My 1948 Chevy truck runs well for a 72 year old vehicle- original engine and transmission. Two things I need some input on. 1- When I'm pulling a hill or pushing 50 miles an hour, the transmission will occasionally jump out of 3rd gear. I just push in the clutch, put it back in gear and go on.This has happened about 3 or 4 times over the last 3 months. 2- Cold or warm engine, sometimes when accelerating or pulling a hill, the truck will run a little rough with reduced power. I let of the accelerator slightly and give then give it more gas and the engine will generally smooth out with normal power. Any ideas out there?
VCCA members have access to a list of over 50 Technical Advisors who can help you with your car. It's worth the price of membership! While you can get a lot of information for free in this forum, sometimes the info that you REALLY need is only available from the right person. This is what "The World's Best Chevrolet Club" is all about!
Generally when it pops out of high gear that means the needle bearings between the pilot shaft, and the main shaft are worn. You probably are headed to needing a small parts and a gasket kit for it. That's assuming it is a 3 speed and 3rd is high gear. Art
Often the "jumping" out of top gear is caused by a misallignment ot the pilot and main shaft, worn bearings between the 2 shafts (as Art suggested) or the gears worn angled. If the transmission has recently been out the misallignment is the easiest to rectify as the trans wont need to be fully removed but moved rearward and clean the face of the gearbox and bell housing and bolt back up but the others will be complete removal and dismantle. Tony
Your second question about engine performance might be addressed by your manuals article on your exhaust manifold. I happened upon a description of a similar problem in my 41 manual which alludes to the symptoms of the heat viser staying open letting excessive fumes and heat into the intake manifold. If you have an exhaust manifold problem the article explains the resulting poor performance. Have you ever checked whether your heat rise valve is in working condition? To easily verify if it is you merely need to move the weight on the riser up and down on a cold, not running, engine. If it doesn't move smoothly or something is wrong with the spring or possibly the spring is missing the problem can be usually corrected without removing the manifold. The SEARCH process can lead you through that repair.
As an engine warms up you should see the weight on the heat riser shaft shift its position as the spring gets hot. The pictures below might clarify the function of the heat riser valve.
Here is a copy of the manual page I am referring to on an online manual:
I got the small parts kit for the transmission. The problem is intermittent. If it gets to apoint where it happens often, I'll get the transmission worked on. I would really like to replace the three on the column for a 4 speed floor shift. Some Chevy pickups at that time had 4 speed transmissions.
The 4 speed is less diserable and the low or first gear is very. very low and will only take the truck up to about 5 MPH wide open. Often refered to a stump puller. Prior to 1948 was not syncronized in any gear and reqired double clutchng as it was designed in 1928. Yor 1948 would have been syncronized but designed for heavy duty truck use
I prefer the 4-speed transmission for the 1948 and later trucks. The venerable SM420 four speed is more robust than the 3-speed and 2nd, 3rd, and 4th gears are synchronized, eliminating the need for double-clutching as on the 1947 and earlier 4-speeds. As mentioned by Chevgene, the 1st gear on the 4-speeds is very low and is simply not used for normal every day driving. Standing starts are made using 2nd gear, which I believe is actually a lower ratio than 1st gear on your 3-speed. And, if you ever need to climb over a curb or navigate over some rough terrain or pull another vehicle out of a ditch, etc.,etc., the super-low 1st gear is really handy and will save wear and tear on your clutch.
Since the 4-speed was an available option on light trucks, it should be interchangeable with the 3-speed, with only minimal modification. You'd have to cut a hole in the floor pan for the shift tower, though it may already be there from the factory. You would also probably need to remove the 3-speed shift linkage to get it out of the way.
A couple of drawbacks regarding the 4-speed: It's way heavier than the 3-speed, making it more difficult to remove and re-install, especially if you're a one-man shop. Also, when riding three abreast in the cab, the floor shift lever can interfere with the legs of the middle passenger.
The SM420 four-speed transmission was used from 1948 into the early 1960's I believe, so it should be relatively easy to find one, however, you'll want a 1954 or earlier, as the later models had a different rear bearing housing to accomodate the open drive line used on the later models.
I have NOS one still in the crate but I think it is the pre-48 one. Let me know if you are interested.
A simple search reveals there are 15 Crestwood sites. Farthest from me is up in Ohio. Where are you?
Best,
Charlie
BTW: I'm thinking the amount the truck will pull (including itself) is limited to right rear wheel before it starts to spin or clutch disc slip. Except for the novelty of it, I can't figure out the advantage of the four-speed except being easer on the clutch. I may have a heavy duty one of those too.
BTW2: I'm not one to tell you what is best for you without you asking or putting down what you think you want for your truck. Your call and your truck.
FYI, the 1947 and earlier transmissions won't bolt up to your 48 bell housing, as the bolt pattern is different. Also, the brake and clutch pedal pivot arrangement is different. Besides that, the pre-1948 four-speeds are not synchronized in any gear, so, from strictly a drivability standpoint, your 48 3-speed is superior to the earlier 4-speed. Therefore, if you do decide to get a four speed, make sure it's a 1948 to 1954. I'm not 100% positive, but I believe 1955 was the year the 1/2 ton trucks switched to the open drive line, requiring a different rear bearing housing on the transmission.
Do not confuse a 4 speed truck transmission wiith a 4 speed later car transmission.
The truck has an ultra low first gear that is never used in normal driving. The seconed and third speeds are the same as a 3 speed and 4th is a 1 to 1 ratio.
