Hello guys n gals,

We I decided on going down to the track this morning with my 93 Auto heads and cam LT1 for the test and tune. Car ran great on the way. During the first pass I hooked hard and had a good launch. I shifted from 1st to 2nd at 6250ish and she kept pulling then on the 2 to 3 shift I knew something went wrong. It felt like the car was missing. A visual inspection led me to believe it was valvtrain related. Jay confirmed that it was probably a roller rocker that slipped off. I babied it home and tore into it.

When I took off the passenger side valve cover I noticed I was missing both pushrods on the number 2 cylinder. One of the rods was visible from there and the number 4 cylinder opening. Half of the other one was also sticking out of the number 4 cylinder opening. So I fished them out with a magnet but was unable to find the rest of it. Later on I pulled the intake manifold off and found the other half laying in the lifter valley.

So, my questions are:

How/Why did these break?
What should I replace these with?
Should I also get new lifters? If so, what kind?

The car currently has 59K miles and had 55K when the heads and cam were installed.

Thanks,

Dave

Pictures will come in the morning.

 

I guess you used the stock push rods? Bad idea...I would recommend using hardened push rods.

If those were hardened that broke, that's bad...

 

They were stock pushrods. I've been searching for casues on here and I see mention of the poly lock loosening but mine are still tight. I've also tried searching for LT1 pushrods on Summitracing.com but have had no luck (I know I'm probably doing it wrong).

Thanks,

Dave

 

Coil bind perhaps?

 

You need chrommoly one piece construction push rod. 7.200 long . It is under $100 in Summit

Bruce

 

Are you using self aligning rockers? I have the stock pushrods and so far no problems.

 

If you have 1.6 rockers 7.150 length might work better you'd have to check and see where your tip is riding on the valve head. The magnum push rods would be more than adequate for you application.

Disregard Summits description about "ball tip" on the magnums, they are one piece.

http://www.compcams.com/technical/Ca...07/252-253.pdf

http://store.summitracing.com/partde...5&autoview=sku

http://store.summitracing.com/partde...5&autoview=sku

 

Stock push rod's @ your almost 6300 RPM shift not good they just failed. Purchase as some have suggested a set of chrome mily case hardened rod's and you wont have this happen again. I'd leace the roller lifter's in,but if you still have the manifold off I'd pull each and do a visual and roll them in your hand to see if any bind and replace if necessary. Others have helped w/your part number so no comment there.

Let us know what comes of it!

Thanks,Matt

 

CentralCoaster - What exactly is coil bind? How does it happen/What causes it?

93 ragtop - I am using the roller rockers that came with the GM Hot Cam Kit. From what I understand, they are self aligning, correct?
I did research before I did the heads and cam last year and found many guys run the stock pushrods without problems and that is why I used them.

ALLT4 - I do have 1.6 roller rockers. What are the benfits of running the shorter pushrod? I will buy one of the sets you recommended.

madmatt9471 - I expected the rear or the trans to blow but never this. I have had the engine up to 6500rpm many times over the past year and a half without any problems.

Thanks for all the info guys,

Dave

What about the lifters, do you guys recommend I replace them too? If so, with what kind?

 

You need to make a solid lifter out of a hydraulic one. Any machine should just be able to "give" you an old one. Take it apart and make it into a solid lifter with some small washers inside, meaning the pushrod cup will not move down in the bore.

Use a Sharpie to paint the tip of the valve stem black. Install your homemade solid lifter, pushrod, and rocker. Adjust to zero lash, no preload, impossible with a solid lifter. Turn the motor over a few times so the rocker roller tip makes a mark on the valve stem tip.

Take the rocker off and look at the mark it makes, more than likely it will be towards outside of the tip a bit. A shorter pushrod will bring that mark more centered on the stem tip.

As some have stated before, I'd bet you're running into coil bind. If you have the Hot Cam springs in stock pockets they're probably at the hairy edge of coil bind.

You need to find your installed height of the springs, you can remove one of the springs and install a checking spring or buy the height gauge. I just leave the spring totally out and pull up on the retainer with locks installed. Measure from the spring pocket or shims whatever the case may be to the underside of the retainer. That is your installed height. Note that measurement.

Stick your Hotcam spring in a vise and totally bind it. (be careful here) Measure the fully compressed height.

Subtract the compressed height from the installed height, that is your total static lift capability. Subtract the total lift of the Cam, it should be .525 with the Hot Cam and 1.6 rockers. You need to wind up with .060 for a safety margin. Meaning you'll still have .060 to go before the spring "binds" fully compressed.

Hot Cam Springs Part Number 12551483
Material Chrome Silicon SuperClean
Outside Diameter 1.337”
Inside Diameter 0.906”
Installed Height 1.780”
Seat Load 101 lbs.
Average Spring Rate 331 lbs./inch
Solid Height 1.215”

Using above we see an installed height of 1.780 and a solid height of 1.215. .565 total static lift With the Hot Cam and 1.6 rockers we need to subtract out .525 lift from that; leaving a .040 safety margin.

So as you can see the Hot Cam springs are already at or over their limit. Why GM does this I don't know.

.040 I still consider safe as a lot of folks just plug the Hot Cam kit in and go with no problems. Hell I've run .010 with no problems, but they were some serious triple springs with superior control of the valve.

If your installed height is less than 1.780 (more than likely it is on an LT1) your in dangerous territory and at high RPM those springs won't cut the mustard.

 

Without a doubt, replace the lifters. Sealed powers run about $10 each. Money well spent. We zinged an lt1 and had the same result. Did not replace the lifters and they failed a month later.

 

The same thing happened to my car. It also colapsed a lifter. One of my rockers came loose and caused the problem. I replaced all of the lifter with GMPP's. I left the stock pushrods, would rather have a bent pushrod than a bent valve.

You may also want to run a scope in that cylinder to check for piston to valve contact. You may have a bent valve.

 

The rockers that come with the hotcam kit are selfaligning. I have those as well.
I agree with the above post on checking the valve. Also, I would pull the springs on that cylindrer and check them for cracks or anything else. By the way, what springs are you running? I have a set of comp. cam springs that the head porter installed, not the std. LT4 spring. But I am using the same rockers as you, stock push rods and orig. lifters.

 

 

Pete K - Can you give me a part number from Summit please?

STL94LT1 - Do you have a GM PN for those.

93 ragtop - Standard LT4 springs that came in the kit were installed by my head porter.

Thanks,

Dave

 

The lifter kit is cheaper than the individual lifters. The kit includes lifter guides, spider holdown, and bolts.

GM part # 12371042, $203.53 from Dal at Vandevere Olds 1-800-362-9494

 

Would you guys say the GMPP Lifters are better a better bang or is an aftermarket set?

 

good luck, and i would reccomend replacing all of the rods, not just some

 

What heads are they? You shouldn't have any problems with a stock pushrod with just a hotcam kit. Might be time to check the geometry though.

 

Corvette for life - I plan on replacing them all.

Alvin - They are LT1 heads that were ported and polished by www.precisionporting.com I guess I should consider pulling the timing cover and checking things out? And the amazing tune was ofcourse done by you!

Here are the flow numbers:

Intake

.100 81.5
.200 137.2
.300 195.5
.400 243.1
.500 266.6
.600 279.0

Exhaust

.100 63.2
.200 113.8
.300 167.5
.400 189.4
.500 221.1
.600 225.7

Thanks again,

Dave