Machined boss on piston - any purpose ?

aSTerix

aSTerix

Joined
Dec 18, 2014
Messages
681
Location
Oman
Bike
ST1100AY
Is there some esoteric purpose for the round machined boss in the MIDDLE of the piston head ? (the boss just above the MT3 stamping which I've ringed in yellow highlighter)

The reason I ask is that if I have to continue my engine strip down to the point of removing the crankshaft I figure I may aswell polish the piston heads (assuming I dont need to rebore and go oversize) and this boss is an obvious hot spot candidate

IMG_0627.jpg
 
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jfheath said:
Isn't it to allow the valves to come down before the piston has got out if the way ?
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The cutouts towards the outside of the pistonhead would give the valve clearance

Basically I would like to polish (grind) away the small round boss right in the middle of the piston head unless it serves so function
 
Ah. Middle. That's what that word means !

Apologies. I didn't read your post carefully enough.

Haven't got a clue about that then. Would removing it reduce your compression ratio ?
 
Bikeric said:
I would leave well alone!
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Unfortunately I rarely follow this approach !

If I can find a good engineering principle behind something I tend to do it and see if I can make things better, doesn't always work out that way (usually because I missed an even better engineering principle that the designers thought of !!)

In this case, removing a potential hot spot is a good thing

On the negative side, it is possible that the rough head surface and the boss in the middle are craftily designed to create air/fuel mix swirl and keep the atomisation, however, I think I may be giving Honda too much credit there !
 
heffly said:
center pin for lathe to turn them to spec
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A machining mark was my first guess. I suspect there was literally a boss - a cylindrical projection cast on the head of the piston that was grabbed by a chuck (or other fixture) on the lathe when it was turned. This boss was later machined away and those are the marks left from its removal.
Asterysk said:
Is there some esoteric purpose for the round machined boss in the MIDDLE of the piston head ?
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A boss is usually a cylindrical projection. This appears to be flat machining marks.
 
How come nobody mentioned accommodating a lathe spindle for turning the piston (a cylinder) to true and a specific diameter?? :rofl1:
 
What would grinding away the boss accomplish, in reality please ? It would be better to cc the heads, port and polish them while your at it. Then swirl the intake valves.1586744584409.png
 
SMSW said:
A boss is usually a cylindrical projection. This appears to be flat machining marks.
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An interesting theroretical debate, when is a Boss not a Boss !! It is raised and to me looks like its best described generically as a Boss.

Its because its raised that I want to remove it i.e. remove a source of a hot spot. if it was just a machining mark then I wouldn't bother
 
The small circular machined spot is where a casting boss was sticking out of the piston dome that was used to hold the piston while it was machined and shaped. After the piston was shaped and machined the boss in the center of the piston was machined off leaving this small circular shape. Pistons are not a true cylinder but are tapered, larger at the bottom and cam ground oval in shape narrower at the sides with the wrist pin holes. This is to allow proper fit of the piston when it is heated from combustion as the top of the piston will grow more at the top from being exposed to combustion heat and having more material at the top and the thicker sides of the piston will also grow slightly more. Air cooled engines typically have more taper in the piston skirt than a liquid cooled engine due to running hotter and more unevenly heat distribution.

I have polished piston domes before as well as cylinder heads - I've never noticed any performance advantage but it can help a little in preventing carbon buildup. As far as preventing detonation the ST1100 engine is in a mild state of tune and detonation is not an issue at all. It won't hurt anything but don't remove much material but it really won't buy you anything.
 
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Asterysk said:
It removes a source of hot spot and therefore reduces tendency for knock
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Doing a poor job of this to all four pistons might also screw up the weight balance between the rotating masses and increase engine vibrations so proceed with caution.
I would just leave them be, they haven't caused a problem thus far.
 
You will also find steel expansion struts in cast aluminum pistons allowing for less noise on startup when cold. The struts control skirt expansion and cause the piston to grow on the pin axis. Most Forged Pistons will be round with no expansion struts lending to more piston noise when Cold.
 
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