Technical Tip - Model A Overheating

Water inlet baffle changed mid-1929

Model A overheating seems to be an increasing problem in recent years. Although the intent of this page is to emphasize something that has nearly faded into obscurity, more common issues will be touched on briefly.

Let me start with the common issues in order of frequency and significance.

1. Radiator - There are multiple problems encountered in this area. Seventy years of corrosion buildup in the engine block will rarely be cleaned adequately during an engine rebuild. Particles continually dislodge within the water jackets and work their way to the top of the radiator and block tubes. Back flushing the system will remove most of the recent deposits.

Loose radiator cooling fins dramatically reduce the effectiveness of the radiator. The fins were originally attached to the tubes with solder. Anywhere the fins are loose on the tubes, the heat cannot dissipate from the tube to the cooling fins.

Many replacement radiators and cores have an insufficient number of tubes and fins. This means they will never cool as required for normal operation. For original configurations see the following table.

Year / Model Tubes Fins
1928-29 models 94 round tubes in four rows* Six fins
per inch
early 1930 "AA" truck 132 oval tubes in four rows
1930-31 models 102 oval tubes in three rows
late 1930-31 "AA" truck 136 oval tubes in four rows
*Most common of three types

Don't be fooled by flow testing! Flow testing your radiator as suggested in several publications will only confirm EXTREME problems. You can block more than 1/3 of the cooling tubes on an original radiator and pass the flow test. The reason for this is the lower water outlet of the radiator is the restrictor. It is designed to limit the speed the water travels through the tubes. More time in the tubes equals cooler water.

2. Ignition timing - There are generally two problems in this area. First, modern timing gears lack the deep impression required to easily locate top dead center (TDC) of piston #1 with the timing pin. This can easily be corrected with a 1/4" drill or countersink bit prior to installation. I've heard a few creative methods for doing it later and containing the timing gear chips created during the process, but can only recommend removal of the front timing cover to be sure the correct location is drilled and the chips contained.

Second, as simple as it is to properly set the timing on the 'A', I find that very few people set it correctly. Most of those with many years experience are fortunate to get within a few degrees, but this is usually adequate to avoid overheating.

3. Fuel mixture - Running with the fuel adjustment too lean will also contribute to overheating.

NOTE: Retarded timing and lean mixture will only CONTRIBUTE to overheating. When these conditions are present the car will provide sluggish performance.

4. O.K., now for the obscure! As shown in the drawing at the top of the page, there is a baffle cast into the block to direct much of the water to the rear of the engine before being drawn forward by the water pump. It originally had a small passage to allow a small portion of the cooled water to flow directly towards the front to cool cylinders 1 & 2. In early-mid 1929 the size of this passage was nearly doubled in size to 5/8" wide to improve overall cooling. Following this change, service letters were sent by the various branches to their dealers. This included instructions to grind the baffle to the new dimension when complaints of overheating were received and all the usual checks were made.

I've done this with a small pneumatic grinder and rotary file. It provided some gain in overall cooling.


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©1999 Marco Tahtaras