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Mechanical

Engine Vacuum Leaks

Thursday, June 4th, 2015


Even the smallest vacuum leak on an internal combustion engine can prevent it operating to the level of its capability.

No matter how well you rebuild the carburetor, adjust the timing, or clean the gas tank, the engine will continue to operate below what it should even with a small vacuum leak.

On older engines a quick, easy way to check for leaks near the cast iron manifold will often uncover the problem.

    1. Place a large piece of cardboard behind the radiator cooling fan. See photo. This stops fan air flow in the area of the meeting point of the manifold, carburetor and engine head.1
    2. Use a spray can of starting fluid (available at auto parts stores). Let engine idle, and lightly spray in areas of where air flow will let into the carburetor through.2
    3. If you have even a small vacuum leak the starting fluid will be pulled incorrectly into the engine combustion chamber. The engine RPM will instantly increase. Your engine problem has been found! A new gasket or insulator plate will usually make all well.3

Casting Numbers

Thursday, February 11th, 2010
Casting Number Make Year CID
1970 Chevrolet 1964-1967 292
2135412 GMC 1946-1954 248,270
2193980 GMC 1952-1954 302
2324003 GMC 1955-1963 270
2324004 GMC 1955-1962 302
2404929 GMC 1955-1963 270
2192402 GMC Military 302
289890 Chevrolet 1963-1977 292
328575 Chev/Buick/Olds/Pontiac 1968-1984 250
328576 Chev/Buick/Pontiac 1968-1976 250
328880 Chevrolet 1963-1977 292
329990 Chevrolet 1963-1977 292
358825 Chevrolet 1966-1976 250
3629703 Chevrolet 1950-1952 235
366855 Chev/Buick/Olds/Pontiac 1966-1984 250
3692703 Chevrolet 1950-1952 235
3692708 Chevrolet 1950-1952 235
3692713 Chevrolet 1950-1952 235
3693374 Chevrolet 1942-1949 235
37001481 Chevrolet 1953-1955 235
3701946 Chevrolet 1953 235
3703414 Chevrolet 1954-1956 261
3733340 Chevrolet 1955-1957 261
3733813 Chevrolet 1958 261
3733946 Chevrolet 1954-1955 235
3733949 Chevrolet 1953-1955 235
3733950 Chevrolet 1954-1955 261
3737012  Chevrolet 1955-1957 261
3738307  Chevrolet 1958-1962 235
3738365 Chevrolet 1960-1962 261
3738476 Chevrolet 1958-1962 235
3738813 Chevrolet 1955-1963 261
3739365 Chevrolet 1958-1962 261
3739716  Chevrolet 1958-1962 235
3759365  Chevrolet 1959 261
3764476  Chevrolet 1958-1962 235
3769716  Chevrolet 1958-1962 235
3769717  Chevrolet 1959-1962 261
3769925  Chevrolet 1958-1962 261
3773949  Chevrolet 1954 235
3782856  Chevrolet 1962-1967 194
3782858  Chevrolet 1962-1967 194
378307  Chevrolet 1960-1962 235
3783949  Chevrolet 1953-1954 235
3788378  Chevrolet 1962-1974 292
3788406  Chevrolet 1962-1969 230
3788514  Chevrolet 1962-1970 153
3788813  Chevrolet 1955-1959 261
3789404  Chevrolet 1963-1976 292
3789412  Chevrolet 1963-1966 292
3789716  Chevrolet 1963-1972 292
3792852  GMC 1962-1966 194
3792858  Chevrolet 1962-1967 194
3821970  GMC 1967-1972 292
3833057  Chevrolet 1962-1970 191
3833067  Chevrolet 1963-1970 194
3833340  Chevrolet 1955-1957 261
383340  Chevrolet 1955-1957 261
3835253  Chevrolet 1942-1953 216
3835309 Chevrolet 1942-1949 235
3835335  Chevrolet 1942-1949 235
3835353 Chevrolet 1948-1952 216
3835363  Chevrolet 1954 235
3835374  Chevrolet 1942-1949 235
3835491  Chevrolet 1954 235
3835497  Chevrolet 1942-1953 216
3835527  Chevrolet 1951 216
3835692 Chevrolet 1950-1952 235
3835794  Chevrolet 1942-1953 216
3835846  Chevrolet 1953 235
3835849  Chevrolet 1942-1953 216
3835894  Chevrolet 1953 216
3835911  Chevrolet 1953-1955 235
3835917  Chevrolet 1954-1955 235
3835946  Chevrolet 1953 235
3835949  Chevrolet 1954 235
3836012  Chevrolet 1955-1957 261
3836223  Chevrolet 1955-1957 235
3836233  Chevrolet 1955-1957 235
3836340  Chevrolet 1955-1958 261
3836386  Chevrolet 1955-1957 235
3837004  Chevrolet 1955-1957 235-261
3837012  Chevrolet 1955-1957 261
3843363  Chevrolet 1953-1955 235
3850817  Chevrolet 1962-1978 230-250
3851656  Chevrolet 1963-1972 292
3851659  Chevrolet 1963-1976 292
3851859  Chevrolet 1963-1972 292
3854036  Chevrolet/Olds/Pontiac 1962-1976 230-250
3855914  Chevrolet 1963-1966 292
3855987  Chevrolet 1963-1971 292
3855991  Chevrolet 1963-1970 230
3856233  Chevrolet 1955 235
3858190  Chevrolet 1954-1955 235
3877178  Buick/Olds/Chev/Pont/GMC 1962-1978 230-250
3879875  Chevrolet 1962-1970 194
3886061  Chevrolet 1963-1966 292
3890011  Buick/Chev/Olds/Pontiac 1968-1972 250
3890013  Chevrolet 1968-1972 250
3892858  Chevrolet 1964-1967 194
389770  Chevrolet 1942-1951 216
3897702  Chevrolet 1942-1953 216
3921770  Chevrolet 1966-1976 292
3921967  Chevrolet 1964-1969 230
3921968  Chevrolet 1964-1976 230-150
3921970  Chevrolet 1963-1976 292
828575  Chevrolet 1972-1977 250
837751  Chevrolet 1942-1949 235
839770  Chevrolet 1942-1953 216
8397715  Chevrolet 1942-1949 235
839910  Chevrolet 1942-1951 216
839931  Chevrolet 1942-1949 235
8994256  Chevrolet 1964-1977 292
9890043 Pontiac 1968-1969 250

