Bent valves on new rebuild

[Steve V.]

This week was planned to be a celebration of nearly two years of frame off restoration of a '68, 327, 4 spd convertible. Instead it is back to engine basics trying to find out why on its first hard acceleration highway test drive I think I bent the new valves and pushrods. I would appreciate others thoughts on what caused this.

The engine is the original numbers matching block, heads and large journal crank. The engine left the factory as a 327/350, but I rebuilt as 300HP. The engine rebuild included boring .060", crank ground .010" new 300HP pistons, new 300HP cam, new lifters, push rods. The block was not decked to preserve the stamping. The heads were rebuilt and leveled with new valves/springs and screw in rocker studs, I reused the original rockers. The engine/head machine work was done by local shop and I assembled.

The engine valve train assembly included manually pumping up the lifters while submerged in oil then letting them sit in the oil for a couple of days before putting in the engine. Upon assembly into engine the rockers were adjusted to zero lash plus 1/2 turn. When the engine was first fired (several months ago) it can up without a hitch, was run in for 20 minutes at ~2000rpm, came up to temp, no noise or smoke or backfiring, and idled smoothly. Engine was periodically started and driven over the intervening weeks as the rest of the car was put into driving condition, it ran smoothly for occasional drives around the block to verify adjustments to clutch and transmission, linkage, alignment, A/C etc.

Then on the day of final completion I took it out to the highway for a first highway drive and on the first hard acceleration the engine stumbled badly then began back firing. After limping back home and verifing that timing was still on target but none of the plugs were burning cleanly then to my amazement when the valve covers were removed I found the #5 intake and #4 exhaust rockers were no longer on the valves and that most other valves were off center on their valves by a significant amount.

The 5I and 4E push rods are for sure bent and most if not all others appear bent (but not yet removed and verified). I did a leak down test on all cylinders and all eight cylinders are evenly leaking way too much so I am guessing bent valves. Have not yet removed the heads to see if any piston damage.

Not sure yet what caused this major foul up, but of course my first guess is that I did not get all the air out of the lifters before installing which led to adjusting them too tight. But this idea seems at odds with the engine running correctly through several full warm up runs and about 6 miles of driving around the block in the weeks preceding the failure. Any other ideas on the cause??

[Dick W.]

Re: Bent valves on new rebuild

This week was planned to be a celebration of nearly two years of frame off restoration of a '68, 327, 4 spd convertible. Instead it is back to engine basics trying to find out why on its first hard acceleration highway test drive I think I bent the new valves and pushrods. I would appreciate others thoughts on what caused this.

The engine is the original numbers matching block, heads and large journal crank. The engine left the factory as a 327/350, but I rebuilt as 300HP. The engine rebuild included boring .060", crank ground .010" new 300HP pistons, new 300HP cam, new lifters, push rods. The block was not decked to preserve the stamping. The heads were rebuilt and leveled with new valves/springs and screw in rocker studs, I reused the original rockers. The engine/head machine work was done by local shop and I assembled.

The engine valve train assembly included manually pumping up the lifters while submerged in oil then letting them sit in the oil for a couple of days before putting in the engine. Upon assembly into engine the rockers were adjusted to zero lash plus 1/2 turn. When the engine was first fired (several months ago) it can up without a hitch, was run in for 20 minutes at ~2000rpm, came up to temp, no noise or smoke or backfiring, and idled smoothly. Engine was periodically started and driven over the intervening weeks as the rest of the car was put into driving condition, it ran smoothly for occasional drives around the block to verify adjustments to clutch and transmission, linkage, alignment, A/C etc.

Then on the day of final completion I took it out to the highway for a first highway drive and on the first hard acceleration the engine stumbled badly then began back firing. After limping back home and verifing that timing was still on target but none of the plugs were burning cleanly then to my amazement when the valve covers were removed I found the #5 intake and #4 exhaust rockers were no longer on the valves and that most other valves were off center on their valves by a significant amount.

The 5I and 4E push rods are for sure bent and most if not all others appear bent (but not yet removed and verified). I did a leak down test on all cylinders and all eight cylinders are evenly leaking way too much so I am guessing bent valves. Have not yet removed the heads to see if any piston damage.

Not sure yet what caused this major foul up, but of course my first guess is that I did not get all the air out of the lifters before installing which led to adjusting them too tight. But this idea seems at odds with the engine running correctly through several full warm up runs and about 6 miles of driving around the block in the weeks preceding the failure. Any other ideas on the cause??

