Piston to pin clearance question

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Mark O'Neal
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Re: Piston to pin clearance question

Post by Mark O'Neal »

If it has that much clearance, and you still feel resistance..the pin holes are probably not properly aligned.

If you shove the pin in, and hear a "tink" when it hits the other pin tower....
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Re: Piston to pin clearance question

Post by stock z/28 »

I would have to agree with Mark on a possible alignment issue.

If the pin feels loose when tried from each end but binds when placed in both bores I would really think there was an issue, esepically with that much clearance?
I have 2 AG 300s, and I usually just leave 1 set to .927" pins to quickly check stock rods before I even mess with them. The other one I set with the comparator/setting fixture pictured and it duplicates several small dial bore gages that I have, so I trust the Ag300 if its calibrated and used properly.

I guess you could check the pins in some "V"s, but since they are new, you would think they were straight?

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Re: Piston to pin clearance question

Post by needforspeed66gt »

stock z/28 wrote: I have 2 AG 300s, and I usually just leave 1 set to .927" pins to quickly check stock rods before I even mess with them. The other one I set with the comparator/setting fixture pictured and it duplicates several small dial bore gages that I have, so I trust the Ag300 if its calibrated and used properly.
Yep - I have two of them as well, one is setup for big ends, the other stays setup for small ends.

There is a definite "feel" for any kind of measuring done in this job, and that includes the setting of the measuring tool it'self.
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Re: Piston to pin clearance question

Post by Strange Magic »

Its hard to get a piston square to the face of the AG-300 and we have found a sunnen bore gauge works the best.
Yes it is....it takes some time to get the "feel" for it...more of a pivot and sweep across the fingers..
The AG 300 is the only way I have ever check piston pin bores...20 plus years
Yep.

P.S. I wouldn't set anything up tighter than .0009 at a minimum. If soo, many of times it will come back with a few stuck or hard to turn. At .0011 is about the area of minimum and you will never, ever have issues using this as a minimum. The majority of my stuff that leaves my shop is .0013-.0018, depending on the application. Never an issue, ever.
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Re: Piston to pin clearance question

Post by grant6395 »

times 2 on the .0012-.0015..
never an issue then.
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Re: Piston to pin clearance question

Post by hodge »

I have the ag 300 with the short fingers which will not measure pistons so I have to set my ag 300 then i size the rod and check then use a bore gage to measure the rod bore then use the bore gage to chech the piston pin bore, my bore gage checks to .0001" so my accuracy is rite on. just some other ways for you to check.
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Re: Piston to pin clearance question

Post by apprentice1 »

Today I checked the pin bores by first measuring the pins (.9902-.9903), and then using the same mic, set at .99025, to set the mitutoyo dial bore guage. Checking the pin bores this way gave me .0005" clearance on the pistons that felt "tight", and up to .0007" clearance on the "looser" pistons. I think this is the most accurate way I can measure the clearance with the tooling at my disposal. Makes me wonder about the numbers the ag300 gave me...gonna have to do some playing around. It appears that I will have to hone approx .0005" out of the bore then to loosen them up and get them to your "specs." Thanks for all the replies, much appreciated
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Re: Piston to pin clearance question

Post by hodge »

there you go, you will be rite on
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Re: Piston to pin clearance question

Post by Ronbz455 »

Hey I'm new here but trying to get this problem answered is tough. I have been racing Buicks for years. The 455 I built 10+ years ago failed on rebuild because the cam ate a grove in the block but it was running 10.80's with stock compression. I havent built another one for a while but now I'm building a Buick 350 and I have a set of used Venolia pistons that I pressed off the rods with the OEM tool and I'm putting them on full floating Hershe rods, may be Eagle, I beams and the piston to wrist pin clearance is tight. I measured with calipers, pin .9375 and the piston pin bore .9380 which is telling me that I have .0005 clearance. I'm thinking on that 455 Fred told me to have the clearance set at .0015 which after reading your post here that seems loose but that engine was used for drag racing only. These pistons have marks on them where the last guy used a socket or round object to mount them. I might need to run a hone in them to true them up a little. What could I use to hone the pistons out without changing the centerline. I hate taking anything to machine shops cause once I get one I can trust they sell to someone else. Is there a cost effective hone I can get on Ebay to do this to keep the two holes of the piston straight?
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Re: Piston to pin clearance question

Post by Wolfplace »

