Some Important Considerations When Running the LT4 Hot Cam Kit with LT1 Iron Heads

WARNING! - Don't assume (as I did) that because it's a kit from GM that everything is just going to fit your LT1.
So here's the whole story -
The Impala is the only LT1 motor that came with iron heads (as far as I know).  The problem comes from the fact that the  LT4 Hot Cam kit is built for aluminum headed LT1s and LT4s, which is practically the same casting except for one key thing: the outside diameter of the LT4 valvesprings is about 0.1" larger.  Here's what the spring seat looks like on the iron heads:
Notice that the machined area that actually makes up the seat is skirted by a casting ridge.  That ridge looks like it doesn't do anything for you, and my initial thought was to knock it down and use the springs that came with the LT4 kit.  That would have been the wrong move because those ridges actually keep the springs centered on the seat.  My next plan was to take the heads down to the machine shop and have the seats machined to fit the LT4 springs.  That would have worked fine, but my machine shop told me that I'd be wasting time and money and that he had some stock diameter springs that would work even better.  I had the heads with me, so they figured out the installed height and everything and put their springs on the tester and found that the were 100# on the seat and about 350# on the nose, which was better than the LT4 springs which were 101# and 300# respectively.  They also looked at coil bind and everything, so I was convinced.  The other question I had was "why don't the LT4 springs or the original springs have dampers?"  The machine shop owner said he didn't know and that they'd be better springs with the dampers, which seemed logical to me.  So I said "great" and walked out of there thinking I was all set ... but that's where the problem really started. 

In retrospect, I really should have started to suspect something when I got the motor back together and I had a sort of "creaking" noise when I turned it over by hand.  I didn't think much of it at the time because I just blindly trusted that the guy at the machine shop was right - bad idea.  Got the motor back in and started it up and noticed that the valvetrain was awfully noisy, so I emailed Pat Flavin who has a 9C1 with almost the exact same setup I have now (in fact I pretty much stole his set of mods all together) and he confirmed that the GMPP roller rockers are just noisy.  Mine probably was even more noisy because of the gear drive, so I was 99.9% satisfied that everything was OK.  I went out the the strip the next friday to make some runs and it was very consistent - until the last run when I dropped 1/10th and heard something that sounded a little funny at my 6200 RPM shift point.  Went back to the pits and the motor sounded fine, but I decided to knock it off and go home.  The next day I went into the shop and pulled off the valvecovers and here's what I found:
OUCH!  That hurts!  In the upper left are pieces of dampers which were all over the place.  You can see the valve stem seals that got chewed up and the valvespring that broke and caused the funny noise I heard at the track.  I'm really lucky that the spring broke where it did so there was still enough pressure to keep from sucking the valve.  So I initially thought that these were just some crappy springs and I thought the stem seal damage was from floating the valves.  After some investigation I found that this wasn't the case.  Turns out the whole problem was the dampers, and to answer the question of why the stock springs don't have dampers, it's because there isn't room!  Take another look at the picture of the seat and you'll notice that the valve guide flares out below where the stem seal goes.  When we test fit the springs, they fit on there just great, or at least that's what I thought.  Turns out that there wasn't enough clearance in there and the dampers "rode up" on those and bound up and broke.  The broken pieces then got stuck under the retainers and smashed the stem seals and I'm pretty sure a piece of of damper got jammed on the side of the one spring that broke and caused that as well.  What  a mess.

So - to sum up:  

- Don't trust anyone without seeing it in writing on a spec sheet.  The machine shop owner wasn't trying to screw me over or anything, and he learned as much from this as I did. In fact he even refunded my money for the springs he sold me, but it was still a tough way to learn.
- Measure and double check everything in the valvetrain and don't just assume things are going to work
- If you really want to make sure that someone else is responsible for making sure the valvetrain is set up right, then give your cam spec card to a machine shop with your heads and have them set them up.
The springs I ended up with now are Crane #99846 and they work great - with one modification - take out the dampers!  The Crane catalog actually calls for #99845 for LT1 iron heads, but the 99846 is a lot more common and I'm pretty sure they're the same spring but the 99845 doesn't include dampers.  Here's a clip of the spec sheet that came with the Crane springs, and you can see that either one is more than adequate for the Hot Cam's .525" valve lift.  By the way, if you have the Hot Cam kit you can use the retainers and locks with these springs.

Bottom line - if you have an LT1 with the stock iron heads, you can buy all the part numbers I ended up with and it will all work out.  But if you've learned anything from my mistakes you'll still double check everything and don't take my word for it!

Home | Engine | Suspension | Interior | Links