube. The bleed hole would also act a metering device to reduce the oil flow to the rockers to the desired amount. Absent the hole you may be faced with three scenarios. The first is the rods get air bound and reduce the oil supply to the rockers for some period of time. The second is the rockers get an excessive flow of oil which bathes the valve stems and causes excessive oil consumption. The third is that the hole really wasn't necessary in the first place and makes little difference. I'm not so sure I'd count on the last scenario ............Bob

Edited by Bhigdog (see edit history)
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Bob, the straight 8 Buicks feed the lifters backwards--through the rocker shaft and down the pushrods...

Interesting. Thanks for that info. Going by what the manual says I think my comments might still apply. The engine designers seem to be using the weep hole to both relieve a possible air lock situation and to relieve excess build up of pressure, and thus oil flow to the rockers/valve train. It would think an air lock might lead to a collapsed lifter and excess oil would still mean oil burning. I'm obviously no St-8 expert but judging by the manual I'd think the hole was needed for optimal operation. Then again maybe the hole was over kill.........Bob

Edited by Bhigdog (see edit history)
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Hmmmm...those definitely look to be the same as the ones they sent me. Thanks for the picture Aaron. They don't have the shroud either. Bob, I agree with you there. According to the manual, that shroud and bleedhole are there for a reason. Aaron, have you had any trouble out of these? Oil burning or the occasional chattering lifters? About how many miles have you put on it in the last 4 years? Man, I'm really torn on which way to go. On the one hand I have these brand new rods that may or may not harm my engine and on the other I have the long, drawn out task of trying to round up originals that don't leak like crazy.:confused: I guess the first step would be to finish the cleaning process on my originals(I gave up after finding 3 out of 4 that were bad) and see how many I would need to round up to keep the originals and then decide from there.

Edited by shadetree77 (see edit history)
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I've put 6,000+ miles on it since the rebuild. It does have one chattery lifter on cold start...lasts about a minute. I'm going to guess it's not the pushrod causing this, and it could even be something in the rocker shaft assembly. It's not obnoxious, like a totally dry lifter, just a chatter. The only oil it uses is what leaks out of the fuel pump weep holes (I've tried 5 pumps so far; they all eventually leak, whether I rebuild them or someone else does). It uses less oil than any of my old cars. I'm 34, been working on old cars since I was 11, and I'm trying hard not to overthink some of these things. My local old school machinist assembled the engine for me (my Mustang was torn apart in my garage as I cut and welded on it at the time), and he didn't comment that the pushrods were incorrect, so I'd go ahead and run them. If they cause a problem, then you can scrounge around for old ones. Just my opinion, your mileage may vary. Besides, I literally have no idea where you'd get good factory ones; mine were mostly junk, probably like yours.

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One more thing...as you probably know, straight 8 Buicks have no valve stem seals. If the new pushrods were a problem, mine would be burning oil. It's not. I don't add it all year, and I drive it about 1,500 miles per year.

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"The push rod upper end is counterbored to form a shroud around a bleed hole drilled in the side of push rod tube. The bleed hole permits air and surplus oil to escape from the push rod, thus eliminating air locking and preventing excessive build up of oil pressure which would result in an oversupply of oil to the valve stem."

This passage from the manual seems to indicate oil flow direction is up the pushrod, not going down the pushrod. I don't have any manuals from that time period so I find it most interesting that the common knowledge is the oil goes down the pushrod. When I think of oil flow I imagine the pressure within the closed system has to be directed to where there is the greatest risk of high pressue metal to metal contact. It's hard to imagine that risk is higher anywhere else than in the lifter to camshaft contact point. The counterbore in the pushrod relieving pressure means the oil pressure at the lifter to camshaft point is just a dribble? That would seem most unreliable. It's a wonder these engines ever lasted as long as they did.

But to get to the point of the thread, I would use the new pushrods without worry. The upper end most likely cannot form a leak proof seal and any excess pressure would probably just blow by at the rockershaft to pushrod joint. It may increase oil presence at the valve stem but I also vagule remember a 38 owner telling me the rocker itself has a drilled blow-by hole?

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John, I'm thinking the cam to lifter point is oiled by splash lubrication from below. Very little oil is getting past the lifters on one of these, and there are no oil galleries in the lifter bore, so the only place they can be fed is from above: in this case from the pushrods. All very interesting compared to other engines. Looking into one of these things was an experience compared to my '60s cars...except for the Corvair, now that's another story entirely!

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According to Motors Manual, Aaron is correct about the feed to the lifters being from the top down. The book shows a cut away of st-8 & v8 lifters with the oil inlet being on the side for the v8 and top for the st-8. Interestingly the book says some engines had solid and some hydraulic lifters with the 40 Series solid and the 50 & 70 series Hydraulic. The rockers and the ball studs are different for solid or hyd with differing oil passage holes. The rockers for both have adjustable ball studs for valve lash adjustment. If the OP's series 40 engine has solids then I wouldn't see where the push rod weep hole would matter. The rocker arm for the solid lifter has the weep hole. So bottom line I'm thinking that the push rods CARS are selling are correct for a 40 series incorrect for a 50/70. But like I said I'm no st-8 expert ..........Bob

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