427 valve adjustment question for Duke

[Robert P.]

Was reading old threads on 427 solid lifter valve lash specs , in 2003 you advised .020/.024 in one post and a post you replied to in 2016 i think you advised .020 /.022
has anything changed i since then , the old search was easy for me to use this new system will take some time
thanks inadvance Bob

[Duke W.]

To review the whole story the '65 L-78 recommended clearances are .020/.024", hot. For the '66 L-72, same exact camshaft, the recommendation was changed to .024/.028", hot.

Lets discuss "hot" first. I have never measured a difference between cold and "hot" (idling), which makes sense because the difference in thermal expansion rates between steel and cast iron is so small that differential expansion over the length of the pushrod is virtually nil.

Intake valve stem expansion is minor even at high load because it's cooled with every inlet cycle. At high load exhaust valve stem expansion is greater, which is why many mechanical lifter cams have taller clearance ramps on the exhaust side... to ensure that the valve will always have some running clearance allowing it to firmly seat.

So being as how adjusting hot, engine running or not, is a PitA compared to cold, cold is what I recommend. There are internet claims that hot and cold on cast iron pushrod engines are difference, but since I've never measured a difference and theory supports my results I put no credence into such claims. There IS a difference, especially on pushrod engines with iron block and aluminum heads or all aluminum between hot and cold, but I won't get into that here because it applies to only a very few vintage Corvette engines.

A couple of decades ago I was able to obtain all the lobe data from the engineering drawings, which is specified in inches to five decimal places every CAM degree. I input this data to an Excel spreadsheet that I set up to calculate the velocity, acceleration, and jerk, and this allowed me to determine the height of the constant velocity clearance ramps to within less than one thou. Multiply this by the rocker ratio and you have the ideal clearance, right? Well, it's not that simple.

Most of us probably know that the small block rocker ratio is 1.5:1 and big block is 1.7:1, but if you read hot rod magazines you may have read that the ratio "varies", implying that if I measure the rocker ratio of a box of 100 it's going to vary somewhat, but that's not true. These are precision stamped parts and within reason they will all measure the same.

There is actual variation, but it's through the lift cycle and it's easy to measure with a couple of dial indicators, one on the valve retainer and the other on the rocker arm pushrod socket. For the ratio to be constant the pushrod socket and rocker tip will have to move in a straight line, but they don't. The move in a circular arc of different radii on each end.

After measuring several, both inlet and exhaust, back in the seventies with a LT-1 cam (.306" L-78/72/71 lobe on the inlet side on a smaller base circle and .323" 30-30 cam lobe on the exhaust side) the results were consistent showing a lash point ratio of 1.37:1 and 1.44: 1 at peak lift. Multiply the lash point ratio times the constant velocity clearance ramp maximum height and you have the "ideal" clearance that will ensure that clearance is taken up at clearance ramp velocity under any and all operational conditions from a cold start to sustained WOT at the power peak.

All of my SB mechanical lifter cam cold clearance recommendations are in the Hinckley Williams valve adjustment paper (easy Web search, make sure you get the final 09/2008 revision) along with an indexing method to ensure that the lifter is on the cam's base circle for each valve you adjust.

When it comes to big blocks I don't have enough rocker ratio data to determine the lash point and max lift rocker ratios, but since they are a scaled-up version of the small rockers, they likely have similar behaviors, but there are still other questions.

Since exhaust valves absorb heat based on face area, but reject head based on circumference, bigger valves will run hotter under the same operating conditions, so valve stem expansion will be greater than a smaller valve. Then why didn't Chevrolet design the big block mechanical lifter cam like Duntov designed his cam - Identical lobes other that a taller clearance ramps on the exhaust side? Also, why was the recommended clearance increased for the identical cam in '66? We'll likely never know the answers.

Assuming the big block rocker ratio is a constant 1.7:1, the "ideal" lash would be 1.7(.012) = .0204" How about that! And since the designers didn't include a taller exhaust clearance ramp, maybe they just added .004" "Kentucky windage" to hopefully ensure that there will always be clearance even at sustained maximum load and revs.

If we assume that big block rocker ratio just scales up proportionally to small block behavior then we can multiply small block ratios by 1.7/1.5 = 1.133, so the lash point/full lift rocker ratios might be 1.55/1.63, and the "ideal" clearance would be .0186 on both sides or call it .018/.020" if you want to a a couple of thou "insurance" on the exhaust side. We won't know for sure until someone provides a good data set. So... given the above, what we know and what we don't know, my most recent recommendation is .020/.022." Given that these cars rarely see sustained maximum load for any length of time a couple of thou one way or another is not going to make any difference.

Chevrolet's recommended clearance checks are every 12,000 miles, so if you set them at .020/.022 and don't beat on the car you are probably good for double that interval.

I'm not sure if the big block SHP cam's lifter ore on the base circle or not at compression stroke TDC of each cylinder. The 30-30 cam lifers are not. The LT-1 exhaust valve is still on the clearance ramp. The inlet lobe is on the base circle, but just barely. Both Duntov cam lifters are on the base circle,, so you can adjust both on a cylinder that is at compression stroke TDC.

To be on the safe side, I recommend the 30-30 cam indexing method - do all the inlets at 90 deg. ATC and all the exhausts ar 90 deg. BTC. Either way you have to rotate the crank seven times from TDC #1. With the 90 ATC/90 BTC method you have to swap sides more times, and write up a "cheat sheet" with the sequence and fill in the data with each adjustment.

I keep records on all my cars valve adjustments (one pushrod, three shim over bucket DOHC) and record both the initial measured clearance and final adjusted clearance (if required).. That way I can keep track of any significant valve recession.

Happy New Year!

Duke

[Kenneth K.]

Duke-
Does your recommendation for 0.020/0.022 valve lash apply to a 1969 SHP 435 L-71. Factory lash specs for this SHP BB are 0.024/0.028. I have the engine out to paint it and freshen up the engine bay. I checked my lash while the engine is on the stand and readjusted everything to the factory specs 0.024/0.028 since it’s easy to do while on the stand then I saw your latest post…. Just wondering if you recommend I readjust to 0.020/0.022.

Thanks,
Ken

[Duke W.]

Yes, I still recommend .020/.022", cold, and the reasoning behind this is fully explained in post #2. I hope you used the all inlets at 90 ATC and all exhausts at 90 BTC indexing method as explained in the Hickley-Willliams valve adjustment paper (09/2008 revision), which is on several Web sites.

With your engine out and on a stand you can do a great service to the community by generating a handful of lobe lift-valve lift data sets that can characterize big block rocker ratio behavior as I did for small blocks nearly 50 years ago.

It's very simple. Zero lash a couple of inlet and a couple of exhaust valves while they are on the base circle. Then set up a dial indicator on the pushrod socket of the rocker arms in line with the pushrods and corresponding valves. Rotate the engine until you are just off the base circle. Record the indicator reading pairs and continue to take readings about every ten crank degrees until peak lift is reached. The increments do not need to be precise and you do not need to mount a degree wheel to measure the crank angle. The only important data is the pushrod and valve displacement values at each measuring point.

Send me the data and I can set up a spreadsheet to compute the actual rocker ratio. Since the big block rocker is basically a scaled up version of the small block ratio I expect it will exhibit similar scaled up behavior.

The small block rocker ratio is "specified" as 1.5:1. It actually starts out at about 1.37:1 at low lifts and reaches a maximum of 1.44:! at peak lift of a nominal 0.3" lobe.

Duke