3900 - piston in hole?

[gectek]

I had a project 3900 apart and noticed that the piston was in the bore .020. Every bore I measured came out like this. SO....I need some input. If anyone has a 3900 with the heads off, i need to know piston to deck clearance from you also!

[Joseph Upson]

I had a project 3900 apart and noticed that the piston was in the bore .020. Every bore I measured came out like this. SO....I need some input. If anyone has a 3900 with the heads off, i need to know piston to deck clearance from you also!

I estimated .031 for mine while working with the mistroked crank. The stroke of the crank is 3.43 if you want to calculate it.

3.43/2 + 5.7 rod + 1.304 CH = 8.719

8.819 Deck height - 8.719= .0999 my new piston to deck clearance, with the offset grind and 5.7" rods. It's actually .010 less since my LS1 pistons are 1.314 CH.

Stock should be:
3.31/2 + 5.827 + 1.304 = 8.786

8.819 - 8.786 = .033 is what you should have, but then I believe it was you that said your connecting rods were measured at 5.9" instead of the 5.827" that I have.

[gectek]

I was using GM numbers. I am measuring my rod right now so we will see on that. I have a spare rod here.

[gectek]

I am getting a 5.83 based on this - rod small end to big end length - 4.174 + pin .943/2 + big end 2.3710/2

[Joseph Upson]

I am getting a 5.83 based on this - rod small end to big end length - 4.174 + pin .943/2 + big end 2.3710/2

Is that with or without the big end bearing as my measurement came out to be the same as what was stated a machine shop measured. Measure the piston CH, if it's the same as stock 1.304 the rod should be 5.827. It all has to add up to no more than the deck height.

I measured the far end distance of the big and small end circle and the short distance of the circles with bearing installed and subtracted half of the crank pin diameter and half of the wrist pin diameter from the maximum distance.

[gectek]

It does not matter about the bearing. You are measuring what is there anyway. Meaning center to center. That is why you take half of the big end and half of the small end. The 3.9 piston CH is .835. Not even 1". I am sure with it being 1" or more, the piston would be at zero or a little above, but its not. With that measurement, you can see just by the math alone that it will be under deck.

[Joseph Upson]

It does not matter about the bearing. You are measuring what is there anyway. Meaning center to center. That is why you take half of the big end and half of the small end. The 3.9 piston CH is .835. Not even 1". I am sure with it being 1" or more, the piston would be at zero or a little above, but its not. With that measurement, you can see just by the math alone that it will be under deck.

Your piston compression height is .835, mine is as stated and I have LS1 pistons with a documented 1.314 CH to confirm it as they fit in the bore according to the deck specs I mentioned above. I'm not sure how you can have a compression heigth that low with only .033 difference between the 5.827 and 5.9" rod length.

[Joseph Upson]

Compression height is from the top of the piston to the pin center, that's why I inquired about your measurement associated with the rod length. It appears you have measured from the top of the pin bore to the top of the piston.

[gectek]

Yah, I was measuring it like that which was messing my math up. Sorry. Add the height i got + .943/2 and thats it.