Determining Your Overhaul Parts Needs
by Chris Pratt
To me, one of the real complexities in overhauling an engine is figuring out which parts need to be replaced,
which can be reused and the sizes to order when you do replace them. This is particularly difficult for the Main
and Rod Bearings, Pistons and Sleeves. The help you get from the service manuals ranges from very detailed to non-existent
depending on the manufacturer. You know the feeling... the manual leaves you with a "If you have to ask, you
shouldn't be rebuilding it yourself" feeling. If you have one of these manuals, the following may help.
Serviceable or not?
The factors that go into determining the serviceability are the manufacturers specifications for what is considered
serviceable and the condition of the various surfaces involved. For example, you may have a main bearing surface
that is within the recommended tolerances but it may be scored too badly to use as is. Another good example is
that while your sleeve and piston may be within tolerances at the bottom, the upper portion of the sleeve may be
wider (i.e., instead of cylinders you have upside down cones) and out of specification. An extreme example may
be a piston that looks and measures perfectly but has a hairline crack. These types of examples show that when
you are measuring your existing surfaces, you may need to measure the entire surface and not stop at a single reading
all the while watching for damage that goes beyond simple wear.
Your measurements will be done with different tools for different surfaces but most will be given in thousands
of inches. The primary tools are the calipers, plastigage, and inside micrometer and feeler gauges. Some manufacturers
also require you to use a spring scale to measure pounds of pull since they show specs such as piston wear in terms
of the pull required to remove a certain size feeler gauge from between a piston and sleeve wall. Don't be too
concerned about the cost of these tools, depending on the quality and where you buy them you may only invest $20
to get all these.
The first step is to locate the specifications and how the manufacturer expects you to arrive at them. The owners
and service manuals of most tractors show the required specifications or in some cases these can be obtained from
the dealer that you will use to purchase the parts. Dig out the specs, grab the tools and start your measurements.
Main and Rod Bearings
These to items can be lumped in together since the methods used to measure them are identical. The measurement
you are looking for here is the existing size of the crankshaft pin or Journal. With this, you can calculate which
undersized replacements will bring the crank or rod journal tolerance back to factory specification. Wear on the
journals or damage that requires they be turned by a machine shop, causes the journal to be undersize to its original
specification. Your measurements determine just how much undersize it now is and tell you what size to order.
The first step in measuring the crankshaft and bearings is to look the surfaces over for scoring. If scoring
is present on the crankshaft, it must be turned or replaced. After looking the crankshaft over carefully, use a
caliper to determine if it has equal wear across the surface. If the journal has more wear on one side than the
other it will also have to be turned or replaced. Lastly, check for see if it has worn unevenly around the diameter
of the journal (oblong!). This can be done by taking a measurement, rotating the caliper around 1/4 turn and taking
another measurement. Repeat this process a few times and you will quickly get an idea whether it is serviceable.
The better manufacturers manuals will provide what are acceptable tolerances in this respect. If the crankshaft
does exceed the manufacturers wear tolerances, you have two options. The first is to purchase a reground crankshaft.
While this is more expensive, it has the benefit of simplifying your measurements since nearly always, new bearings
will be provided with the replacement that are premeasured for the shaft you receive. In this case you are done
with this job. The less expensive alternative is to have it ground by a local machine shop. If you can find a shop
to do this, you will find their prices reasonable. The difficulty is that few places will grind a shaft now and
those that do usually are very busy with specialty engines such as those for racing.
A word of warning about grinding the crank. Be sure you can get the right sized bearings to cover
the amount of material the machinist will remove before you have the shaft ground. If you don't, you may have wasted
your money on the machining.
The other journal and bearing measurement process is accomplished by putting the whole assembly back together
with plastigage inserted between the bearing shell and the crank journal. Some manuals will talk about using shim
stock to measure the clearance. This is not practiced anymore because it is so easy to use the plastigage. You
purchase plastigage at an auto parts store. It comes in a few sizes depending on the clearances you are measuring.
If you take your service manual, the parts person will help you find the right size to purchase. The cost is minimal.
Plastigage is an impregnated string that squishes when you insert it between the journal and bearing shell and
torque the bolts down. The width that it squishes to is measured against a scale on the package to determine your
exact clearance. Once you find this value, you can determine how much oversize will be required to bring the clearance
back to that required by your manual. To use plastigage, cut a strip wide enough to go across the journal, reassemble
the shell and cap with the strip placed across the journal, torque the bolts, and disassemble. You can then measure
the surfaces against the package that the plastigage came in. Never turn the crank during this process.
