On Sat, 11 Jan 1997 20:13:51 -0500 (EST) RatiganJ@aol.com
(Joe Ratigan) writes:
> I am in the process of rebuilding an MGA 1500 engine and have
>been guided by the excellent responses to my questions obtained
>through this List. I have the engine disassembled and ready to go
>to the machine shop. I know that I will have to have one crank journal
>welded, the crank ground, may have to bore and put in new pistons, etc.
>
> Now to my question. Most of the Mailing List responses having to
>do with engine rebuilds suggest that the engine be "balanced".
>Clearly, this is a process that results in an engine with minimal
vibration.
>How is it done by the machine shop? Do they actually assemble the
>engine (with the pan off) and rotate the crank at relatively high speed?
>Do they balance individual components? Can anyone tell me a little bit
>about the proceedure?
To get this out of the way first, IMHO, don't be afraid of welding the
crankshaft for the purpose of rebuilding a damaged journal. But you
didn't ask, and that's a subject for another day.
When balancing an engine, you may hear that you have to supply all the
moving parts to the shop doing the work, crankshaft, harmonic balancer
(front pulley), flywheel, clutch cover, clutch disk, bearings, connecting
rods, pistons, wrist pins, piston rings. This may lead you to believe
that they balance all the parts together. This is usually not so, and
for an MG engine in particular this is not necessary.
The key factor here is that the MG crankshaft is symmetrical when viewed
from the front, so can be balanced alone with no other parts attached.
To state that differently, #1 and #4 rod journals are symmetrically
opposed by #2 and #3 rod journals, and everything else on the crankshaft
is round and concentric to the main axis of rotation. All other parts
are either also concentric to the main axis, or come in pairs offset by
180 degrees on the crankshaft to counterbalance each other.
All that means that the rotating parts can be balanced individually, and
the reciprocating parts can be balanced as sets. In this case it is also
best to do it that way. If you balanced the whole assembly but ignored
the individual parts, then changing any one part later could unbalance
the whole system.
Balancing engine parts differs from balancing road wheels in that weight
is not added to the light side of the parts, but is removed from the
heavy side. For the crankshaft, the factory finds it quick to do this by
drilling holes in the outer edges of flanges and counterweights. It can
also be done by grinding off a bit of any non-machined surface at some
distance from the center of rotation, again usually at the extremities of
the counterweight areas.
Since the crankshaft is not symmetrical from front to back, it cannot be
statically balanced like bubble balancing of a road wheel. The
crankshaft needs to be spun in a machine to produce a vibration. The
location and direction of the vibration is indicated by the machine, and
the operator can then remove weight from the appropriate location on the
crankshaft to reduce the vibration to some specified acceptable maximum.
Main bearings are non-moving parts, so are not required to be balanced.
Rod bearings are machine made, nearly identical, and small, so are
usually well within 1/2 gram of each other as shipped, so are usually
ignored. If you were to carefully check the rod bearing shells and find
them not matched to your satisfaction, weight can be removed from a
bearing shell by grinding a bit off of the non-contact surfaces. That
would mean only at the sides, not on the ID or OD and not where the flat
ends come together.
The front pulley on an MGA is not a harmonic balancer type (except very
late production MK-II 1622 engines), but is an assembly of two pieces of
stamped sheet metal riveted to an iron hub. Here it is impractical to
add or remove weight from the sheet metal pulley, so if balancing is
required, a bit of material can be removed from the edge of the iron hub.
This type of pulley is usually quite symmetrical and has low mass at the
rim, so generally does not upset the balance significantly and is usually
ignored.
Later MG engines used harmonic balancer type front pulleys, these having
a heavy iron rim section attached to an iron hub with a vulcanized rubber
joint. These pulleys do have enough material to balance at the rim,
usually done by drilling into the face of the rim section. These pulleys
also have significant mass at the rim by design, and should be checked
for balance. Spin balancing is the preferred method here.
