Well, as some of you may know by my posts, I have been in the process of purcasing a '91Z28. Well, I bought it, got it home, and cracked the valve covers off assuming that I had a bad pushrod or two, as I was told by the previous owner, only to find 3 of the rockers loose, and not properly lined up over the valves, but one literally having been pulled through the stud it was riding on. Here is what you might need to know to make a diagnosis.

- All Bore 383 w/TBI (350 bored .40 over) Forged TRW Pistons, rods, and Crank, New 1.6 Rockers, Balanced Rotating Assembly with high volume oil pump.
- Ported and Polished Vortech Street Heads
- Mild Cam ("Mild cam 218 Duration 246Lift or something like that" is what I was told)

Now, is this damage due to over reving the motor, or is it likely a setup issue? If so, what can I do to help fix this problem? Go with 1.5 Rockers instead? A couple of the studs the rockers were riding on were broken in the process. We plan to remove the heads so we can get a good look at the pistons and the valves and to ensure we don't have any bent valves. What Rocker/Pushrod combo do you suggest I use so this won't happen again? Thanks in advance!

 

did it pull the stud that the rocker bolts to out of the head? The vortec heads need work to be used in performance applications. they should definately be converted to screw-in studs and have bigger valve springs installed. I would have that stuff done to the heads and find out for sure the specs on teh cam in that motor. good luck!

 

Probably too much cam for the springs. Probably bottomed out the springs and broke the parts. I also wouldn't trust the cam anymore.

Could you please explain how on earth he bored a 350 out to 4.4 inch bores. Are you sure he didn't use a 400 block, or simply lie to you. Maybe he meant .040 over standard bore, which would make sense to me, and wouldn't be a 383, more like a 357 or something (don't want to do the math). Or, maybe he (or you) confused .4 inch bore with .4 inch stroke increase?

 

He probably just doenst know it's REALLY 0.040 over. As you can see from the rest of his post he doesnt know much about engines. Stop and let him learn first... then make fun.

 

As I mentioned in another post (maybe yours?) on this identical subject:

Look at the top of the valve guides and the bottom of the retainers, and see if the retainers have been hitting the guides. Those heads are famous for only tolerating about .475" of lift before parts start slamming into each other.

Also, don't forget they are NOT PERFORMANCE HEADS as they come out of the box, regardless of their flow numbers. They are REPLACEMENT TRUCK PARTS. The valve springs on them are total garbage: they are not made for high lift and fast ramps, they are REPLACEMENT TRUCK PARTS. Low-perf valve springs are made of a few turns of fat wire; they have relatively low seat pressure, but their spring rate is very high, and the rate increases rapidly once they get past their design lift of .400" or so. You need to get rid of them if you haven't already, and put some real springs on there. Real performance valve springs are made of more turns of thinner, stiffer wire. You don't have to go wild on them; any of the major cam mfrs. entry-level performance spring will do, such as Comp 981.

There are 2 problems with really working over those heads. First, it doesn't do a whole lot of good because their flow is already fairly decent as-cast, with just a little bowl work they're nearly maxed out; and second, their one shining virue is their economics. They offer great bang for the buck. Once you start spending money on screw-in studs, guide work, spring pocket milling, etc. etc. etc., you'll end up with as much money in them as you'd have in a set of Iron Eagles or some other superior head. The economy disappears.

 

I can't argue with that... I know LS1 applications almost exclusively, this is forigen territory... Never been involved or learned about bore/stroke before, and thus, not used to the lingo... .040 is right... not .4 My bad.

So, what do you all suggest I do? stronger springs, stonger pushrods and better rocker's? Should I go with roller/rockers?

Does anyone else agree with the "wouldn't trust that cam" statement? I can pull it and take a look now, not that much more work, as the heads and intake have to come off anyways, but I'd be surprised if the cam was damaged in any way.

...someone asked about the studs, I think they are aftermarket studs, and the heads have 4800 series CompCams guides in as well... I THINK the springs are aftermarket, but I'm not opposed to going with new springs, that way I know what I've got. Keep the ideas coming... as it has been pointed out, I don't know much yet, so I'm depending on you guys to learn... I can learn quick, and I know my way around a set of tools, so I can do it... I just need your expert guidance. Thanks everyone!

 

One last thought... can I put a less agressive cam in this thing and use the springs that are in it (LT1 or a stock cam?) I'm just trying to get it running... We're building a motor for it long term, so this doesn't have to be a high performance application, simply a reliable one. Thanks!

 

Welcome aboard!

First off, try to find the cam specs. A mild cam is not very descriptive. Once you find out what the cam is, then base your spring pressure from there.

What type of rockers are on the heads. Aluminum roller, roller tip, stamped steel?

Do you have screw-in studs or pressed-in studs?

 

They are screw in studs, stamped steel rockers. I think I am going to get some stock parts and go from there... Stock Cam, rockers, pushrods, etc... The cam is the question mark, and I have no way of finding the spec on the cam short of tearing it out... as I said, we're going to build a motor for this car on a stand, so my motivation is to get this car running in the meantime... Power isn't the utmost concern.

I can get my hands on a Stock LT1 cam in a big hurry... as in today... does anyone anticipate any problems if I go with that cam, or should I hold out for a stock 350 cam?

Thanks guys!

 

You have a serious valvetrain interference problem.
Probably interference between the retainer and top of the valve guide.
You can determine the specs of your cam by degreeing it and measureing the lifter rise in the block without removal
to see if it is useble.
Stock vortecs need the springs upgraded and the valve guide
tops shortened. Otherwise the stock valve train is more them
adequate for mild perf use. 1.5 ratio rockers are all thats nessessary for your use here.
The seller sold it to you for a reason. I'd inspect the motor over carefully 'cause it sounds like it was abused or not assembled right.

 

Ok... So, what to you think the results would be if I kept the springs I have in the car now, and replaced the rockers, pushrods, and went with a stock L98 or LT1 cam? Should I do something to the heads forst, like "shorten the valveguide tops?" or is that not going to be nessicary with a stock cam? Thanks for your guidance guys... It's REALLY helpful...