Venting the Differential

Thursday, February 11th, 2010

Sometimes overlooked by mechanics and restorers is a small vent in the rear axle housing. This part is necessary to keep internal pressure equal to the outside atmosphere. Thus, as the internal temperature of the differential warms during use, any expanding heated air is vented and no pressure occurs. This saves wheel and pinion seals from leaking.

Check for this vent in your truck. From years of abuse many vent assemblies are missing. A sliding log chain wrapped around the axle housing for pulling is a way many vent assemblies were accidentally removed. The owner usually didn’t know the damage has been done or that a vent ever existed. Now, the small hole that once held the vent assembly is able to take in water. This will certainly cause long term damage to differential parts.

Differential Vent 1

Differential Vent 2

1946-1972 3/4 Ton and 1 Ton Ring and Pinion

Thursday, February 11th, 2010

One series of the famous “drop out” GM differentials was used between 1946 and 1972 on 3/4 and 1 tons. The complete assembly (often called a pumpkin) will interchange during these years with no alteration.

The highest gearing in this series is the 4.10 ratio and is found in most 1967-72 3/4 tons with automatic transmissions. Therefore, those “low gear blues” often associated with 3/4 and 1 tons during the late 1940’s and 1950’s can be greatly improved with no visible exterior changes. Originally these older trucks had a ratio of 4.57 in the 3/4 tons and 5.14 in the 1 tons.

Once a 4.10 pumpkin is located (usually in a local wrecking yard) it is a basic interchange requiring little more than new gaskets and gear grease. Your truck’s personality is now changed!

This interchange will fit perfectly if the “complete” pumpkin is used.  The 1963-1972 carrier is necessary and it will be part of this total assembly.  The change-over will not work if you only use the ring and pinion.

The only negative to this changeover is if you are hauling a ton of gravel up a mountain road with the original smaller six cylinder!! In this example a lower geared differential is best.