A few months ago I swapped the cam on my car. As part of this process I installed hardened pushrods (7.400"). A short time after I made a few more changes (including a thinner head gasket) and did not give any thought to the length of my pushrods and whether they would still be the correct length. Shortly after finishing all of my modifications I realized I was burning a lot of oil. The only thing I could think of was that perhaps my valve stem seals/scrapers had not been seated far enough down and had perhaps come loose. I removed my valve springs and sure enough 14 of my 16 valve stem seals/scrapers had popped off and were riding high on the valve stem. I reseated the valve stem seals/scrapers and was grateful the burning oil issue could be accredited to such a minor issue. The question of "why" the valve stem seals/scrapers popped off in the first place was nagging me though. I told myself I must not have put them down far enough the first time I did the valve springs and that pushing them all the way down the second time would ensure I would not run into this issue again. Lo and behold though, the next time I check my oil I am burning oil again! I pop the valve covers off and without even removing the springs I can see a number of valve stem seals/scrapers riding high on the valve stem. It dawns on me that perhaps the 7.400" hardened pushrods I originally installed are no longer the correct length. Perhaps the rocker arm is not evenly putting pressure onto the tip of the valve stem and thus the valve stem is putting unequal pressure onto the valve guide and possibly the valve stem seals/scrapers.

I rushed out to a local performance shop and purchased a Comp Cams Pushrod Length Checker. This tool comes in a variety of lengths and the 6.800" - 7.800" length unit (great for the LS2 engine) has a part number of: 7702-1


Here are the steps I took to check the sweep area of a rocker arm onto a valve-stem tip.

- Select a valve that is fully seated (valve spring all the way up)
- Remove rocker arm
- Wipe oil off of spring retainer (top of valve spring)
- Mark entire tip of valve stem with black Sharpie (felt pen)
- Re-attach rocker arm
- Wiggle rocker arm side-to-side (this action will scar the black Sharpie markings)
- Remove rocker arm and observe the start position of the rocker arm tip
- Using the Comp Cam Pushrod Length Checker, set the tool to the length you would like to test. Replace pushrod with tool, redo steps above.

As a secondary step you can rotate the engine a few times which will cause the rocker arm tip to sweep and scar its entire range of motion into the black felt markings. Caution: I would not perform this step while using the Comp Cams Pushrod Length Checker as the tool appears to snag the head a bit while traveling up and down within the head.

It looks like a hardened 7.350" pushrod would be the correct length for my engine.

 

Interesting. My builder is finishing up a comp cam install with ported / milled heads and .045" cometic head gaskets. We originally had 7.40" hardened push rods and he informed me that we would need 7.35". I was not sure how he came to that number, but hopefully he did his homework and all will be good.

 

If the exact sizes of all components (cam base circle, gasket thickness, etc) were known, you should be able to calculate it. From what I have read though the recommendation is to put the engine together and simply measure it. Measure it with an adjustable push rod or have a collection of different pushrod lengths and do the trial the error approach.

 

Great post. Much appreciated as you find problems and solutions that you share them with us.

Will the pushrod checker tool overcome the pressure of the regular spring, or do you need to purchase and use a weak checker spring? Also, does the lack of oil pressure in the hydraulic lifters while checking this way affect the reading when using the regular spring to check this?

 

Comp cams has the correct push rod checker, unfortunately it does not come in the kit your purchased, I found that out the hard way. If you get the correct checker you will not the problem with the locking nuts.
Are you using aftermarket rocker arms? If so you may wish to set the wipe pattern prior to measuring the pushrod length.
Their are also checker springs available to replace the valve springs

 

Great job MrDrezzUp

 

No one mentioned preload. How much preload have you got with the pushrods?

 

was the very very very very tiny increase in compression worth all this?

 

You don't determine required pushrod length by 'wiggling' the rocker across the valve tip. You have to mock up the rock with the correct pre load and rotate the engine a complete revolution.

The rocker will start on the intake side of the tip, sweep across to the exhaust side. Basically, you want the tip centered on the tip at half lift.

Anything more or less will increase guide wear.

The correct length will have the shortest travel across the tip.

 

He wasn't wiggling the rocker to show swipe, he wiggled it to determine rocker position on the base circle of the cam.....or the beggining of his wipe pattern.

 

Good post. I know I am going to have to do this with my odd ball super long PRs that I have to run with the ETP heads it hink they were like 8.3s er so. Going to try to do it on the dyno.
Had a customer that just changed springs and PRs and we picked back up the 20RWHP he lost and shifted the power back up where it was supposed to be. didnt have much time on the springs he took off but the geometry was a bit off w/ the Prs.
Thank you and God Bless
Jeremy formato

 

 

As near as I can tell it looks like the pushrod checker is made from a regular Comp Cams pushrod! :-) The feel and weight of it is the same and it even has the oil hole in it! Heavy springs...no problem! I used it with the Comp 921 springs without issue. My only caveat is that the tool should probably not be used when turning the engine over with the starter as it felt like the narrow area (where the two parts of the pushrod separate) catch on the head a bit while traveling up and down in the head.

