Doug Braun writes:
>My `72 Spitfire (1300cc) makes a tapping/knocking noise that sounds
>like one worn out big-end bearing.
>isolated the noise to the #4 cylinder by shorting out one
>spark plug at a time until the noise went away.
Good technique, bit it may not tell the whole story.
>I could not detect any play in the
>big-end bearing by wiggling the connecting rod on the crankshaft.
>I took off the bearing cap, and removed the cylinder head, and
>took out the piston and rod. There were no broken rings, the
>gudgeon pin had no apparent looseness, and the cylinder bore
>had no detectable ridge or scoring (I was able to get the piston
>out the top without reaming the ridge and only gently tapping it.)
>I noticed that the big-end bearing shells were worn enough to
>expose the copper-colored underlayer.
First, yes, that may be enough wear to cause a knock. And, once the
babbit (white metal) surface is worn off the bearing surface, wear
can occur faster. The real test is to measure the bearing clearences,
using plasti-guage. (You don't even need to remove the head to check/
replace rod bearings. "As long as you're there now, you might as well
check/freshen everything." :-) ) Plasti-guage is thin strips of plastic,
(spaghetti), available in ranges, ie: .001-.003", .002-.006". A
short piece is placed accross the (clean) bearing shell, which is then
installed and torqued to spec. (do not turn the crank!) When the
bearing is then removed, the clearence is indicated by the width of
the now flattened plasti-guage. (Unless you have *extreme* gaps!)
The flatter it is, the wider it is, and the smaller the clearence.
The package has a scale to indicate clearence by checking the width.
Something like this:
|| | | | | | | | |
.003 .0025 .002 .0015 .001
If you are at/past the max reading, you'll need to remeasure with the
next size range up. (or just declare the clearence excessive)
Proper clearences for the Spit are probably in the range of
.0015-.003 or so. It doesn't take too much more to develop a knock.
Small amounts of excess slop may not be easily detected by hand.
The babbit surface of the bearing serves a couple of functions- it
is considered a "sacrificial" metal, being much cheaper to replace than
the crankshaft. Also, small particles can become imbedded in it, rather
than remaining on the surface to score the crank. *All* new rod and
main bearings, should be checked before final assembly. This will
double check the machinists work, as well as correct fitting of all
parts. I have seen mismarked replacement bearings! (why does this seem
too loose? They're stamped .020 under, and the crank measures out right...
Surprise- plastigage showed that they were .010!) The actual dimension
of the crank can be varied (tuned) by a good machine shop to achive
a desired clearence. Racers may desire a larger space to reduce friction,
on an engine which will be rebuilt frequently.
Once you've checked the big ends, then you can consider all
those other things which might cause a knock too:
-main bearings (use plasti-guage)
-thrust bearings (end to end play)
-Piston slop (measure with a T-guage, check diameter in both directions, top
and bottom. Look for ovaling, and taper.)
-Valve train- Rocker shaft/bearings, lifters, cam, push rods, valves....
________
/___ _ \ Roger Garnett (garnett@BATCOMPUTER.TN.CORNELL.EDU)
/| || \ \ Agricultural Economics |
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| | \ | | | Cornell University | Wayward Sports Cars
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