07NBSChevy

Hey guys, I had a delay on some parts for my rocker arms and finally got them in. I am trying to get a general idea and close estimate using "mathematical" calculations for pushrod length before I use the pushrod checking tool.

I have a Gen 4 LMG 5.3 Iron block with 243 heads. Stock Pushrods were 7.325" long. I have since added a BTR Truck Stage 2 cam, New LS7 lifters, Smith Brothers Trunnion Upgrade, and removed all physical AFM/DOD hardware. I sent my 243 heads off to TSP-PRC to have them do a Stage-1 Port job with a 5-Angle Valve job with the stock valves. I also had them mill the heads from 64.5cc down to 63cc Chambers for more compression. They said they took 0.00975" of material off to achieve 63cc Chambers.

I am doing my calculations before using my Pushrod length checker just to get a basic understanding and a close estimate, I will measure before I order the pushrods. So stock pushrods were 7.325", I have now removed 0.00975" of head material from milling as well as went from a stock 0.051" head gasket to a Cometic .0.045" head gasket, difference of 0.006", so new pushrod length should theoretically be 7.310" (7.30925"). The Gen 4 LMG 5.3 uses 8x 17122490 lifters for the Non-DOD ones. Which from my understanding is LS7 Lifters. So good news there and I do not have to factor in different lengths for the lifters being a slightly different lengths. GM lists the #17122490 lifter preload to be set at 2.09mm or 0.0823" in the Gen 4 LMG 5.3. Obviously the closest would be 0.080" preload.

So, with the stock pushrod being 7.325" long I now I have subtract the material removed from milling the heads (0.00975") as well as the head gasket thickness difference (0.006") ,which combined is 0.01575", from 7.325" stock pushrod length for a total of 7.30925" or realistically 7.310". Now I would add in the preload of 0.080 for a new Pushrod length of 7.390". Since 7.390" is not a common pushrod since, I rounded up to 7.400" pushrods, and Comp offers a .105" wall in that size, so preload for 7.400" pushrods would be 0.090", (actual preload would be 0.09075 for a 7.400" Pushrod).

I do realize that the cams base circle may be different and will have to factor that in when I use the pushrod length checker tool. I am NOT taking into account for the cams base circle difference in these calculations yet, I do not have that information available to me at this time, so lets just say, for these calculations, that the COMP cam and the Stock Cam have the same base circles.

So 2 questions.
1. If this makes any since to someone, are my calculations correct.
2. If my calculations are right, do you see any issue using 0.090" of preload.

All this will be theoretical and I will be using COMPS pushrod length checker before I purchase pushrods. I just wanted to have theoretical information and an general understanding before I measure as well have a "Cheat Sheet" so I can just "Plug" in the numbers once I measure.

Another question on a somewhat different topic. For torquing down the rocker arms it is recommend to do 22 ft. lbs. which should be 1 to 1-3/4 revolutions. I am using ARP Rocker Arm Studs instead of the OEM bolts. ARP recommends a torque of 25 ft. lbs. Now which torque should I use when using the pushrod length checker to determine to correct length?

MikeGyver

iTrader: (5)

Answers to your questions: 1. Get a pushrod length checking tool. 2. Sorry, I am not positive.
Where in the world did you come up with the idea that "two revolutions equal 22 ft. lbs"? First off, it's in. lbs, secondly, the difference in pushrod length between having the rocker arm bolts seated and being torqued should be zero. Still, they need to be tightened to use a pushrod length checking tool. Tighten them to 22 inch pounds with a torque wrench for final assembly.
Everybody on this forum really, really, really, really believes that you are getting a pushrod checking tool because you mentioned it four times.
Except for me. Since you have already made it OK to use .090 instead of .080 inches, (I don't know what it's supposed to be), I think that you are going to order pushrods and the length checker at the same time, measure the length, and justify that whatever difference that you have is not significant so that you can put the engine together sooner.
I am phsycic, right?

07NBSChevy

My Tech Manual says 22 foot lbs. and I do have a pushrod length tool. You do not seem to be to keen on this do you?

The idea behind the 2 full turns is that if you tighten them down to 22 foot lbs and you get 1-1/4 turn before it clicks, the pushrods are to long, if you get 2-3/4 turns before it licks the pushrods are to short.

Like I said GM recommend .080 preload. The window they recommend is .060-.100. or that is what my GM Tech manual says.

07NBSChevy

Comp Pushrod tool.....

terravast4

iTrader: (25)

foot lbs. is not measured in revolutions. use the correct pushrod and torque to 22 ft lb. done.

Beatdown Z

iTrader: (18)

You're supposed to finger tighten the rockers until you have zero lash. Then from there you count the turns required to achieve 22 ft-lbs.

Beatdown Z

iTrader: (18)

07NBSChevy

Terravast4, I am trying to determine the correct pushrod length first.

BeatDownZ, ok Say I do that, when do I add in the preload?

I am going to search and search to help get this through my head. I have heard different methods. I need a watch a video on it.

Also if it matters, I have Cylinder Number 1 set at TDC on Compression stroke, and am using a checker spring on both intake and exhaust to make it easier for checking.

07NBSChevy

BeatdownZ thank you, I was just reading that as well. very good information. I did not know the counting the turns is basically setting the preload. Makes since now.

I am hoping my ARP Studs do not effect any specs since I no longer have the stock bolts.

I am assuming that "counting the turns of the torque wrench" method is not a common thing since some people did not understand that when I posted it.

07NBSChevy

This video that COMP Cams sent me has good info on pushrod length measurement and setting preload.