The flywheel is by far the most massive moving part of your engine, so
the most critical to balancing. Even if you use a light flywheel that
weighs less than the crankshaft, the bulk of the mass of the flywheel is
much farther from the center of rotation and will affect the balance
proportionately. Spin balancing is the preferred method here as well,
with material being removed from the non-contact face at the rim. The
factory always balances flywheels (within reason), so you will generally
find some drill holes in the front face of the outer rim just inside of
the ring gear.
The factory balancing of the flywheel may not be close enough to satisfy
some people (especially for a race engine). With considerable age,
warping by thermal stress or wear on the friction surface may require
refacing of the flywheel. It is a good time to have it rebalanced if any
machining is done. Also, while changing a ring gear should not
significantly affect the balance of a flywheel, if you're superstitious
or critical or if it's a race engine, this is also cause for rechecking
the balance.
The clutch cover (pressure plate assembly) and clutch disk are usually
well balanced as shipped from the factory, and can generally be ignored.
However, if you're concerned, or if it's a race engine, the clutch cover
can be mounted to the flywheel and balanced after the flywheel has been
balanced alone. The MGB diaphragm type pressure plate assembly is not
likely to loose the balance over time. The MGA three-arm pressure plate
assembly is more likely to get out of balance if somehow misadjusted,
especially if it is disassembled for reconditioning. After
reconditioning and reassembly the MGA pressure plate should always be
re-balanced.
The connecting rods are the next critical item to balancing an engine.
These are statically balanced on weighing scales. Two scales are used,
placed under the extreme opposite ends of the rods (one rod at a time),
and carefully located in the same position for each rod. The method here
is to weigh all the rods first, find the rod with the lightest big end,
also find the rod with the lightest small end. Then you remove weight
from the ends of the heavier parts until the big ends all weigh the same
and the small ends all weigh the same. Again material is removed from
the non-contact surfaces. Maximum imbalance here is usually specified as
all parts to be equal within 1/2 gram.
As a side note, for racing engines an effort is often made to
considerably lighten the big end of the connecting rods to reduce bearing
loads and allow higher engine speeds. When carried to extreme, some
material is removed from the bosses with the tapped holes and the outter
shoulders are rounded off. The seating surface for the bolt heads is
sometimes cut thinner or counterbored and shorter bolts with smaller
heads can be used. Much material is sometimes removed from the big end
bearing cap. Do not try this at home if you value your engine, it is not
a procedure for the faint of heart.
Pistons are also critical to good balance, and the specification here is
also usually all weighing the same within 1/2 gram. Material can be
removed from the bottom edge of the piston skirt, or from the interior
surfaces around the large boss that holds the wrist pin. I would not
recommend removing any material from the interior in the vicinity of the
ring grooves, as I have personally witnessed a piston failure because of
this.
As another side note, many racing pistons can be lightened considerably
by removing material around the wrist pin bosses. Forged pistons
especially can be lightened by cutting away material between the wrist
pin boss and the inside of the crown. Removing only a minimal amount of
material for balancing is easily done. However, if you have in mind to
remove considerable material for lightening the pistions, especially if
you want the lightest parts possible for a race engine, you had best give
this job to someone with a great amount of experience. When it comes to
high stressed and high speed internal engine parts, it's not nice to fool
mother nature.
For my MGA 1500 I just bought a new set of +.020" pistons. These are
Aerolite 4-ring and came from Moss Motors inventory. Inside the cover of
the box is printed the proclamation that these pistons are weight matched
within 1/2 gram. My engine guru just smiled and said not to bother
bringing these into his shop, that's as good as it gets.
Now, my new spare MGA 1500 engine has the original style sheet metal
front pulley, I don't have a flywheel for it yet, and the pistons are
already balanced. So I took in just the crankshaft and connecting rods.
For this I was quoted a price of $35 for balancing. Now I've always
thought that balancing was a waste of money for a street engine in normal
service, but I autocross a lot and often run the engine to 7000 rpm, so
for the price I couldn't resist balancing this one. For the curious,
SCCA does allow engine balancing in all classes, but lightening of the
internal engine parts puts you squarely into Modified class.
The floor is now open to further questions, but I've already pretty much
spilled my guts on this subject, so anything more may have to be referred
to a professional.
Barney Gaylord
1958 MGA with an attitude
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