1966 427 425HP Valve Clearances

[Joe T.]

I am beginning to notice a tap or ticking from under the valve covers on both sides at low idle (1000) to medium (2500) RPM. Seems to have started after going through the gears at a higher RPM. Only have about 600 miles on a rebuild. Valves have not been set since the rebuild. I purchased the car after it had a rebuild and I am not aware of the initial settings. When looking at the 1966 service manual, I see a setting of 0.024" intake 0.028" exhaust cold and 0.020" intake 0.024" exhaust at operating temp.

Am I looking at the proper settings to resolve the tap? Should I consider another issue or other settings?

[Joe L.]

Re: 1966 427 425HP Valve Clearances

I am beginning to notice a tap or ticking from under the valve covers on both sides at low idle (1000) to medium (2500) RPM. Seems to have started after going through the gears at a higher RPM. Only have about 600 miles on a rebuild. Valves have not been set since the rebuild. I purchased the car after it had a rebuild and I am not aware of the initial settings. When looking at the 1966 service manual, I see a setting of 0.024" intake 0.028" exhaust cold and 0.020" intake 0.024" exhaust at operating temp.

Am I looking at the proper settings to resolve the tap? Should I consider another issue or other settings?

Joe------


Those are correct.

[Carl N.]

Re: 1966 427 425HP Valve Clearances

Would run the lash for sure. Be sure and inspect springs, push rods and studs for wear. You will need new valve cover gaskets. IMO it is best to set the lash with motor running and at operating temp. Messy but best results if your going to drive it. Also you need a quality set of individual feeler gages.

[Joe L.]

Re: 1966 427 425HP Valve Clearances

Would run the lash for sure. Be sure and inspect springs, push rods and studs for wear. You will need new valve cover gaskets. IMO it is best to set the lash with motor running and at operating temp. Messy but best results if your going to drive it. Also you need a quality set of individual feeler gages.

Carl------


Perhaps, but keep in mind that the engine plants never adjusted lifters hot and running. All were adjusted cold and static.

[Carl N.]

Re: 1966 427 425HP Valve Clearances

Sorry, will not offer my opinion again!

[Joe L.]

Re: 1966 427 425HP Valve Clearances

Sorry, will not offer my opinion again!

Carl------


Why not? I offer opinions all the time. Folks can choose to agree with them, or not. Folks can choose to offer alternate opinions. So what?

In this case, I stated a fact. That fact does not contravene your opinion or suggestion.

[Jim D.]

Re: 1966 427 425HP Valve Clearances

I recently assembled a new L-72 and broke in the engine prior to putting the body back on the frame. I tried the .024/.028 setting and the .020/.024 recommended by Duke. First off, after checking both settings MULTIPLE times cold vs. hot, there is no difference between the two as Duke has posted many times. I have also confirmed the hot vs. cold setting on my solid lifter small block is the same. I feel the engine runs better and has less valve train noise with the .020/.024 setting.

[Duke W.]

Re: 1966 427 425HP Valve Clearances

Valve clearances for the '65 L-78 were specified as .020/.024", but were increased to .024/.028 for L-72 even though both used the EXACT same camshaft. Why, I don't know.

From my analysis of the camshaft drawing data, the constant velocity clearance ramps (same lobe on both sides) max height is .012" above the base circle. Multiply this by 1.7 and you get .020. Since big block rockers are basically scaled-up versions of small block rockers they should yield similar behavior. Small block rockers are not 1.5:1. In fact, they are not constant. Based on my measurements, they start at about 1.37 at the lash point and achieve a maximum of about 1.44 with an approximate 0.3" lobe.

Unfortunately I don't have enough reliable data to pin down BB rocker behavior, so my default recommendation is to use the L-78 specs and maybe even set the exhaust valves at .022". Due to the large BB exhaust valves they run hotter than the smaller SB exhaust valves, so this may be the reason GM recommended slightly greater clearance on the exhaust side, but I still have no idea why they loosen the specs for L-72.

There is no need to do them "hot and running". You can do them cold, which is so much easier, but the trick is making sure the lifter is on the base circle.

Do a Web search - Hinckley Williams valve adjustment and make sure you download the latest Sept/08 revision of the paper. It was originally written for small blocks, but it has a sure fire way to index the crank for each valve that GUARANTEES you are on the base circle for ANY cam. You adjust all the inlet valves at 90 deg. ATC (relative to TDC of the compression stroke) and all the exhaust valves at 90 deg. BTC. So starting with #1 at TDC of the compression stroke you adjust #8 inlet and #2 exhaust. The paper specifies what valves to adjust at each TDC position.

The reason you cannot adjust both valves of each cylinder at TDC is that the cam has low velocity clearance ramps that are very long, and at TDC compression stroke of each cylinder the lifters are still on the clearance ramps, which means the actual running clearance will be MORE than you actually set them at, and like I said adjusting all inlets at 90 deg. ATC and all exhaust at 90 deg. BTC ensures that the lifters are on the base circle.

Duke

[Ray K.]

Re: 1966 427 425HP Valve Clearances

I also like the .024 & .028 cold lash settings as suggested for the L72. This will will give you the .020 / 024 hot lash operating adjustment, which I have confirmed after engine warm up and a double check on the adjustment. Have even switched to using this adjustment on my L-78 as well as I believes it runs better. The above settings should work well for cast iron heads, but if you should happen to have aluminum heads you may find that you may need to tweek the cold adjustment settings to yield the desired hot operating adjustment. So with aluminum heads, I prefer to do a hot adjustment after the engine has been thoroughly warmed up.

