I noticed that some builders have stated after finding the correct pushrod length they add another .025" inch to the push rod length to combat the flexing of the pushrod under load. I guess the theory is while the engine is running at rpm the pushrod is flexing and the .025" makes up for the difference and you have the "correct length" at rpm.

Anyone do this and have any thoughts? I'm getting ready to assemble my motor and of course the pushrods will be the last thing to order. I'm going with the Terry Manton #4 pushrods so I only want to order them once.

 

Nope make them the correct length and then make them larger in diameter. Just put a set of those exact Manton PR's in a motor, very nice stuff. Upped them to a 3/8ths diameter to prevent the flex that you are speaking of.

Bret

 

I wouldn't have to worry about flex with comp chromoly pushrods and a gm846 cam on crane dual springs (~130 seat pressure) would I?

 

HOW are these builders measuring this .025 flex? I want to see the machine that does it.
3/8" heavy wall won't flex,like Bret said.

 

Thanks for the replies. Here is the thread where a couple of the builders use longer pushrods than the correct static measure. Also and interesting read in it's own right.
http://www.speedtalk.com/forum/viewtopic.php?t=3731

 

That thread is mostly about geometry and what is the best way to achieve it.

 

True, but Awesomebill in post 11 and Ed-vanced in post 12 mention the extra pushdrod length. In fact they say .025/.050".

Bret I'm going to take you up on the 3/8 inch Manton #4 pusrods. I think that is the way to go and I will get the thicker wall version at that. The 3/8" guideplates are only like $25.00 so not that much more money for valve train stability. Might have to relieve the heads a little to clear the pushrods. They are Dart Pro One 200cc heads. Thanks guys.

 

It's also about engines with 0.700 lift and 9k rpm...

 

A Spintron will show flex,but who has one but the cam manufacturers.
I have run more than double your spring pressure with 3/8" and not had a problem. All my customer engines have the "correct" length push rods and not .025 over/under.

 

I will be ordering the as push rods as measured without any extra length except for the lifter preload. I do think the tool that andyf came up with in post #14 using the dial indicator is the cats a$$ for measuring rocker arm travel. Should be able to get the correct pushrod length down to the tee with it.

The zero lift point and the maximum lift point should measure the same on the dial indicator. Got to make one up. You can tell if you are over arching or under arching in a heart beat. For most of us that is good enough as long as the roller is riding correctly on the valve tip.

 

Not having read that thread do they suggest setting valve lash .025-.050” tight also, that would be necessary if they’re really expecting that the pushrod will flex enough to loose that much length.

BTW, that’s a bow of about ½” in the pushrod, I don’t think that most engines have enough room to flex that far without hitting something resulting in a catastrophic failure.

 

This is exactly what happens in an IHRA ProStock caliber engine with a very tall deck. The long pushrods can flex enough to rub the block and waller out the head gaskets where they pass through. This is with substantial clearance all the way around a 7/16" pushrod. Changing to manton 7/16"-1/2" tapered pushrods fixed the flex issue and completely changed the sound and response of the engine though overall WOT power did not change as expected. The rest of the valvetrain was more durable and springs went longer between service on the intake side. An engine such as that runs very little if no lash when cold due to block and cylinder head growth at operating temperature. In fact some engines have negative exhaust lash when cold and it makes for a nasty running cold engine as the valve grows before the case and head do.
This is why some of those guys attach a burner to the frontend of the car in the pits.

I have always had good results with 3/8" pushrods in a small block.

 

dumb question, those of you running 3/8" pushrods in a small block... what guideplates do you use/like?

 

Comp Cams raised 3/8".

Isky also has a nice set of adjustable guideplates.

 

Ditto on the Iskys