Wobbly Pistons
#1
Posted 01 July 2015 - 09:33 AM
#2
Posted 01 July 2015 - 09:34 AM
#3
Posted 01 July 2015 - 09:41 AM
#4
Posted 01 July 2015 - 09:49 AM
It's probably fair to say that it's not at all easy to say if there's a problem or not. They may have some movement and that could be normal. Pistons do expand when under normal use (from heat mostly) and they will expand the most where there is the most heat and material in the piston, so the Crown, which is what you can see with the Head removed, will expand the most. The Pistons are shaped (said to be Cam Ground) to allow for expansion of it's irregular shape. Typically, when cold, most 'movement' will tend to be along the same line as the Crank.
While the Piston to Bore Clearance is 'fine' (typically in the range of 0.0015 to 0.004") on these Engines, it's normally measured at the lower end of the Piston Skirt at 900 to the Crank Line
#5
Posted 01 July 2015 - 09:52 AM
#6
Posted 01 July 2015 - 10:02 AM
#7
Posted 01 July 2015 - 10:05 AM
#8
Posted 01 July 2015 - 10:09 AM
#9
Posted 01 July 2015 - 01:52 PM
You dont want 12 thou movement, they have to be firm.
Investigate further.
#10
Posted 01 July 2015 - 03:20 PM
#11
Posted 01 July 2015 - 03:24 PM
Would a compression test show me if there was a problem?
It may point to worn rings so worth doing a leak down test.
#12
Posted 01 July 2015 - 03:51 PM
#13
Posted 02 July 2015 - 05:01 PM
I had my head off to replace stems seals as I had quite abit of smoke on start up, so at the same time I gave the top of the Pistons a clean up but whilst doing so I noticed a slight movement of the Pistons from side to side. Is this normal? Or are the stem seals the least of my problems? It's a 1275 a+ and it's apparently had a recent rebuild according to the person who supplied it.
You will find a lot more clearance at the top of the piston, and it will rock quite a bit when cold, needs to be this way to allow for expansion.
I wouldn't worry about this, but do check the bores for any sign of a lip. Looking down the side of the piston you should be able to just make out the ring gaps, these gaps should looks similar for all 4 bores.
For the valve stem seals are you fitting them on both inlet and exhaust?
#14
Posted 03 July 2015 - 08:18 PM
#15
Posted 03 July 2015 - 10:35 PM
Pistons MUST have some free-play or else they would seize up when the engine got hot. it's the rings which control the compression.
Machine shops bore the engine to suit the piston skirt to bore at the bottom of the piston, not the top.
However, if the engine has been newly & incorrectly bored the pistons can 'rock'' in the bores and this causes a 'ripple' finish on the bores and scores the piston skirts which will show as excessive oil consumption and lots of 'blow-by'. Unless this is happening on a new-build engine there should be nothing to worry about.
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