Topic: Rebore is home from the shop but....

[DavePhipps]

I  got home and did a test fit of the pistons and there is a slight rocking along the axis of the wrist pin. My first thought was " Great the bore is out of round". Then I turn the piston 90 degrees and it's still rocking on the wristpin axis. It would seem that my pistons are not perfectly round. Is this normal? I don't have much experience in this area so I'm sweating it a bit right now.

[754]

Top of piston means nothing, it grows as it heats.
The skirt at 90degrees to piston is what it was fitted to.

[DavePhipps]

The skirt area seems fine. I checked the ring gap last night and there is no gap near the top of the cylinder but halfway down there is a gap. Do I just measure near the top?

[iron_worker]

If the ring gap changes down the bore then it means your more is not a constant diameter. Sounds like something is awry.

IW

[MCRider]

To answer your question, a cold piston is not round. IT is oval with the narrowest part being along the pin axis and widest perpendicular to pin axis. IT grows to round when underway.

AS to end gap, it should be the same throughout the cylinder, but a few thou is probbably OK. There is an allowancee of end gap, so as long as your not tooo big throughoout, should be OK, I think.

[iron_worker]

Personally, if I found the diameter of my bore was not consistent I would be worried what else may be of issue? The piston to cylinder clearance is pretty tight. For cast pistons I think the recommended clearance is around .001"  for pistons of this size range and maybe around .002" for forged pistons. Others can chime in on exact clearance specs but I'm in the ball park anyway.

How much does the ring gap change from top to bottom?

IW

[iron_worker]

Also, are you using the piston to push the rings down the bore and ensure they are square in the bore? If you get them even slightly crooked then that will skew your measurements.

IW

[DavePhipps]

Will be double checking everything tonight after work.

[Oldtech]

When pistons heat up they expand more along the axis of the wrist pin due to there being more material in that area ie: the wrist pin gugeons. So when they are made they are ground oval and expand to round when the engine reaches operating temp. That's why a piston diameter is always measured below the wrist pin on the skirt and at 90 degrees to the wrist pin axis.

[DavePhipps]

Sorry for not replying to this. I couldn't find my feeler gauges. I found that everything is good in the bores. I found that I didn't have the rings square. Now the top end is ready to go back together.

[MCRider]

Sorry for not replying to this. I couldn't find my feeler gauges. I found that everything is good in the bores. I found that I didn't have the rings square. Now the top end is ready to go back together.

Good news!

[iron_worker]

^ x2

IW