Before you start tearing the engine apart, do your leak down test again and listen to the exhaust and intake. I suspect that your leakage is going past the new rings that have not seated to the walls. You can remove the oil fill cap and listen to the crankcase leakage

[Loren L.]

Re: Bent valves on new rebuild

Question, not an answer - the 350HP '68 used the big 8" balancer/timing mark - does the 300 mark occur at a different spot?

[Duke W.]

Re: Bent valves on new rebuild

VALVE SPRINGS - manufacturer and part number, please.

Did you install the pushrods/rocker arms in matched sets as they were removed from the engine?

How high did you rev it?

If any valves are bent they will be hung open and there will be no compression on those cylinders. So....?

Duke

[Duke W.]

Re: Bent valves on new rebuild

Question, not an answer - the 350HP '68 used the big 8" balancer/timing mark - does the 300 mark occur at a different spot?

The balancer timing marks are in the same postion relative to the keyway, but the "big balancer" results in a timing tab error - a bit on the low side, so 10 degrees indicated is about 8.5-9 true.

Duke

[Joe L.]

Re: Bent valves on new rebuild

This week was planned to be a celebration of nearly two years of frame off restoration of a '68, 327, 4 spd convertible. Instead it is back to engine basics trying to find out why on its first hard acceleration highway test drive I think I bent the new valves and pushrods. I would appreciate others thoughts on what caused this.

The engine is the original numbers matching block, heads and large journal crank. The engine left the factory as a 327/350, but I rebuilt as 300HP. The engine rebuild included boring .060", crank ground .010" new 300HP pistons, new 300HP cam, new lifters, push rods. The block was not decked to preserve the stamping. The heads were rebuilt and leveled with new valves/springs and screw in rocker studs, I reused the original rockers. The engine/head machine work was done by local shop and I assembled.

The engine valve train assembly included manually pumping up the lifters while submerged in oil then letting them sit in the oil for a couple of days before putting in the engine. Upon assembly into engine the rockers were adjusted to zero lash plus 1/2 turn. When the engine was first fired (several months ago) it can up without a hitch, was run in for 20 minutes at ~2000rpm, came up to temp, no noise or smoke or backfiring, and idled smoothly. Engine was periodically started and driven over the intervening weeks as the rest of the car was put into driving condition, it ran smoothly for occasional drives around the block to verify adjustments to clutch and transmission, linkage, alignment, A/C etc.

Then on the day of final completion I took it out to the highway for a first highway drive and on the first hard acceleration the engine stumbled badly then began back firing. After limping back home and verifing that timing was still on target but none of the plugs were burning cleanly then to my amazement when the valve covers were removed I found the #5 intake and #4 exhaust rockers were no longer on the valves and that most other valves were off center on their valves by a significant amount.

The 5I and 4E push rods are for sure bent and most if not all others appear bent (but not yet removed and verified). I did a leak down test on all cylinders and all eight cylinders are evenly leaking way too much so I am guessing bent valves. Have not yet removed the heads to see if any piston damage.

Not sure yet what caused this major foul up, but of course my first guess is that I did not get all the air out of the lifters before installing which led to adjusting them too tight. But this idea seems at odds with the engine running correctly through several full warm up runs and about 6 miles of driving around the block in the weeks preceding the failure. Any other ideas on the cause??

Steve-----


I don't think the problem had a thing to do with not getting the lifters full enough of oil. Soaking the lifters is not even really necessary but if it has any value, at all, it's only for initial engine start-up. After that, the lifters will fill on their own. Once you got the engine up-and-running, you immediately "left behind" anything having to do with pre-filling the lifters with oil.

Has this engine ever had push-rod guide plates installed?

[Rich P.]

Re: Bent valves on new rebuild

You said you reused the rockers did you put new lock nuts on the rockers or the old ones?

[Steve V.]

Re: Bent valves on new rebuild

Many thanks for these ideas, a glimmer of hope that now that maybe only pushrods are needed to get engine back on the road.

I will rerun the leak down test as suggested, air leakage could be heard during test and I confirmed not out the carb or tailpipes but did not make check at the oil filler. Hoping the leak is indeed unseated rings, that would explain the odd result that all cylinders leaked at the same high level. Dick thanks for offering this glimmer of hope.