 

I am not a fan of the checker tool that uses the locking nuts. The reason is that you do not know what length you have set the tool to and you have no way of setting it to exactly a certain length. With the 7702-1 model, every complete twist of the upper portion is exactly 0.05". Thus you can easily set the tool to 7.350" and test, and then set the tool to 7.300 inches and test (which is what I did). To be perfectly honest a few pushrods of various lengths would also achieve the same thing. I guess a selling point of the adjustable unit is that you literally have 20 different lengths (0.05 increments) in one pushrod as the adjustable length is 1".

If you used the locking nut style and trialed and error-ed your way to success you would then need a pair of 6"-7" calipers to even figure out what length your checking rod was presently set at!

 

My compression ratio increased from 10.9:1 to 11:3:1.

The reason the gasket was swapped was that my motor came apart shortly after a cam install. http://forums.corvetteforum.com/c6-t...ne-update.html
I was in the middle of nowhere, and it was only by extensive help from Louis and the gang at LG Motorsports (and a large care package of various Corvette parts that they over-nighted to me) that I got my car back to life in short order! The gasket they sent me was a Cometic 0.40" which bumped up my compression ratio. For the record, when I called LG Motorsports regarding the problem I had encountered (car suddenly stopped and wouldn't start) they immediately sent me a collection of various Corvette parts (everything they could think of) that I might need (it was unknown at that time what had gone wrong). I was not charged upfront for the parts and Louis told me the only thing that mattered was that I got my car back up and running as soon as possible (I suspect he could hear how stressed out as I was as the car was my only vehicle and I was in middle of no where!). Louis told me to use whatever parts out of the package I needed, and that I could send back whatever I did not use and that we would figure out the costs later. Louis even told me that he would personally drive the package to the airport (it was getting late when I called him) to get it to me as fast as possible. Still to this day I am amazed at Louis, and how unbelievably helpful he was to me (a crazy Canadian no less) for an issue that had nothing to do with him other than the cam I was using happened to be one of his! This forum is filled with amazing vendors and it is great to be part of such a great community!

We now return you to your regular programming of: "Why pushrod length matters".

 

Exactly.

The first picture was taken with 7.400 pushrods and the engine was turned over so the entire sweep area could be observed.

For the next pictures I was using the pushrod length checking tool at 7.350" and then 7.300". I was not comfortable cranking the engine with the pushrod checking tool in place (as it felt like it might snag on the heads while moving up and down) so I made sure I was on the base circle of the cam and then wiggled the rocker arm so that I could see where the starting point was.

By reviewing the 7.400" picture I could deduce that the starting point of a 7.350" pushrod looked good, whereas the 7.300" looks to be a bit to far towards the intake side.

 

My bad, but that's not what your post is saying ......

 

I think the confusion is coming from me loosely using the term "wipe" and "wiggling".

There is two things I needed to figure out.

1. Where the rocker arm tip starts on the valve stem (when the lifter for that rocker arm is on the cam circle base).
2. The total range of sweep that the rocker arm tip travels when moving back and forth across the valve stem tip.

Both of these steps can be accomplished at once by marking the valve stem with the sharpie, reinstalling everything, and turning over the engine.

Due to my reluctance to crank the engine when using the pushrod length checker, I was only able to see the starting point of a 7.350" pushrod and a 7.300 pushrod. I determined the starting point by ensuring the lifter for that rocker was on the cam circle base, and then wiggling (the word "twisting" would probably be more accurate here) the rocker arm once it was seated.

 

only way to check to adjust the geometry of the rocker is to raise the rocker support by shimming. or milling the rocker support. Lengthening the pushrod only changes the preload. You are running shorter pushrods now. You are simply running less crush.So now for eg, you may only have .050" preload whereas before 0.100" Its the distance at the bottom that is changing. The geometry at the top is exactly the same.Understand? If you did have to much preload the lifters could be pumping up and causing problems, but in general pushrod lengths, milling heads, do diddly squat to wash on the valve stem. I have had the same issues as you, and i shimmed the rockers to fix the issue. Typical of those seals which is why many engine builders stick with single springs.

Only time pushrod length will have an effect is on solid lifters. With hydraulics Get your preload right first, install light springs so the piston won't drop from excessive spring pressure, wind ur engine over if the rocker rides to high mill the support, to low it needs shimming, then the pushrod will need changing again to ensure preload is correct.

 

Sub'd