Odd Deck Height

[AUSSIE427SOHC]

I just thought I would show the members an odd thing I found when checking the deck height in a side oiler that I am building for a mate of mine. We are using a 4.25 stroker crank from Doug at Precision, Lunati 6.660 I beams and a set of Wisco flat tops. As you can see by the measurements, they are all over the shop. Piston to bore is at the top end they recommended, 0.005". I would not have though this would be enough to have the pistons cock over that much. Appreciate and thought's and experiences.

[blykins]

How are you checking?

On a piston like that where you can't put your bridge directly over the wrist pin, you need to rotate until the piston is as high in its travel as it can be, then put your bridge on one side of the piston.  Push down all the way, record the measurement, then push down all the way on the opposite end of the piston so it will rock up in the other direction, then record your measurement.  Average the two and you can see how far down it is. 

I wouldn't expect the numbers to be so different on adjacent holes, even on factory blocks that have not been square decked, it will be similar, but you'll see extreme measurements on opposite ends of the decks. 

[blykins]

BTW, you've got the pistons in upside down....

[fekbmax]

BTW, you've got the pistons in upside down....

I was thinking,    hummm, new trick cylinder head set up..  ??  LoL.
Coulda just been a late night.. those aussies are full of new tricks though, maybe flipping the heads.. lol.

[AUSSIE427SOHC]

How are you checking?

On a piston like that where you can't put your bridge directly over the wrist pin, you need to rotate until the piston is as high in its travel as it can be, then put your bridge on one side of the piston.  Push down all the way, record the measurement, then push down all the way on the opposite end of the piston so it will rock up in the other direction, then record your measurement.  Average the two and you can see how far down it is. 

I wouldn't expect the numbers to be so different on adjacent holes, even on factory blocks that have not been square decked, it will be similar, but you'll see extreme measurements on opposite ends of the decks.

Thanks for pointing that out Brent, a big opps. I machined an ally bridge that I bolted down to the block for rigidity and consistency. The worst of them is 0.028" to -0.004", a total of 0.032". That seems a bit excesive? I am going to put each piston on the CMM at work next week and see what I get. Cheers.

[blykins]

I think you're seeing tolerance stack, plus piston rock.  Doubt there's anything wrong with your pistons, rods, or crank.....I'd say it's in the block, plus the piston rocking in the bore. 

Try to measure off of one side of the piston, and rock it from extreme to extreme, then take the average.