Bent valves on new rebuild

[Jack H.]

Re: Bent valves on new rebuild

The pushrod holes have been modified (opened-up) for use with guideplates, but I don't see any guideplates. Stock pushrod holes are slotted, but these have been opened up to a round shape. This is a recipe for the disaster you encountered. The rod needs to be supported (side-to-side, i.e. long direction of head) due to it's length in order to maintain geometry and not kink. You need to run guideplates or get a set of unmolested heads.

[Joe L.]

Re: Bent valves on new rebuild

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Thanks to all for the great suggestions. Taking in the many thoughts I have a new theory on the cause. I have some pic's attached that hopefully knowledgeable eyes can help confirm the theory. First the pic's show that all so far checked rocker studs have scaring on one or both sides where they interfered with the stud hole on the rocker. Second the new push rods all have evidence of rubbing on guide hole through the head.

As suggested I reran leak down test and have confirmed leakage is into crankcase not intake or exhaust and I have checked found no interference with valve spring retainer. I have confirmed the cam as EGT ES274 (68 300HP cam), since I cannot ID the spring PN/mfg is there a visual inspection possible or do I need to replace with know PN parts.

My current best guess as to the cause is the machine shop machined the threaded stud boss for a push rod guide that I did not insert thereby changing the geometry of the valve train.

If this idea is correct can someone tell me how to confirm correct stud height?? The machining work was done over six years ago and I doubt machine shop has memory or records. If stud height is made right is it possible to expect success by reassembling with new push rods and rockers (springs) and readjust valves or do I have other things that likely need attention?

Stephen------

The rocker arm pictured has excessive wear on the pad. I would not have re-used these. The rocker ball looks fine, but your photos don't show the ball socket in the rocker. No matter, though, since if the pad is worn like this, it doesn't really matter how good the rest of the rocker is. However, I don't think that rocker arm pad wear had anything to do with your problem. It might have eventually caused a problem, though.

As far as abrasion from contact of the pushrods in the holes in the head, I don't see anything significant on the pushrod pictured. Also, given the size of the pushrod holes in the head, I don't see how any contact could have occurred at any point in the rotation of the engine.

The rocker bosses look like they've been cut down to a point consistent with the use of guide plates. However, I don't see where not using the guide plates ought to create a problem with valve train geometry, especially based on where I see the wear mark on the rocker stud body.

These push rods look to be the welded ball type. Those are fine pushrods and, actually, what GM now sells in SERVICE for most small blocks. However, they must be of the correct length or there will be a problem with valve train system geometry. These should be 7.78" in length, from end-to-end.

If you add guide plates, check to be sure the pushrod tubes are hardened. You can do this by running a file over the surface. If hardened, the file will not leave marks. However, almost all pushrods sold in the recent past and present do have hardened tubes. I have never seen a welded-ball type that did not, but it's still wise to check them if using guideplates.

I can't tell much about the springs from your photos. Do you have any that you've removed? They do look pretty much like a stock spring, though, from what I can see of them and they do have the stock-type oil shields installed. Do the retainers have any embossments on them?

[Duke W.]

Re: Bent valves on new rebuild

It's tough to tell from photos, but if you can feel any kind of lip or ridge on the valve end wear surface of the the rocker arm, they should be replaced.

You don't need guideplates if you get a new set of self aligning rocker arms, which is about all that is available new, nowadays.

If fact, my suggestion is to get new OE replacement Sealed Power self-aligning rockers, balls, and pushrods, so you know that you have compatible parts. You can buy everything from NAPA.

The machine shop that let this kluge out the door is run by a hack. Add another one to the list!

OE valve springs have a paint blotch to ID them, but I don't know the color code or whether the Sealed Power VS677 have the same paint color codes as the GM 3911068 versions, but they are the exact same parts. Maybe someone with any of these springs laying around can chime in.

The OE springs are single with a flat wire damper inside the coil.

For reference to others who read this thread, if you are restoring an engine that has not already been messed up by some hack and are going to run ANY OE cam:

1. You don't need screw in studs if the engine has no history of pulling them.

2. You don't need guideplates if the OE head pushrod holes have not been opened up, nor do you need self aligning rockers. (But self aligning rockers will not work with mechanical lifter cams.)

3. If a thoughtful inspection of the valve train parts show no wear - just burnishing - no galling or ridges, all the OE parts can be reused, especially if you maintain the matched sets and put them back in the original locations.

4. Maintain records of the parts you buy including from the machine shop. If the invoice doesn't show the manufacturer and part number, give it back to them and tell them to do if right. You may be spending hundreds or maybe even thousands or dollars with these clowns. Demand at least a minimum level of quality and professionalism.

Duke

[Michael B.]

Re: Bent valves on new rebuild

Typically if you have valve contact at high speed it will be exhaust only. Since you have intake issues also it is not valve to piston contact.

Here are some other valve train minimum clearances to check.

Rocker to Rocker stud. .030 at full open
Spring coil to coil. .012 at full open
Retainer to valve guide .060 at full open

Looks to me that the stud damage you have is on the side and caused by the rocker binding after the tip slipped to the side of the valve tip.

Depending on the work done, there are several potential clearance problems i.e. valve guides not set deep enough, too many spring shims installed, wrong springs and/or retainers installed.

Machining for screw in studs is 0.250 for no guide plate and 0.320 with guide plates. That may not help, your looking for a number from the bottom up, not the unmachined top down.

These number may have been superseeded.

Rocker stud 3973416
Pushrod Guide 3973418

As Joe has said, part matching, pre-oiling and blueprinting could have benifits for longevity but is not going to cause the problems you have.

[Joe L.]

