What would cause this piston slap on a newly rebuilt engine?

[grannyknot]

I believe there is only a few minutes running time on an engine I'm tearing down at the moment. This is the #1 cylinder and  is the worst one, there is a minor amount on a couple of others. Also, the rod bearing shells.

 

 

Edited July 28, 2023 by grannyknot

[Patcon]

Piston pins are pressed, not free floating?

Do the piston pins move freely?

Do you have a dial bore gauge and some mic's? What is the ID of the bore versus the OD of the piston?

[Zed Head]

Looks like overheating and/or lack of oil.  Are they all like that?

p.s. I wouldn't call that piston "slap".  Slap is from loose pistons.  That one got too tight.

Edited July 28, 2023 by Zed Head

[grannyknot]

4 hours ago, Patcon said:

Piston pins are pressed, not free floating?

Do the piston pins move freely?

Do you have a dial bore gauge and some mic's? What is the ID of the bore versus the OD of the piston?

Not sure yet if the pins themselves spin but the conrod moves freely on the pins, will get out the measuring tools tomorrow and get some hard numbers.

2 hours ago, Zed Head said:

Looks like overheating and/or lack of oil.  Are they all like that?

p.s. I wouldn't call that piston "slap".  Slap is from loose pistons.  That one got too tight.

I was told something about the oil pressure sender not being in place on start up and the oil gushed out, and I have seen signs of gasoline flooding the cylinders, washing the carbon off and pooling as in these pics so the gas would have washed any lubrication off the walls.

 

[jonbill]

A brake fluid leak from the MC into the brake booster into inlet #6 caused this a few years ago. It doesn't take much to remove the oil leading to heat leading to expansion and more heat and expansion!

 

[Patcon]

Those were nice pistons 😞

[jonbill]

6 hours ago, Patcon said:

Those were nice pistons 😞

It wasn't too much of a disaster, Mori san was happy to supply a single piston (and the bore cleaned up ok with the flexi hone).

Edited July 29, 2023 by jonbill

[grannyknot]

I spent a few hrs today getting all the measuring done, as you can see it's pretty consistent as you would expect from a freshly rebuild engine, the piston to bore clearance is suppose to be between .0010 and .0018", my clearance is working out to be about .0020 so I'm good with that.

I noticed that all the scuffing on the sides of the piston and cylinder wall are on the carb side of the engine so that's making me think the gasoline flooding is what washed the lubrication away and caused the scuffing 

 

[jonbill]

The carb side is the thrust side so will always get the worst of any scuffing. 

[grannyknot]

5 hours ago, jonbill said:

The carb side is the thrust side so will always get the worst of any scuffing. 

Right, and I found the reason for the gas flooding, the idle mixture screws on the triple Webers are suppose to be somewhere between half to one and half turns open, if the carbs need more than that you have to start changing jets.  On these carbs the idle mixture screws were turned out 5 1/2 turns, so the gas was just gushing in.

[SJZ]

 I would be checking ring gaps too just for fun.

[grannyknot]

7 hours ago, SJZ said:

 I would be checking ring gaps too just for fun.

Yeah, I checked, they were good, the work the machinist did is excellent, very consistent,   it's the guy who reassembled it🤡