I am not aware of matching push rods to rockers, the pushrods were new and verified same length as originals, the rockers were the originals but I did not focus to make sure they went back in original location thinking that with new push rods and valves there was no need. Is there a matching need here such that on reassembly I should use new rockers as well as push rods?? The valve springs are dual springs purchased at local parts supplier but I cannot find my receipts on the manufacturer/PN, they looked the same as ones on the original heads (but I cannot confirm the springs were original). Duke, can you give me insite to what your thinking is regarding matching and spring PN's as to how these elements might have contributed to the problem. The engine revved only to about 4000 RPM in second gear before failure, I had revved to higher RPMS unloaded previously. Your insite to bent valves sounds right on, and gives me further hope that only the pushrods are bent, if true does that mean the the push rod is simply the weakest link in the valve train (not the valve)?

I heard from a local advice giver that filling the lifters with oil before assembly was the wrong thing to do, and that the lifters should be installed and adjusted collasped. Can someone confirm this?? This gives rise to my limited knowledge of hydraulic lifters, once filled with oil why are they not essentially solid, how does oil ever get out? I have rebuilt a few engines in the past similar to this one and never did the lifter oil fill thing, this is the first time I did this on the advice of a creditable source. Joe, no push rod guides are used or appear to have every been.

[Steve V.]

Re: Bent valves on new rebuild

The old lock nuts and spherical washers were reused but on new screwed in studs. How might this contribute to the failure??

[Ray G.]

Re: Bent valves on new rebuild

Hello Steve;
Two things pop into mind.

Were the valves readjusted after running the initial 20 minutes ? Possibly the adjustment is at the tightest setting. Causing a valve float at the 4000 RPM.

Possibly the valve guides are fit to tight to the point of scoring the guides and/or valve stems. Holding the valves open longer.

Have seen several newly overhauled engines brought to me over the years with these problems.

Hope these suggestions help.
Ray

[Joe L.]

Re: Bent valves on new rebuild

Many thanks for these ideas, a glimmer of hope that now that maybe only pushrods are needed to get engine back on the road.

I will rerun the leak down test as suggested, air leakage could be heard during test and I confirmed not out the carb or tailpipes but did not make check at the oil filler. Hoping the leak is indeed unseated rings, that would explain the odd result that all cylinders leaked at the same high level. Dick thanks for offering this glimmer of hope.

I am not aware of matching push rods to rockers, the pushrods were new and verified same length as originals, the rockers were the originals but I did not focus to make sure they went back in original location thinking that with new push rods and valves there was no need. Is there a matching need here such that on reassembly I should use new rockers as well as push rods?? The valve springs are dual springs purchased at local parts supplier but I cannot find my receipts on the manufacturer/PN, they looked the same as ones on the original heads (but I cannot confirm the springs were original). Duke, can you give me insite to what your thinking is regarding matching and spring PN's as to how these elements might have contributed to the problem. The engine revved only to about 4000 RPM in second gear before failure, I had revved to higher RPMS unloaded previously. Your insite to bent valves sounds right on, and gives me further hope that only the pushrods are bent, if true does that mean the the push rod is simply the weakest link in the valve train (not the valve)?

I heard from a local advice giver that filling the lifters with oil before assembly was the wrong thing to do, and that the lifters should be installed and adjusted collasped. Can someone confirm this?? This gives rise to my limited knowledge of hydraulic lifters, once filled with oil why are they not essentially solid, how does oil ever get out? I have rebuilt a few engines in the past similar to this one and never did the lifter oil fill thing, this is the first time I did this on the advice of a creditable source. Joe, no push rod guides are used or appear to have every been.

Steve------


There should be no problem with using original rocker arms and after installing new pushrods and new valves there's absolutely no need to ensure that the rockers go back in the original positions. It's entirely moot at that point. As long as the original rockers were serviceable, they are fine for re-use. Serviceable means no wear on the rocker pad (the part that contacts the valve), no wear in the rocker ball socket and no wear in the pushrod seat. By "wear" I mean obvious ridges, grooves, spalled metal, erosion, or other such things. I don't mean just shiny metal; that's to be expected on used rockers. However, even if you were to use rockers which shouldn't have been re-used it would not cause a problem like you experienced. The only effect would be long term durability.

As far as lifters being pre-filled with oil or not, lots of folks out there do it both ways. Personally, I don't think it's necessary. I don't think that GM ever pre-filled the lifters with oil and they started a LOT of engines for the first time. However, whether you filled the lifters with oil, or not, the effect of doing that or not doing it lasts only a matter of a second or two. After that, the engine doesn't know one way or the other if the lifters were originally pre-filled with oil.

