Small End clearance for Con-Rods
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Laughing Gas
Joined: Aug 2008
Posts: 840
Likes: 8
Small End clearance for Con-Rods
Oh my......putting my engine back together and noticed 1 rod seems to have a bit of play in the top bushing. It's barely perceptible, in fact, I have to clamp the rod big end in a vise and try twisting the piston. I can just barely tell there's some movement. Seems to me the spec calls for less than 0.001" of clearance, something like 0.0003 to 0.0008". Not the easiest thing to measure, hence the "twist and feel" test.
There is no movement of the wrist pin in the piston that I can tell. Seems to be in the rod.....but it's so small.
I already installed the piston/rod assmebly, but as it's not too late to take it back out, I was thinking of checking it again. I guess the Machine Shop could get a bushing and put it in for me.
Anyone know anything about checking the con-rod small end clearance and what's OK, and what's not?
There is no movement of the wrist pin in the piston that I can tell. Seems to be in the rod.....but it's so small.
I already installed the piston/rod assmebly, but as it's not too late to take it back out, I was thinking of checking it again. I guess the Machine Shop could get a bushing and put it in for me.
Anyone know anything about checking the con-rod small end clearance and what's OK, and what's not?
Lead Driver
Joined: Jan 2003
Posts: 9,893
Likes: 12
From: Ohio
I would bet that if you could even FEEL movement, then it's got way too much clearance. Even at .001 clearance, that's only .0005/side and you shouldn't really feel that "twisting".
The only proper way to check small ends is with a Sunnen bore gage that is set up for doing rods. Every shop that does rod work will have it bolted to the side of their pin hone machine. They set the ZERO using the actual pins(that they measured) in a fixture and work from there.
The only proper way to check small ends is with a Sunnen bore gage that is set up for doing rods. Every shop that does rod work will have it bolted to the side of their pin hone machine. They set the ZERO using the actual pins(that they measured) in a fixture and work from there.
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Laughing Gas
Joined: Aug 2008
Posts: 840
Likes: 8
It must have been my imagination. I pulled that piston/rod asssembly and removed the piston. Put rod in vice and tried to wiggle wrist pin. Fit seemed fine. Compared it to others. Same thing. Using two different measuring techniques, I repeated measuremtns about 20 times. Worked out to 0.0005" clearance, the max of the "new part" spec. Max wear spec is 0.001", so I'm fine.
I think I must have been accidently sliding the piston against the side of the rod and was feeling that impact when I did it before.
I'm not really a nut about all engine specs, but I feel some need to be exactly right. Thanks for the input.
I think I must have been accidently sliding the piston against the side of the rod and was feeling that impact when I did it before.
I'm not really a nut about all engine specs, but I feel some need to be exactly right. Thanks for the input.
