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piston/deck clearance variance along pin axis, 4.6"bore

General engine tech -- Drag Racing to Circle Track

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Calypso
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piston/deck clearance variance along pin axis, 4.6"bore

Post by Calypso » Sun Mar 22, 2020 5:44 am

Is it common to see 0.01" deck clearance differences side by side on the same piston (measured at pin-axis) in 4.6" bore builds with quality components? Pistons not being square in the bores...
Piston pin bores accurate, crank bryant, three sets of quality rods, carillos & hi end callies. Block A460 BBF.

After all trials in two different blocks it seems like a combination of rod and bore machining accuracy. Not my build, but the owner will get the bores measured next.

Curious if this is common in the big bore builds?

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Re: piston/deck clearance variance along pin axis, 4.6"bore

Post by PackardV8 » Sun Mar 22, 2020 12:11 pm

Have you tried sliding the rod side-to-side on the crank and then pushing the piston down on the side with side-clearance gap? Then move the rod to the other side of the gap and push down on the other side of the piston. My experience is .001" can appear/disappear by rotating the crank, moving the rods, et al.
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Re: piston/deck clearance variance along pin axis, 4.6"bore

Post by Calypso » Sun Mar 22, 2020 1:08 pm

PackardV8 wrote:
Sun Mar 22, 2020 12:11 pm
Have you tried sliding the rod side-to-side on the crank and then pushing the piston down on the side with side-clearance gap? Then move the rod to the other side of the gap and push down on the other side of the piston. My experience is .001" can appear/disappear by rotating the crank, moving the rods, et al.
That max is .01” not .001” and is an average from manipulating clearances back & forth.

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Re: piston/deck clearance variance along pin axis, 4.6"bore

Post by PackardV8 » Sun Mar 22, 2020 1:27 pm

Sorry for the decimal fat finger. Yes. .01" is more than one would expect with today's CNC parts.

Is that an average for all eight pistons on eight rods in eight holes or one piston in one hole?

Has the block been square decked?
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Re: piston/deck clearance variance along pin axis, 4.6"bore

Post by Calypso » Sun Mar 22, 2020 4:27 pm

Square decked, two holes off by that much, only one perfectly straight. However, one rod/piston combo may get different result in different hole, so it seems it’s a combination of block and reciprocating determining the way it goes.
Just wondering how much off rod small end machining has to be to show .01” at 4.6” bore edge. Not much I suppose. Hence wondering if builders of big bore engines are familiar with this.

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Re: piston/deck clearance variance along pin axis, 4.6"bore

Post by PackardV8 » Sun Mar 22, 2020 4:40 pm

You mention "three sets of quality rods" and "Just wondering how much off rod small end machining has to be to show .01” at 4.6” bore edge." Can we neglect the rod big end as a source of angularity? Today''s setup jigs and CNC boring usually produce perpendicular holes. Back in the bad old days, rods resized on a hand-held Sunnen could easily go off true.
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Re: piston/deck clearance variance along pin axis, 4.6"bore

Post by modok » Sun Mar 22, 2020 9:40 pm

I don't know if we are talking about play (rod side to side) or alignment.
yes the rods have play, pushed sideways you can get .008 tilt of the piston either way.
If the alignment is perfect it will have the same play both ways.

If the crank is level and engine rotated several revs it usually centers at .004 difference or less. more than that, I would say is beyond "new tolerance" but maybe to .006"-.008 is not beyond the "running limit"

I check the parts separately instead of assembling it. It's been a LONg time since I assembled an engine and had any problem with it.

Rods honed by hand is NOT a problem, if they are honed in pairs, held against eachother, and rotated several times,
then the alignment of the housing bore to the sides of the rods corrects itself as you hone.

the pistons can be checked quickly by several methods, ball end micrometer from pin bore to crown, or on a surface plate or machine table upside down, especially easy if they are flat. If not flat may have to set on a spacer that contacts the areas of the crown that ARE flat.

The rods are a bit more tricky, I prefer to use levels. If the rod big end is placed against a leveled surface, you can use a level on the end of the pin to see that the pin is 90 degrees. if the end of the pin isn't square to the pin, that needs to be fixed first OR just mark the high and low spots and place the level 90 deg to that. Or make a special pin for it with a flange on one end.

if you level the rod vice itself, or use a leveled machine table AS the rod vice, then the alignment can be checked IN the vice.....which I should not need to point out how handy THAT is.
There are several more commonly known ways of checking the big end bore to pin for being in parallel, but the sides of the rod are less often discussed. Whatever way works. If the pin is aligned with both the sides and the bore, then the bore must be aligned to the sides. Although that could be checked ALSO, tho I never have.


The crank of course also could have been ground funky to cause this, but I'm not GEARED up to check that. Let the crank specialist handle it.

Bore being 90 to deck can be checked with....a tool to check that, it's basically a parallel with two stops and an indicator on a stick. Not hard to make. I have one by port-a-tool, MAINLY for diesels, but very handy to adapt for whatever.

ALTHOUGH how much clearance to YOUR pistons have? If they are a very lightweight slipper skirt with basically no skirt area on the sides, the PISTON itself may have a ton of clearance side to side in the bore, and that may prevent the piston from aligning itself when rotated.

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