By now you must have noticed that we have measured the journals twice, once with a caliper and once with plastigage.
In practice, with old well-used tractors, you will do both these measurements. First the caliper finds the irregularities
and gross undersizes that mean crank welding, grinding or total replacement. The plastigage process is still necessary
to determine any possible shimming that will be required in reassembly or may be used when there is very little
or extremely even wear. If plastigage is the only measurement you use, you may find it difficult to find certain
important facts like conical or oblong journals. On the other hand, you virtually can't get your final clearance
right without plastigage.
Most manuals will offer another alternative for bearing shells that are not out far enough to justify the purchase
of new bearings in the smallest undersize (.002). The process that is used is to carefully file the edge of the
bearing shell to achieve the small difference between what you measured and what the factory manual requires. This
can be somewhat difficult to maintain the perpendicular surface nessecary on the end of the shell. This would not
be the recommended method of correcting the clearance but has been practiced for many years. Never file down the
bearing caps as this permanently ruins the cap.
Pistons and Sleeves
What you are looking for with the piston and sleeves is any visual damage such as scoring, out of roundness
and greater width on the sleeve at the highest point of piston travel (not including the ridge the forms above
the high point of travel). Scoring will indicate the need for replacement regardless of the measurements. The other
conditions are measured as follows.
Out of roundness is measurable by placing the inside micrometer at the top and bottom of the cylinder and then
repeated at right angles to the original measurement. With these numbers, you can see how conical the cylinder
is and how oblong it has become. Repeat the process using your caliper on the piston. There will usually be a factory
specification for what is acceptable. Some pistons are cam ground meaning they are supposed to be slightly out
of round but how much determines servicability.
How conical the bore of the sleeve is, contributes to whether it can be reused. There is more wear at the top
than at the bottom. If the differences exceed the manuals specification, the piston and sleeve will need replacement.
Based on the measurements you did for out of roundness, you would have already have written these down.
If the differences at the top and bottom are acceptable, you will still need to determine if there is too much
distance between the piston and the sleeve. This will be most noticeable at the top of the stroke. Most manuals
suggest using a long feeler gauge of a certain size placed between the piston and sleeve with the piston inserted
in the sleeve. The amount of drag on the feeler gauge when pulled out, determines its serviceability. Fortunately,
some manuals provide what it should "feel" like to pull it out but many require that you connected a
spring scale and read the pounds of pull as the feeler gauge is removed. Do this process without the rings. Usually
this measurement is taken 90 degrees from the piston pin hole.
You may be wondering what measurement could possibly be necessary on the sleeve outside of what we have done
already. The important issue here is that once you have determined you need new sleeves, you may have a couple
of unpleasant experiences with purchasing new pistons and sleeves, receiving them, sending them back, receiving
them again ... This type of thing is frustrating for you and dealers alike. You see many tractors had more than
one size for a given model. A few tractors even had different sizes embedded in the middle of their serial number
ranges. Arrrgh. To carry this problem further, it takes some serious research to determine that your engine was
not replaced with one 2 years older and thus has a different block. The helpful measurements here are the bore
and stroke. This is the distance across the piston and the distance the piston travels. Then measure the thickness
of the sleeve. Lastly a few engines can be identified by measuring the outside diameter of the sleeve or the outside
diameter of the base of the sleeve (where it seals to the block). Tractors that should always be checked for these
types of differences are the Farmall A, B, & C, Super A, A-1, Super C and the Ford N-series machines. These
are ones that I have commonly had the serious problems with but there are probably many more. Simpler problems
occur with the D series ACs and the B, C, IB, and CA where the sleeves will all fit but you may end up with a smaller
CID and less power when your overhaul is complete.
The piston pin is difficult to measure outside of using the caliper to measure the pin and and the results of
this may be meaningless to the home mechanic. If your tractor does not use bushed piston pins then bearing replacement
may be inexpensive and quick. If it does use bushings then see what the manual says are the indication of wear.
As an example, one manual suggests that if you hold the piston rod vertically and center the piston it should balance.
A slight but quick movement should cause the piston to fall to the left or right. Confusing? Kind of, but what
they seem to be suggesting is that there should be no detectable looseness. The saving grace for most of us is
that rebuild kits normally alway have oversize piston pins included.
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