Bent push rods

[Fred Rawling]

I coasted to a stop about 2 miles from Prim, Nevada and found 2 bent push rods in my 1935 90 series. What could cause them to bend? I had some backfireing about 100 miles prior to the failure. Intake valves were set at .0098 to .009 and exhausts at .010 to .011.

[Guest imported_NikeAjax]

Could you have sucked in some water; on an extremely rare case, it might cause this to happen. You'd need quite a bit of it, but I've heard of water blowing out the tops of cylinder heads. How is your timing chain, might you have slipped a gear? Someone else might have some other ideas, but you may want to check the above.

Jaybird

[Old Crabby]

Hot valves seizing in the guides? this happened to a Nash I did once where the clearance was too small.

I have heard of it happening due to too much carbon buildup on exhaust valve stems as well.

Noticeable loss of power happen as well?

[Fred Rawling]

There was no water in the oil. The temperature did get up to 200 but it did not boil over and it cooled down when I slowed down to 55 mph. I have not checked for frozen valves. I will try that. I just got it home last night and I did not have time to work on it.

[NTX5467]

Not sure about the clearance specs, but what you quoted sounds a little "tight" to me, considering what solid lifter valve clearance usually is. Even if they were adjusted too tight, it would not cause them to bend anyway, just a loss of power (remember the wheezing VWs? that's what happens when you don't run the valves regularly on them). Backfires usually don't bend pushrods either. Stems seizing in the guides? Seems kind of remote considering modern oils, but if enough oil is not getting down the guides, it'll be easy to tell due to score marks on the stems from metal transfer. In some cases, engine mechanics might try to compensate for oil consumption via the valve guides by putting scraper seals in that keep the guide/stems too "dry", thereby causing more wear to happen sooner, instead of really fixing the problem of stem to guide clearance with new or reconditioned guides plus new valves.

When you do get it apart, please let us know what you found.

Just some thoughts,

NTX5467

[37sedan_mod47]

I had 2 push rods bend on my 40 year old International tractor due to lack of lubrication at the rocker arms. It took me a while to figure out what happened, but when I pulled the rocker shaft off while performing the repairs, I found some sludge built up in the oil supply passage were the rocker shaft tower and cylinder head meet.

[Matt Harwood]

The spec for tappet clearance is 0.015" making your current adjustments a little tight. When things heated up, your clearance could have been reduced to 0 and you had some valve spring bind. I don't think carbon build-up would be a "hard" enough stop to bend a pushrod. Regap your tappets to the specified 0.015" and hopefully the problem will go away. You might also want to check the cam and lifters for any damage from spring bind.

Hope this helps.

[NTX5467]

Something else to consider is if the valve clearances are spec'd at "cold" or "hot" engine temps. "Cold" is usually what the spec is for whereas "hot" would be at operating temperature (after running for about 15 minutes after the thermostat opened up).

Just some thoughts,

NTX5467

[Matt Harwood]

Good point, NTX! Most of the specs on the older Buicks, especially the tappet clearance, is done hot. I think some people even do it with the engine running!

[Fred Rawling]

The specs on this engine are .008 on the clearance with the engine well warmed up. The cylinder head, oil pump, rocker arm, valve push rods and lifters all came from the engine that I had been running. I changed the block because this block had good rod bearings and a round crank shaft. The above items had been running for many many miles on long and short trips with no problems. The head was rebuilt about 25,000 miles ago. I had plenty of oil and the engine was running at 180-190 degrees when I had the problem.

I have not checked the valve stems to see if there are score marks. That is a good idea. There was a lot of oil dripping from the engine after it broke down but as I said, I had plenty of oil in left in the crank case.

Also, does anyone have any 90 series 31 to 35 push rods or has anyone made new ones from the old parts?

[Guest 53and61]

If both bent rods were on intake valves, you might consider contaminated fuel leading to sticking of the valve stems. On my list of possible contaminants would be sugar.

[Guest]

I agree partially with the sticking valve theory, but during assembly the 2 bent ones may have been caught off to one side just enough to put a little curve into them. When run, they failed due to extra loads from the new springs. I used to buy adjustable pushrods that could be fixed for any length, but don't know who has them anymore. Maybe www.oldcarparts.com ?