The reason that I asked about the pushrod guide plates is that guide plates are hardened and must be used with pushrods with hardened stems. Most pushrods these days have hardened stems but if you happen to get a set of old pushrods without hardened stems and they are used with guide plates, the guide plates will wear the pushrod stems rather quickly. Then, they bend at the worn area. In this same regard, you might want to check the pushrod-to-head clearance in the pushrod holes in the head. Just look at the pushrods you took out to see if there is any evidence of serious contact or abrasion in this area.

There may be a problem created by the valve springs and/or retainers. Personally, I would not use anything but GM springs and retainers or high quality aftermarket that you KNOW are exactly the same as GM. The original valve springs used on your engine were not dual springs. They were single spring and inner flat wire damper. Used on unmodified heads, you know that the GM pieces or IDENTICAL aftermarket will work properly. When one gets into re-engineering this part of the valve train, extreme caution needs to be exercised. The GM springs and retainers will provide all the performance capability that you need for an engine like yours. The exact same valve springs and retainers used on your original engine were the same as those used for 70-72 LT-1 and they worked pretty well there.

Another thought that I had involves the timing set. Are you sure that there has been no failure of the chain or sprockets?

[Rich P.]

Re: Bent valves on new rebuild

The old lock nuts and spherical washers were reused but on new screwed in studs. How might this contribute to the failure??

Steve,

I like to start with the simplest thing first. You mentioned that 2 of the rockers were off I did not know if that ment off compleatly.

Rich

[Duke W.]

Re: Bent valves on new rebuild

The valve springs are dual springs purchased at local parts supplier but I cannot find my receipts on the manufacturer/PN, they looked the same as ones on the original heads (but I cannot confirm the springs were original). Duke, can you give me insite to what your thinking is regarding matching and spring PN's as to how these elements might have contributed to the problem. The engine revved only to about 4000 RPM in second gear before failure, I had revved to higher RPMS unloaded previously. Your insite to bent valves sounds right on, and gives me further hope that only the pushrods are bent, if true does that mean the the push rod is simply the weakest link in the valve train (not the valve)?

Since you did a "stock rebuild" with a "300 HP cam" the valve spring requirement is OE 3911068 or Sealed Power VS677 OE equivalent, which cost about a buck apiece. These are single springs with a damper, not dual.

Without part numbers (cam and valve springs), who really knows what you have, but the wrong valve spring - such as a high rate dual spring could bend pushrods, and not all "300 HP cams" are actually ground to OE spec.

OE rockers, balls, and pushrods are very long lived parts and can usually be reused, however, they have "run in" together, which is why I recommend bagging them as matched sets during disassembly and marking the bag with the location in the engine. At some point, they should be inspected for wear and if okay, they should be reinstalled as "matched sets" in the same engine location as they were removed from.

Is this absolutely necessary? No, but it's good practice and minimizes the chances of problems compared to jumbling everything up.

The same applies to cam and lifters - only more so. Often they can be reused, but if the lifters are not bagged/tagged and installed back on the same lobe, the chances of a surface compatability problem that can lead to a "wiped lobe" increases greatly.

Pre soaking lifters is not necessary. I don't think GM did it. Using a "preoiler" - turning the oil pump while rotating the engine will fill up the galleries and lifters, but, again, GM didn't do this nor did they do a cam break-in. They just fired up the engine at hot test and the oil pump filled the galleries and lifters within a few seconds. The "cam break-in" consisted of starting the car at the end of the line, running through the roll test, and then driving it out to the parking lot. The rest was done with 50 cold starts and a few minutes of operation as the car went through shipping and was moved around the dealer's lot.

The Web has most guys believing that if they don't do a "cam breakin" a wiped lobe is inevitable, but if you use OE equivalent parts it's not necessary. At the same time guys don't sweat little details that can make a real difference.

Bending a pushrod on a 300 HP engine at 4000 revs is a 6 sigma event. You probably have some kind of parts compatability problem (or possibly a collapsed,/defective lifter), and you need to do a forensic investigation beginning with an audit of exactly what valvetrain parts are installed and whether or not they are actually OE or OE equivalent.

Duke

[Cecil L.]

Re: Bent valves on new rebuild

What type of threaded studs did you use, shouldered or straight? If shouldered, did you cut the boss to compensate for the shoulder?

[Steve V.]

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?