Ray

[Joe T.]

Re: 1966 427 425HP Valve Clearances

A big thanks to all of you for your comments and recommendations.

Joe T.

[Duke W.]

Re: 1966 427 425HP Valve Clearances

I also like the .024 & .028 cold lash settings as suggested for the L72. This will will give you the .020 / 024 hot lash operating adjustment, which I have confirmed after engine warm up and a double check on the adjustment. Ray

As I've stated many times before, I have never measured difference between "hot" (idling) and cold lash on a cast iron block/head engine (all small blocks) and Jim so stated for his L-72 in post #7.Cast iron and steel have virtually the same thermal expansion rate, and at idle the exhaust valves are very cool relative to their temperature under sustained heavy load.

About 80 percent of heat transfer from the valves is through the seat, so stem temperature at idle is relatively cool and there is no significant increase in stem growth over the surrounding cast iron. Inlet valves run much cooler because in addition to heat transfer though the valve seat, they are cooled by the incoming fresh charge during the inlet stroke. The same argument applies to the pushrods since they run at about the same temperature as the surrounding cast iron.

Lash will tighten up under sustained high load, but it's tough to measure at 6000 revs, WOT. That's why mechanical lifter cams have generous constant velocity clearance ramps, so some lash is always maintained, and the valve is gently moved off the seat and returned at clearance ramp velocity and will not hang open due to insufficient clearance.

On an aluminum component engine (heads and/or block) it's easy to calculate the difference between hot (idling) and cold using the various lengths of the affected components (cam to rocker, and valve stem length) and the different thermal expansion rates. That way you can compute the appropriate cold lash to achieve your target hot lash. Aluminum's thermal expansion rate is about three time steel/cast iron.

Duke

[Jack C.]

Re: 1966 427 425HP Valve Clearances

Hi Duke, Jack Cox here (shared space with you at the Petersen with my 69). A lot of good information here. I have searched the threads and thus far have not been able to fine the part number or specific lift height / duration numbers for the L72 cam. Can you point me to these or provide from your records? Many thanks in advance.
Jack

[Joe L.]

Re: 1966 427 425HP Valve Clearances

Hi Duke, Jack Cox here (shared space with you at the Petersen with my 69). A lot of good information here. I have searched the threads and thus far have not been able to fine the part number or specific lift height / duration numbers for the L72 cam. Can you point me to these or provide from your records? Many thanks in advance.
Jack

Jack------


The information can be found in many sources. Alan Colvin's "Chevrolet By The Numbers" is one. The 1966 L-72 camshaft casting number was 3863144. This number appears on original cams. The finished part number was 3863143 but appears nowhere on the camshaft.

Later SERVICE 3863143 camshafts do not include the casting number although the camshaft specs are the same. This is because original camshafts were manufactured from specific, "near-net-shape" cores in order to minimize machining cost. In later SERVICE, with camshaft manufacturing volume way down, the cams were ground from "generic" cores.

[Duke W.]

Re: 1966 427 425HP Valve Clearances

Hi Duke, Jack Cox here (shared space with you at the Petersen with my 69). A lot of good information here. I have searched the threads and thus far have not been able to fine the part number or specific lift height / duration numbers for the L72 cam. Can you point me to these or provide from your records? Many thanks in advance.
Jack

Hi Jack - The big block SHP mechanical lifter camshaft, manufactured from the GM print is Federal Mogul (Sealed Power and Speed Pro brands)
CS-165R. As delivered I don't think this cam has a rear journal groove, so one must be machined on '65-'66 blocks (assuming they have not been modified) and used with the "three hole" rear cam bearing; '67-up don't need the groove, but I believe it's okay to run a cam with the groove with the single hole bearing for either.

The same lobe is used on both sides. Gross .050" lifter rise duration is 242 degrees with POMLs of 108/120 for a LSA of 114. Gross lobe lift is 0.30572" and .012" of this is clearance ramp. So to compare to a hydraulic cam .062" lifter rise duration should be used, which is 231 degrees.

BTW, this same lobe, on a smaller base circle is used on the LT-1 cam inlet side along with the 30-30 lobe on the exhaust side.


As a FYI for everyone else Jacks '69 convertible and my Cosworth Vega were "roommates", next to each other in the Petersen Museum from May '18 to February this year. The worse feature of my CV is the paint, which is mostly original with some dealer paint and my own repaint of the grille panel and headlight buckets. We all know how bad GM paint jobs were in the seventies and the flaws really showed under the LED lighting.

When I first saw Jack's car, I was certain that it was acrylic lacquer and was virtually perfect. Later when I met jack he confirmed that it's acrylic lacquer and he painted it himself!!!

Duke

[Jack C.]

Re: 1966 427 425HP Valve Clearances

Thank you Joe and Duke. I'm aware of the third bearing groove issue on the 65-66 big blocks. I have done a number of small blocks (thanks for the kind words on my 69 Duke, and I'm familiar with the LT-1 cam in my 70) but this will be my first L72 project so I'm doing my research. I wanted the specific numbers because I was considering installing a solid-roller lifter cam to help it live but I would want to otherwise match the factory numbers. Due to today's oils and the fact that I drive my cars this is a consideration.