Machined boss on piston - any purpose ?

Also due to the forged pistons having denser material than cast pistons the forged pistons expand more from heat. This requires a looser piston clearance to allow for more expansion for forged pistons which also make them noisier when cold until they expand a bit. I bore a lot of cylinders and have to be careful when fitting forged pistons. Too tight and you will get scuffing or possible engine seizure when breaking in. I've never seen steel expansion struts in motorcycle engines. Maybe some larger bore BMW's have them - I know the old ones don't. I've seen them on truck and automotive pistons.
 
Andrew Shadow said:
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.
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I was going to get them balanced anyway so will have that done after the polishing, we have a local performance garage who do this day in day out on way more powerful engines
 
Bikeric said:
It may be a bit left over from the casting process possibly? I would leave well alone!
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Agreed. Unless you can see another similar feature on the piston somewhere else, this is where the molten material entered the mold or cavity when the piston was cast. Think of it like the plastic runners on injection molded plastic parts that get trimmed off later. It may also have served as a means to hold the piston for secondary machining processes. I would leave it alone. Most pistons are designed to change shape (become more round) as they come up to normal operating temperature. Because the piston is not heated uniformly in use, the cold shape is designed to be non-uniform in shape. This shape change prediction is very carefully calculated based on the thermal expansion coefficient and dimensions (thickness, length etc.) of the material when cold. It's possible removing extra material could change the way the piston expands when it heats up in operation.

On another note, these ST1100 engines run perfectly fine on regular grade 87 octane and don't have a history of spark knock problems so there's absolutely no benefit to removing that feature and there's plenty of risk.
 
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Asterysk said:
I was going to get them balanced anyway so will have that done after the polishing, we have a local performance garage who do this day in day out on way more powerful engines
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So they will do it with the rods, installed on the crank...
 
Al st1100 said:
in a hi performance engine not this one
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The main objective is to reduce the speed of carbon build up by having a polished head, the boss sits proud by about 0.5mm and with carbon build up it will naturally run at a higher temperature than the rest of the head and therefore reduce the margin for pre-ignition, this happens regardless of whether the engine is high performance or not, even lawnmower engines suffer it (mind you I've seen some pretty high performance versions of those !!)
 
Asterysk said:
The main objective is to reduce the speed of carbon build up by having a polished head, the boss sits proud by about 0.5mm and with carbon build up it will naturally run at a higher temperature than the rest of the head and therefore reduce the margin for pre-ignition, this happens regardless of whether the engine is high performance or not, even lawnmower engines suffer it (mind you I've seen some pretty high performance versions of those !!)
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Let us know how it works out...:rolleyes:
 
Asterysk said:
It removes a source of hot spot and therefore reduces tendency for knock
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Consider this. How many miles are on the engine? How much evidence of prior knocking is there? But then, you are putting way more energy and probably money into an engine that might be replaced for around $500 USD than a lot of the guys on this site would do. So, have at it. If nothing else, it keeps you occupied during this global pandemic and out of trouble. :thumb:
 
SMSW said:
Consider this. How many miles are on the engine? How much evidence of prior knocking is there? But then, you are putting way more energy and probably money into an engine that might be replaced for around $500 USD than a lot of the guys on this site would do. So, have at it. If nothing else, it keeps you occupied during this global pandemic and out of trouble. :thumb:
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Life is a bit different here in Dubai ! I doubt I could get a decent used engine and If I found one it would be a lot more expensive than $500 and most likely have similar issues. The environment here is like a summer Texas desert in the city and a sand blaster in the countryside. I literally have had to replace every single rubber part on the bike.

I enjoy the spannering work on the ST, its a nice change from my classic bikes, a sort of modern classic and ultimately it will be a test bed of sorts for lots of extra tweaks that I teach at a local makerplace
 
While a lot of info presented is factual the majority of conclusions while based on facts are merely opinions with a lot of if/then maybe tossed in.