Re: Bent valves on new rebuild

You don't need guideplates if you get a new set of self aligning rocker arms, which is about all that is available new, nowadays.

Duke

Duke-----


Yes, as far as GM-availability is concerned, the only rocker arms available for all 1955-96+ small block applications are the "guided" type, GM #10089648. These replaced, for SERVICE, the last unguided rocker arm, GM #3974290. The 10089648 were used in PRODUCTION on 1987 and later small blocks.

Unguided type rockers can be obtained in the aftermarket, though. The best of these are the Crane Nitro-Carb rockers which may or may not remain available.

[Joe L.]

Re: Bent valves on new rebuild

Typically if you have valve contact at high speed it will be exhaust only. Since you have intake issues also it is not valve to piston contact.

Here are some other valve train minimum clearances to check.

Rocker to Rocker stud. .030 at full open
Spring coil to coil. .012 at full open
Retainer to valve guide .060 at full open

Looks to me that the stud damage you have is on the side and caused by the rocker binding after the tip slipped to the side of the valve tip.

Depending on the work done, there are several potential clearance problems i.e. valve guides not set deep enough, too many spring shims installed, wrong springs and/or retainers installed.

Machining for screw in studs is 0.250 for no guide plate and 0.320 with guide plates. That may not help, your looking for a number from the bottom up, not the unmachined top down.

These number may have been superseeded.

Rocker stud 3973416
Pushrod Guide 3973418

As Joe has said, part matching, pre-oiling and blueprinting could have benifits for longevity but is not going to cause the problems you have.

Michael------

The GM #3973416 rocker stud has been discontinued and replaced by GM #10168410 which remains available.

The GM #3973418 guide plate remains available.

[Steve V.]

Re: Bent valves on new rebuild

Once again many many thanks for this great assessment and advice. I lose track of exactly how novice I am at mechanics until I see the depth of knowledge and experience displayed on the discussion board.

I have my plan... going to NAPA tomorrow to get recommended rods/rockers/balls and nuts. I will post follow up results after installed and tested.

[Duke W.]

Re: Bent valves on new rebuild

Go to www.napaonline.com and look up the numbers for your specific year/engine using their online catalog. Go in to the store armed with the correct part numbers.

If the catalog search yields any part number confusion, work it out with the store, but do your due diligence and don't rely on a store clerk to look up the correct numbers.

Duke

[Joe L.]

Re: Bent valves on new rebuild

Go to www.napaonline.com and look up the numbers for your specific year/engine using their online catalog. Go in to the store armed with the correct part numbers.

If the catalog search yields any part number confusion, work it out with the store, but do your due diligence and don't rely on a store clerk to look up the correct numbers.

Duke

The Sealed Power part numbers from NAPA or otherwise are as follows:

rocker arms---stock, unguided------R826 (includes ball and nut)

rocker arms----guided type---------R973 (includes ball and nut)

push rods---welded ball-type--------RP3093

guide plates--------------------------MR1896

rocker studs--------------------------MR1865RS

The guided type rocker arms must not be used when guide plates are also used.

[Timothy B.]

Re: Bent valves on new rebuild

Steve,

Here is a picture of heads that have not had the stud boss cut so you can visually compare how much has been taken off your heads. Your heads clearly need the push rod guides, my thinking is the geometry is not correct so you need to be careful here..

If it's rebuilt as a stock 300hp with the hydraulic camshaft, at this point push rod guides with new rockers and push rods may be all you need..

[Joe C.]

Re: Bent valves on new rebuild

Assuming that you did not direct your machinist to deck the block and/or the heads, the installed head gaskets do not vary from original thickness by more than (about) .030, and the installed spring height is close to 1.70", then the stock push rod length should be acceptable. The point of contact of the rocker arm to the valve tip will move from outboard to inboard as the valve travels from its seated position to its position at full lift.

In order to check the geometry, the valve tips should be marked with machinist's blue or a felt tipped marker. With hydraulic lifters, the lash must then be set to spec, the engine run for a short time, and the marks inspected for contact pattern. With solid lifters, set the lash to zero, and rotate the engine manually for a few cycles. If you are using guide plates, be sure to set them so that each pair of guided push rods keeps the rocker arms as close to centered in the fore-and-aft direction AS IS POSSIBLE. Centering one rocker will skew the other, so this becomes a balancing act.

Here is a photo of the contact patch that you are looking for on all of your valve tips:

[John N.]

Re: Bent valves on new rebuild

I had a simular experience with new GM aluminum Bowtie heads in the early 80s. I was adjusting the lifters and the valves started to freeze open bending pushrods etc. The cause was the valve to valve guide clearance was to tight.
Regards

[Clem Z.]

Re: Bent valves on new rebuild

i did not read all the posts BUT from the pictures the shop open up the push rod guide holes in the heads and you MUST use after market guide plates to align the rocker arms. NEVER fill hyd lifters with oil because they will not allow you to pre load the lifter push rod cup seats the .020/.040 as they are locked with the oil in the lifter

[Duke W.]

Re: Bent valves on new rebuild

The other choice is self aligning rockers - one or the other, but definitely NOT BOTH!

Duke

[Joe L.]

Re: Bent valves on new rebuild

The other choice is self aligning rockers - one or the other, but definitely NOT BOTH!

Duke

Duke------



One of the things that confuses folks on this score is this: 1987-1992 small blocks with aluminum heads used self-guiding rocker arms AND WHAT APPEARED TO BE GUIDE PLATES. However, these guide plates, GM #10111771, were not true guide plates. They were primarily an assembly aid and were non hardened.

I believe these plates were eliminated after 1992, but it's possible they were around longer.