I finally got the '37 running again after I had the rocker arm tips faced. There was enough wear that I could never get the valves set properly with a feeler gauge.
I set the initial valve clearances with feeler gauges when I reassembled the rocker arms. I know I need to make a final adjustment with the engine hot.
I am interested in learning about the method to use a vacuum gauge to set the valve clearance. The '37 shop manual states that it is an alternative to using feeler gauges. I can understand that if you get the clearance too small it will make the needle jump because the valve is not closing.
Is the proper technique to reduce the clearance until the needle jumps and then back off slightly? What happens to the vacuum reading if the clearance is too large?
I have never heard of adjusting valves this way. I guess it would work, but the engine would have to be running. It doesn't sound like fun.
You could check the thread pitch of the adjuster and calculate how far to back the screw out to get the proper clearance.
A dial indicator might be an easier way to get it right than a vacuum gauge.. You wouldn't need the engine running, nor would you need to measure the thread pitch.
I did it years ago so have pretty much the same image in my head that you do Rusty.
Basically you don't worry about the lash too wide as you just stay at the other end where your goal is to get the lash gap as small as possible and then add a wee bit of a margin to handle some hotter temps than the setup temp. The "wee bit" I don't recall but you could experiment and set a few valves with a 1/4 or 1/8 turn looser or so after the maximum vacuum point and then use a feeler gauge to see where your gap is.
If the materials didn't expand, we could target almost zero lash as the valve opens sooner, opens further and stays open longest which should result in the most power in this finely engineered engine. The lash range is there to account for some wear and extreme temp increase on its hottest/hardest drive.
1938 Canadian Pontiac Business Coupe (aka a 1938 Chevy Coupe with Pontiac shaped front sheet metal - almost all Chevy!) 1975 4-speed L82 Vette
Suspect someone will have a more accurate and proven amount to back off the lash. Let us know how it works and what you see.
I bought a set of those ball/spring loaded rocker adjustment studs to experiment with someday and vacuum gauge would likely make the most sense and would show immediately if these devices are indeed "quiet as a mouse" like the box says lol...
1938 Canadian Pontiac Business Coupe (aka a 1938 Chevy Coupe with Pontiac shaped front sheet metal - almost all Chevy!) 1975 4-speed L82 Vette
Set each valve to the maximum vacuum. Doesn't matter in what order. Slow idle is best speed. In the process if you get a valve too tight the engine will tell you either by running rougher, missing or stalling.
Just wanted to follow-up on this post and close the loop.
This week I got the right combination of events (car was nice and hot after quite a bit of driving around town, the weather was great, and I had time) to do a final setting of the valves on my '37.
I used the vacuum gauge method and was totally impressed with the results! And it was so easy!
I connected my vacuum gauge to the wiper motor tube on the intake manifold. Then I used a piece of wire to hold it in place on the cross brace for the horns on the right side of the car. I set the idle fairly slow and then did a final adjustment on the mixture screw to get the highest. vacuum, about 20 3/4".
I simply started at the front end of the engine and adjusted valves working to the rear. It was immediately obvious if I got things too tight. The vacuum would drop slightly (1/4") and then come back as I backed off. When the vacuum dropped you could also hear a little pop in the exhaust or out the intake. (I had removed the air cleaner so I would hear that better.) I was also able to tell if the adjustment changed when I did the final tighten on the lock nut. If it dropped I just loosened the lock nut, turned the screw slightly, then locked it again.
It was fast with no trying to hold feeler gauge while making the adjustment or need for a second person. I was pretty close with my initial adjustment so I did not change things very much. It did quieten things a little more and there are no signs of overtight valves.
Somewhere in my library is a page about using a vacuum gage for tuneups. Can anyone remember where that information is located? It showed pictures of each result with different things wrong. I thought it was in my Motors Manual, but can't find it. HELP!
That is exactly the chart I was looking for. Years ago that was my bible, however my work has changed and I had forgotten where that information was located. I feel sure this information will assist me in getting a 235 with juice lifters running as designed.
