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  #1  
Old 1 Week Ago
marioserafica marioserafica is offline
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Default 1968 xlch

1968 XLCH engine number 4424 ……51 years of love and hate !

Hi friends,
Considering what below.. I have to change pistons ( and to rebore one cylinder) , but the rotating assembly was already balanced with the previous piston weight, therefore :
a) how far from this piston weight could I go before needing to rebalance everything again? The new pistons ( forged TRW ) are about 35 grams ( !!) heavier than what my assembly is balanced for.
b) Is it advisable to reduce pistons weight down to former conditions, by milling f.i. the bottom of the pin bosses ? ( only few grs !) … or what to do ?
I don’t yet know the reasons for these seizures, as well as the damages to piston pin hole on both sides of the piston ( only rear one) and consequently “wound” on cylinder wall


I am afraid that heavier pistons can affect connecting rod and bearing life and nullify the previous balancing ( I am not planning to open crankcase for checking new balancing conditions), moreover with 35 gr for each piston , what about vibration amplitude and critical RPM point ?!
Well any advices will be appreciated.
Thanks and best regards ciao
Mario
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  #2  
Old 1 Week Ago
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What do you mean when you say “seizure”? Did you have a piston get stuck in the bore and damage the cylinder wall and piston skirt? Or did the piston pin retainer clip fall out? Are they spirallox or wire clips? I will let someone more knowledgeable speak to the balancing question regarding your switch to forged pistons...
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1Scooter 1Scooter is offline
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Forged pistons are equal weight +/_ .015 grams. Do not need to re-balance. Don't over think it.
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Quote:
Originally Posted by 1Scooter View Post
Forged pistons are equal weight +/_ .015 grams. Do not need to re-balance. Don't over think it.
pretty sure he's saying that it used to have cast pistons and he now has forged pistons to go back in (?). hopefully he didn't buy new pistons before boring/honing the cylinders to final size - he may not have the correct piston size.
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needspeed needspeed is offline
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Quote:
Originally Posted by 1Scooter View Post
Forged pistons are equal weight +/_ .015 grams. Do not need to re-balance. Don't over think it.
I agree that there's a good chance those pistons could be used without rebalancing. The rider probably wouldn't notice any difference in the vibration characteristics of the bike. But he says that the pistons are 35 grams heavier. That's fairly significant. Certainly more than what you posted. Did you mean +/- 15 grams? I remember an old thread that mentioned how heavy the TRW's are.

Changing the reciprocating weight without balancing effectively changes the percentage used. Adding weight lowers the percentage. In this case 35g x 2 pistons = 70g. That will lower the percentage about 3 points. So if its currently balanced at 60% it will become 57%. If it's at 55% it goes to 52% etc. Don't know if its known what it is now. So it's hard to say if that would make it better or worse or no noticeable difference. There's a lot of debate on what is the best percentage to use.

Mario, if you feel you need to keep the balance factor the same you might find some cast pistons closer in weight to your current ones. Removing 35 grams from a piston will not be easy and may weaken it. But you're right- lighter is better.

But I suspect you could use them as is with no problem. At least not as far as balance.
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Quote:
Originally Posted by marioserafica View Post
1968 XLCH engine number 4424 ……51 years of love and hate !

Hi friends,
Considering what below.. I have to change pistons ( and to rebore one cylinder) , but the rotating assembly was already balanced with the previous piston weight, therefore :
a) how far from this piston weight could I go before needing to rebalance everything again? The new pistons ( forged TRW ) are about 35 grams ( !!) heavier than what my assembly is balanced for.
b) Is it advisable to reduce pistons weight down to former conditions, by milling f.i. the bottom of the pin bosses ? ( only few grs !) … or what to do ?
I don’t yet know the reasons for these seizures, as well as the damages to piston pin hole on both sides of the piston ( only rear one) and consequently “wound” on cylinder wall


I am afraid that heavier pistons can affect connecting rod and bearing life and nullify the previous balancing ( I am not planning to open crankcase for checking new balancing conditions), moreover with 35 gr for each piston , what about vibration amplitude and critical RPM point ?!
Well any advices will be appreciated.
Thanks and best regards ciao
Mario

To rebalance to heavier pistons, you have to add heavy metal to the flywheels. This makes for a more expensive job. Reciprocating weight calculates for half of rotating weight when balancing. (So 35 grams more isn’t as bad as it seems)

If you had a balanced rotating assembly, it would be nice to keep it that way. I cant say that 35 grams per piston is going to damage anything, prob not at all. You might not even notice any more vibration.

You can remove some weight from the pistons, how much depends on the forging. 35 grams is a lot of aluminum. If you post photos of the new pistons, top and underneath, I could give a better opinion about lightening. you can also use lighter piston pins to reduce the weight. it can be a lot easier to lose 35 grams with different pins.

Piston pin bore damage happens for a few reasons, lack of lubrication, heat, distortion or flexing of the piston pin. Detonation can be a cause, but there are always other signs of detonation to support that.

Piston to wall seizing is usually caused by heat, lack of lubrication (or both) and sometimes, ingesting a lot of dirt.

Good clear Photos of the failed parts would be helpful to see.
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needspeed needspeed is offline
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Originally Posted by Maxeffort View Post
..Reciprocating weight calculates for half of rotating weight when balancing...
Hi Maxeffort, I'm trying to understand this. Would you explain what this means? Thanks.
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Originally Posted by needspeed View Post
Hi Maxeffort, I'm trying to understand this. Would you explain what this means? Thanks.
That balance factor of 50% is really for a 90* Vee Engine. I wrote that out of habit. Harley is typically balanced with a 60% balance factor, and that is a compromise.

Balance Factor x Reciprocating Weight + Rotating Weight

Reciprocating weight is imposing less force than weight rotating at the flywheel.

If there were 100 grams heavier reciprocating weight, it would be Calculated as “the same as” 60 grams heavier rotating weight at the rod pin.

If you cut off the rod at the big end, you could just balance it neutrally. (Like a wheel and tire)

Some engines such as an in-line six, do not require balance factor... all reciprocating weights cancel out.

With reciprocating weight on most engines we have to calculate the balance factor... that additional weight imposed on the flywheel from weight moving up and down.

Weigh everything up, calculate, assemble bob weights (weight which clamp onto the rod pin) and balance it.

As I wrote, 60% is a compromise that seems to work best. The only way to get a 45* Vee Engine engine smooth is with balance shaft systems.
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Dear friends, I thank all of you for your good pieces of advice. I am thinking to follow this way :
a) try to reduce as much as possible the weight of both pistons to the same value
b) I don't know if the silicone content is high or low, therefore I must apply the suggested .007" clearance between pistons and cylinders (.007 !!!)(???)
c) Try to start ....very long runningin and hope.....

...in the while I'll order new +.50 cast pistons
Thanks again mario
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bella Mario!
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