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1937 Buick Special Business Coupe: A Restoration Journal — Part 8

By Chris Ritter

If you are a numbers person you are going to love this journal entry. I try to make my entries as fun and personal as possible but this time I have to get a bit technical. While I am not a mathematician, I do love analyzing data and I had the opportunity to harvest a lot of data when I measured various clearances in my 1937 Buick engine.

As I closed out my last journal entry I left you with the engine in various pieces. What I didn't mention then is that before I removed the cylinder head from my engine I adjusted my valve tappet clearance and performed a cylinder leak-down test. If you aren't familiar with a leak-down test, this is the process of introducing compressed air into a cylinder through the spark plug hole when the piston is at top dead center of the combustion stroke. At this position the intake and exhaust valves are closed so the only place for air to escape is from the valve seals, the head gasket or cylinder rings. The gauge on the tester reports a percentage of leakage and up to 20% leakage is considered acceptable. My cylinders ranged from 10% - 17%. There were no obvious problems to be found with the leak-down test but this gave me some basic information to think about before I removed the cylinder head and essentially "broke the seal".

Measuring tappet clearance before leak-down test.

Measuring tappet clearance before leak-down test.


Leak-down test in progress.

Leak-down test in progress.


With the cylinder head off I removed the valves and started to check the valve guide clearance. The valves, while not burnt or damaged, would all be replaced. Valve stem-to-guide clearance limits on the Series 40 engine range from .0015" - .0035". Of my 16 valves only five would be considered perfect. The rest of the guides each had at least a little slop in them and while it probably wouldn't have a major affect on my performance at 40mph or so I knew right away that I would have to replace all of my valve guides. I would also have to have the head hot-tanked and checked for cracks and flatness at the engine shop. So much for that simple (and cheap) piston ring job I was hoping to do a few months earlier!

With my cylinder head set aside I hoisted my cylinder block on to a couple of sturdy saw horses. At this point the block still contained the pistons, rods, crankshaft, camshaft, timing chain and sprockets. I scraped away decades of oily sludge and grime to reveal the timing marks on the crank and cam sprockets. Each sprocket is clearly stamped and there should be nine chain links between the timing marks. There should be 1/4" — 1" of slack in the timing chain and mine had about 7/8" so I had to add a timing chain and sprocket set to my growing list of parts needed. Pictures and notes were taken and, because I couldn't find the timing chain's master link, the sprockets were pulled off each shaft.

Close up view of crank & cam sprockets.  Red arrows point out the timing marks and green arrows point to 9 link posts between.

Close up view of crank and cam sprockets. Red arrows point out the timing marks and green arrows point to 9 link posts between.


From here I pulled and marked each of my valve lifters, removed the flange that holds the camshaft in place and gently removed the camshaft from the block. Next, I removed each main bearing cap, checked for shims (I will explain those later) and groaned, grunted and swore as I lifted the 88lb crankshaft on my own. After each piston was marked, photographed and noted they easily slid out of the cylinder bottoms.

This is the rear-main bearing.

This is the rear-main bearing.


Each bearing cap had this dimple.  The dimple faces the back of the engine.

Each bearing cap had this dimple. The dimple faces the back of the engine.


Here is where things get a bit technical and to help me with these technicalities I called in my father. The "Old Man" has been a machinist for nearly 50 years and he is the kind of old school guy that can actually run and operate a machine when a fancy computer fails. His touch with hand tools and measuring devices is as second nature to him as breathing is to me. I am good with a ruler down to 1/32" and he is the man to go to when measurements are .001" or less.

While I was relegated to a helper position similar to that of a 12-year old helping his fathers change brake pads, the Old Man measured the outside diameter of each piston in three different places.

My father measures the outside diameter of a piston.

My father measures the outside diameter of a piston.


The first measurement was perpendicular or side-to-side to the block, second measurement was 45 degrees to the block while the third was parallel to the block. Those measurements produced the following data:

Piston # Measurement#1 Measurement#2 Measurement#3
1 3.087 3.090 3.090
2 3.0885 3.090 3.0903
3 3.088 3.0895 3.0901
4 3.089 3.090 3.090
5 3.089 3.0905 3.0905
6 3.0866 3.089 3.090
6 3.088 3.0898 3.090
8 3.0885 3.0985 3.090

After taking the piston measurements my dad measured the cylinders in the same three spots as the pistons with each measurement taken approximately 1" from the top of the cylinder.

