undy

I just bolted a set of PRC 265s on my LS7. The heads have been cut 0.030". The stock pushrods for the factory heads are 7.8". The base circle of the cam is same as a stock cam. What length pushrods for the combo?

Jontall

You really need to measure it.

DietCoke

iTrader: (18)

measure

undy

6.8" pushrod checker, 16 turns out to zero lash @ 0.050" a turn = 0.8".

6.8" + 0.8" + 0.75" (preload) = 7.675" pushrod needed.

Right?

DietCoke

iTrader: (18)

.075 preload might be a little much, LS7 lifters?

undy

I was told the range was 60 to 90 with LS7 lifters? They have 0.200" of travel.

Sales3@Texas-Speed

iTrader: (2)

We typically shoot for right around .050" with the LS7 lifters. The range should be anywhere between .040"-.090".

undy

Do you guys have any 7.650" pushrods in stock. I need them NOW

Do the PRCs in general take shorter pushrods then the OEM heads?

bww3588

iTrader: (49)

Longer if they are aftermarket castings due to the thicker deck.

undy

That's wierd, I checked several valves and with the stock LS7 pushrods (7.8") and there was only 0.017" between the valve tip and the rocker on all of them. If you divide that by 1.8 (LS7 rocker ratio) that means there is only 0.010" of travel left in the lifter before it turns into a solid (fully compressed). I even turned over the engine several times to ensure the lifter and pushrods were seated.

This has me worried a bit as I've already next day aired the new Trick Flow 7.650" pushrods. It looks like I need to call TSP as soon as they open.

Sales2@Texas-speed

iTrader: (5)

Keep in mind even on the cathedral port heads with the thicker decks, the longer pushrod is not due to the thicker deck....the difference in length comes from the pedestals being raised in the head to get out of the way o the ports a bit, and that causes the need for longer pushrods. The thickness of the casting doesn't have anything to do with it.

David, you need to measure after they're installed! There are too many variables to try that kind of math!

Jimbo1367

Count me in for dyno/strip numbers/results!!!

badazz81z28

iTrader: (6)

.040-.090? What happened to .050-.080? I thougth people ran as much as ..100? More or less better??

undy

I went and did some re-measuring last night. Evidently I didn't have the cam on it's base circle the first go-round. Still, I'm not getting readings that say I need a longer pushrod.
The measurements that I'm repeatedly getting is that I need 7.750" pushrods, 0.050" shorter than stock. Remember, the head is cut 0.030".

I used both forms of measurement, tightening the trunion bolt and the feeler gauge under the rocker tip. Keeping in mind that I'm running 1.8 ratio rockers...

With a 7.750" adjusted comp cams pushrod checker it takes just a RCH over one full turn to torque down the trunion bolt on the rocker. I believe that equates to right at 0.075" of preload, exactly what Katech recommends for the LS7.

The second way, with the 7.750" pushrod checker in place and the rocker torqued down I'm getting 0.220" between the valve tip and rocker. If you divide that by the rocker ratio (1.8) you get .122". The LS7 rocker has 0.200" of plunger travel. That means that the plunger in .122" from the bottom of it's travel. When you add the 0.075" of preload that I got from the first test it pretty much adds up to the 0.200" of advertised plunger travel. Both tests seem to collaborate each other.

Having said all this, I'm intending on getting a set of "expert eyes" to check things behind me before calling it a done deal.

Thoughts again???

bww3588

iTrader: (49)

One full turn on a stock thread pitch and diameter bolt is .047 roughly.

undy

Yes, but another "pushrod checking thread" here says you need to do a bit of math to get the actual pushrod movement when you tighten down the trunion, (1.8 + 1) / 1.8 x 0.048 = 0.0746" of actual pushrod movement.

Sales2@Texas-speed

iTrader: (5)

.040-.090, .050-.080, .060-.100....it's all good. Unfortunately, it is not a real specific number they have to have, which is good, because without adjustable rockers you aren't going to get it! It's just a range, and a broad one at that!

Why not use that length checker the way it was intended originally to measure this? On the base circle of the cam, hang snug rocker arm. Run length checker out to zero lash. measure number of turns it takes to get back to short length.

6.800+((number of turns)x.050") = Zero lash length. Now add preload, and you have the length to order.

Keep in stock LS7 pushrods were about 7.785" or so in gauge length, not 7.800 (not that it matters, because you don't know what stock preload was on your vehicle).

Ellesone

The only way to get it proper is to measure. The only way to get it right is to measure intake and exhaust. Glad to see shops finally adopt a proper procedure on here

undy

Matt, that's the same method I've used all along... Look at my second post. The only difference is the cam wasn't entirely on it's base circle as I thought it was.