All of Asteryk's attention to detail may or may not net a palpable gain or one significant enough to be appreciated. But as long as he first does no harm I don't see any reason for him not to follow his particular path. It's not like he's talking about drilling holes through the piston dome to decrease wind resistance or putting silicone buttons in the skirt to prevent piston slap.

Even if the only returns were knowing that much was 'cleaned up' - if that makes someone happy - great. I've seen a lot of forged pistons with the domes beautifully polished that were works of art. If it's only aesthetics so what.

It sounds like he has access to hi-po engine builders/tuners so a tweak here needing a counter-tweak there shouldn't be a problem.

I hope to see lots of pics (great camera work BTW) on the assembly!
 
Asterysk said:
The main objective is to reduce the speed of carbon build up by having a polished head, the boss sits proud by about 0.5mm and with carbon build up it will naturally run at a higher temperature than the rest of the head and therefore reduce the margin for pre-ignition, this happens regardless of whether the engine is high performance or not, even lawnmower engines suffer it (mind you I've seen some pretty high performance versions of those !!)
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it doesn't seem like the honda engineers worried about it besides aluminum transfers heat rather quickly. I would think that the spark plug side electrode causing pre ignition is more of a concern that the boss. Google it
"A pre-ignition combustion event looks something like this…

The fuel air mixture enters the combustion chamber as the piston is on its downward intake stroke. The piston then returns upward for the compression stroke. The more compressed the mixture is, the more difficult it is to ignite it, therefore, with the piston on the low side of the compression stroke, the mixture is actually easier to ignite than when it gets closer to Top Dead Center (TDC). A hot spot in the cylinder such as a glowing spark plug tip can ignite this low compression mixture very early, before the spark plug fires. Now the upward motion of the piston is fighting the combustion force that is expanding. This adds a tremendous amount of heat and load to the engine, and for this reason pre-ignition is responsible for much higher cylinder pressures than detonation. The pressure from pre-ignition is not rapid like it is with detonation. Instead, the pressure is very high and has a much longer duration." from A high performance site
 
There have been some speculations by engine builders as well that highly polished surfaces are not the best for combustion chambers as the rougher surface conduct heat away much faster than a polished surface and could reduce detonation or preignition. It is the very sharp fine edges that promote preignition as they can glow and pre-ignite the fuel. A simple rough cast surface typically is not subject to sharp edges.
 
bdalameda said:
There have been some speculations by engine builders as well that highly polished surfaces are not the best for combustion chambers as the rougher surface conduct heat away much faster than a polished surface and could reduce detonation or preignition. It is the very sharp fine edges that promote preignition as they can glow and pre-ignite the fuel. A simple rough cast surface typically is not subject to sharp edges.
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This is a useful theory, I was wondering about the rough casting on the head and whether it may have had a purpose, the increased surface would indeed offer more opportunity for thermal flow.

The edge of the machining "boss" is very sharp and I was planning on smoothing it but now I wondering if it might be better to dimple it.
 
I believe the rate of carbon build-up doesn't depend on surface smoothness either; carbon molecules are too small to care.
 
For an ST1100 with barely 100HP and only spinning at a few thousand RPMs, this is all a tempest in a teapot. The bike isn't going to care what you do to the top of the piston, within reason. Different story for a race engine running at 250HP @ 18k rpms.
 
paulcb said:
For an ST1100 with barely 100HP and only spinning at a few thousand RPMs, this is all a tempest in a teapot. The bike isn't going to care what you do to the top of the piston, within reason. Different story for a race engine running at 250HP @ 18k rpms.
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Maybe a storm in a closed cylinder?
 
paulcb said:
For an ST1100 with barely 100HP and only spinning at a few thousand RPMs, this is all a tempest in a teapot. The bike isn't going to care what you do to the top of the piston, within reason. Different story for a race engine running at 250HP @ 18k rpms.
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I have to contend with some low quality low octane fuel here and my bike suffers pre-ignition. It could well be carburation and the heat but I want to make sure I have no sharp edges to act as a hot spot. My query was primarily related to what its purpose may be
 
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