My father measures the outside diameter of a piston.

My father is measuring the wall-to-wall distance of cylinder #7.


Those measurements produced the following data:

Cylinder # Measurement#1 Measurement#2 Measurement#3
1 3.096 3.0975 3.0975
2 3.096 3.097 3.097
3 3.0975 3.0975 3.0975
4 3.098 3.0985 3.0985
5 3.096 3.097 3.097
6 3.098 3.0985 3.0985
6 3.0965 3.098 3.098
8 3.0975 3.0985 3.0985

After all the measurements were taken I averaged each piston/cylinder measurement and subtracted the piston diameter from the cylinder diameter to determine wall clearance. Those calculations produced the following data:

Cylinder # Clearance
1 .0075
2 .007
3 .0085
4 .0085
5 .00625
6 .0092
7 .0076
8 .0085

According to the 1937 Buick Shop Manual, piston clearance limits on the Series 40 are .0018" - .0024" with a desired measurement of .0021". Clearly every single one of my clearance limits were beyond acceptable limits. My block would need to be bored and I would need a new set of pistons and rings.

The crankshaft and connecting rod bearings in the 1937 Series 40 engine were originally babbitt. Buick installed thing shims where the top cap connected to the other half so owners or mechanics could easily remove shims and bring clearances back within limits. This process saved the owner from having to have new babbitt linings installed with each adjustment. Unfortunately, when I removed my connecting rods I quickly learned that my rods had no shims. If my clearances weren't within limits I would need to get new linings or find a different solution.

My father measures the outside diameter of a piston.

A close-up of the rod bearing. Note how thin the babbitt lining is. Ideally there would be shims for adjustment.


To get a bearing clearance measurement you need to use a product called Plastigage. It comes in various sizes depending on your application and looks a lot like colored angel hair pasta. Because oil won't compress you need to remove all traces before taking a measurement. With the bearings and shaft clean, you simply lay a piece of Plastigage on the shaft, gently place the bearing cap on top and tighten the connecting bolts to specified torque levels. Then you remove the bearing cap and look at your squashed piece of Plastigage. Its width is compared to an index on the Plastigage wrapper and that will show you your clearance. It is really quite simple to use as long as you don't move anything during the process. While the main bearings can be checked in one location you should really check clearances in multiple locations on your connecting rods based on the fact that different forces are applied to the rod bearings and they may be out of round.

I am using a torque wrench to tighten on of the main bearing caps.

I am using a torque wrench to tighten on of the main bearing caps.


This is what compressed Plastigage looks like.  The index makes readings a breeze!

This is what compressed Plastigage looks like. The index makes readings a breeze!


Using Plastigage I harvested the following connecting rod bearing clearances:

Bearing # Clearance
1 .002
2 .002
3 .0017
4 .0018
5 .002
6 .002
7 .0017
8 .002

The Shop Manual states that connecting rod limits for wear are .0018". If my rods still had shims I would simply remove the required number to bring them back into tolerance. Since I didn't have shims I was forced to decide if I would get my rods relined or come up with a different solution. The cost to re-babbitt rods is about $80/rod. For around half that amount I could send my original rods to Terrill Machine in Texas where they would swap my original rods with machined rods that would accept insert bearings. I chose the latter option and couldn't be happier with the product and customer service from Terrill.

The last measurement I needed was the main bearing clearances. There are five main bearings in the Series 40 engine. One of these bearings (the middle or #B) had shims remaining. Here are my main bearing clearances:

Bearing# Clearance
Front .002
A .002
B (with shims) .002
C .0015
Rear .002

The main bearing clearance limits are .0007" - .0022" so in this case my main bearings are currently ok. However, my crankshaft was sent to the machine shop for cleaning and inspection and, at the time of this writing, I don't know if the shop will recommend grinding or simply polishing the journals. The engine shop findings will determine if I need to replace my main bearings. Unlike the connecting rods there is no insert bearing modification and new main bearing sets hover just over $300.

So, remember when I just wanted to do a simple ring job on my engine? That clearly turned into a total overhaul. My budget for new rings was about $150. My cost for a total overhaul? Well, my wife reads this journal so let's just keep that number secret for now!

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