Bent pushrods

[tinman58]

What would cause two pushrods to bend? I may have hit the 5000 rpm red line once or twice, They are the 3/8" heavy duty rods.

 

[TR3driver]

Coil bind is the first thing that comes to mind. Any chance you've got a high lift cam, or shims under the valve springs?

 

[tinman58]

yes Randell! I have a 270 duration, 410 lift on the exhaust and 400 lift on intake. no shims, plus the supercharger, What is coil bind?

 

[Brosky]

In essence, this is it. When the spring compresses to the max and the rod cannot travel any further, but the cam and rod keep pushing down on it. Something has to give and the weakest point is the rod, then a spring will break and lastly the lobe of the cam will wear. There should always be some space between the coils at max lift of the cam. That is usually determined by the cam maker.

Did you change springs when you put the high lift cam in and were they matched to the cam?

You could also have a weak spring that is collapsing under high rpm stress.

 

[Hap Waldrop]

Another possible cause would be the valve seized in the guide due to lack of proper clearence for the vlave stem to guide ID, more commonly this happen on the exhaust valves and with bronze guides. One thing non British car machine shops and engine builder struggle with is our bronze guides, some of them use bronze liners, but thats nothing more than a sliver of bronze and the clearence is not much different than that of what they are used to working with, but with our bronze guide inserts, they are thicker than the bronze liner and because of this need more clearence at room temps when sizing, due to the fact with the thicker bronze guide they will tighten up at normal engine temps, this has stumped more than one machinist or builder, who is not familar with theses engines and our bronze guides. The bronze guides we use in these engines require about .003"-.004" clearence or you could have a issue, the exhaust is of course the normal culprit due to the extra heat on that side, and if not clearenced enough they would tighten up grab the valve, and from there can/could bend the push rod, break rocker arms, damage cam lobes and lifters.

 

[TR3driver]

Right. The bronze expands faster with heat than the cast iron head, and is also softer. Plus the heat conducted away from the valve stem makes the guide hotter than the surrounding head. So the clearance closes up some when the engine warms up, and then even more when you run it hard. As noted, the solution is to ream the guides oversize after installation. (Has to be after installation, because they also crush a bit when pressing them into the head.)

 

[Brosky]

Interesting info. Thanks for posting those heat and tolerance issues.

 

[tinman58]

Thanks for the info Guys!The head was done by the P.O. so I don't know if the springs were replaced. The cam maker said that is is o.k. to use stock springs. I ordered a set from B.P.northwest. Until I can pull the head and have it checked out I will keep my right foot a little less pressure. Sunday is the Autoum Classic show, The last big show of the year. So Monday evening I might start the teardown.

 

[Hap Waldrop]

FWIW, 99% of time the valves will not even fit into new guides when first installed, they have to be sized to fit, most not familar with these cars just don't size them large enough. This is fairly common problem from generic machine shops.
My guess is this has nothing to do with valve springs, but is a guide fitment issue.

 

[Brosky]

Hap, I know that mine were fit at the shop that did my TR6 head. I just never knew what the actual numbers were. Years ago when I worked in the shop, I swapped out countless heads on 68-70 Oldsmobile 350 & 455's due to badly worn cast iron guides. The factory used to pay for new heads until several dealers with racing machine shop connections got new bronze guides installed, proved that it would last and saved them a ton of money. I've seen the machining to install many, but never the final fit of the valve to the guide. The dealer that I worked for was one of the three that proved the fix was viable.

I was just happy that they used to come back assembled and ready to go.

 

[61tr3]

Last Saturday, you left me and my TR6 in your dust, and I got to smell your clutch for 2 blocks. How many people think you'll really ease the pressure on your right foot.

 

[Brosky]

Ha-ha-ha!!! That is beautiful!!!

 

[Hap Waldrop]

Hap, I know that mine were fit at the shop that did my TR6 head. I just never knew what the actual numbers were. Years ago when I worked in the shop, I swapped out countless heads on 68-70 Oldsmobile 350 & 455's due to badly worn cast iron guides. The factory used to pay for new heads until several dealers with racing machine shop connections got new bronze guides installed, proved that it would last and saved them a ton of money. I've seen the machining to install many, but never the final fit of the valve to the guide. The dealer that I worked for was one of the three that proved the fix was viable.

I was just happy that they used to come back assembled and ready to go.

There's no magic to fitting them, just having the right size reamer, heck a drill bit can work in a bind, and flex hones to finish them.

 

[tinman58]

Just replaced the pushrods (thanks Glen). While adjusting the gap I could not get .12 clearance on #8 and maybe .10 on #4. I think that there is some valve work to be done. I am planning to teardown the engine and have it balanced, But while I am there I ordered the 89 mm pistons and barrels. Hap I am going to your web site to see what is avaible in the way of a good head.... "It's only Money!"

 

[Hap Waldrop]

So both are exhaust valves, and you could not get enough lash, is that what I'm reading, hmmmm, it could be you have serious exhaust valve recession going on , an the valve is working it's way up in the bowl, if the valve lash tightened up because of this, it could indeed cause excessive valvetrain pressures and bend the the pushords, you need to look at the lifter on those valves when you pull the head as well. Exhaust valave recession is somewhat rare, but it happens, normally due to the not having hardened exhaust seats suitable for unleaded fuels, I install hardened exhaust seats in all my cyliner head rebuilds, not only can they develop valve recession over time if hardened inserts are not installed , but more commonly they just will not seal because the metal in the exhaust seat area has been compromised due to hotter burning unleaded fuels.

 

[poolboy]

Were the pushrods the same length as the "originals" ?

 

[TR3driver]

an the valve is working it's way up in the bowl, if the valve lash tightened up because of this, it could indeed cause excessive valvetrain pressures and bend the the pushords,

You lost me there, Hap. Can you explain that a bit more?

BTW, I had severe VSR on a head that had hardened seats installed. WHile it certainly helps, hardened seats are not necessarily a "magic bullet".

 

[CJD]

I would think a lack of lash would lead to valve burning around the edges...and very rough running. And that would be because it will eventually fail to seat at all. I do agree that no lash is caused by seat wear.

Another thought....is there a chance your pushrods are to long?

John

 

[tinman58]

Well all I need is to get to the last British car show on Sunday. It is about a 200 mile round trip all freeway, I belive that the PO had the head redone but installed oversized intake valves so the machinist could not install the harden seats. I kinda knew that and knew that the head was the weakest link. Thank you for all the input. P.S. Do you think that I will make the last 200 miles?
P.P.S. Hap your web site is great. I wish that you were a little closer to this side of the country.

 

[poolboy]

Another thought....is there a chance your pushrods are to long?

John

I had the same thought.. at the top of this page...