Ronbz455 wrote:Hey I'm new here but trying to get this problem answered is tough. I have been racing Buicks for years. The 455 I built 10+ years ago failed on rebuild because the cam ate a grove in the block but it was running 10.80's with stock compression. I havent built another one for a while but now I'm building a Buick 350 and I have a set of used Venolia pistons that I pressed off the rods with the OEM tool and I'm putting them on full floating Hershe rods, may be Eagle, I beams and the piston to wrist pin clearance is tight. I measured with calipers, pin .9375 and the piston pin bore .9380 which is telling me that I have .0005 clearance. I'm thinking on that 455 Fred told me to have the clearance set at .0015 which after reading your post here that seems loose but that engine was used for drag racing only. These pistons have marks on them where the last guy used a socket or round object to mount them. I might need to run a hone in them to true them up a little. What could I use to hone the pistons out without changing the centerline. I hate taking anything to machine shops cause once I get one I can trust they sell to someone else. Is there a cost effective hone I can get on Ebay to do this to keep the two holes of the piston straight?
That must be a hell of a set of calipers :lol:

Define "cheap"
The only correct way to hone the piston pin bore is with a mandrel that is long enough to stroke both sides at the same time & the mandrel must be true before you start
This means you need a Sunnen rod hone or equivalent

Not sure what you mean by an "OEM" tool but it brings up visions of the Sunnen press fixturing which is great if you are throwing the pistons away when you are done.
With pressed pins you never ever press pins into a piston nor do you press them out against the piston if you intend to use said piston again.
You use a fixture that supports the pin end of the rod & the piston floats to remove them & you either use heat or this fixture to install them
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Re: Piston to pin clearance question

Post by Baprace »

Brian B wrote:Its hard to get a piston square to the face of the AG-300 and we have found a sunnen bore gauge works the best.
Yes it is....it takes some time to get the "feel" for it...more of a pivot and sweep across the fingers..
The AG 300 is the only way I have ever check piston pin bores...20 plus years
[/quote]



Be aware that there are two types of AG-300, one has longer fingers and that is the one I use for pistons , it has to do with the serial number something like 13000 and over is the longer fingers.
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Post by dwilliams »

[quote}The AG 300 is the only way I have ever check piston pin bores...20 plus years[/b][/quote]

You can make some stepped plug gauges for an SBC by cutting down some old Buick V6 wristpins on your crank grinder. No need to mount the long jaws on the AG300.

Some old rod caps bolted together also make fine setting rings, so you don't have to keep chasing the low spot trying to set the gauge with a 2-3" mike.
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Re: Piston to pin clearance question

Post by Dave Koehler »

apprentice1 wrote:Today I checked the pin bores by first measuring the pins (.9902-.9903), and then using the same mic, set at .99025, to set the mitutoyo dial bore guage. Checking the pin bores this way gave me .0005" clearance on the pistons that felt "tight", and up to .0007" clearance on the "looser" pistons. I think this is the most accurate way I can measure the clearance with the tooling at my disposal. Makes me wonder about the numbers the ag300 gave me...gonna have to do some playing around. It appears that I will have to hone approx .0005" out of the bore then to loosen them up and get them to your "specs." Thanks for all the replies, much appreciated
I think you just confirmed that the AG needs a tune up. If the points do not have carbide balls in the end (late model) I bet the original points (no carbide balls) are worn (early version) and likely not evenly. Time to get the manual out and study the calibration procedure and maybe purchase some new points.
It's not the simplest of tasks to calibrate these gauges but doable. Fortunately it's not something you have to do every day and gets easier over time. When calibrations, life, the stars and Karma are all in alignment your dial bore gauge, outside micrometer and AG300 should all agree.

You probably already figured this out but unlike a rod big end against the back plate you have to tilt/rotate the piston or any other hole to find the tightest/zero point of the hole. Same thing with using a mike to set the AG points. With the mic firmly/gently held against the right hand side,non moving point...UP/Down/Forward/Back to find zero point. Pretty much a one handed deal with practice.
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Re: Piston to pin clearance question

Post by Belgian1979 »

When I put my pins in my rod/piston assembly I measured them as they came from the factory (Olivers + Autotec). They came with pin clearances in the .0008-.0009 range. I measured with a .001 mm gauge. The fell right through when I held the piston and just dropped the pin through the hole.
If I read comments here, it seems that this would be too thight. I find that rather strange, since the alu of the piston will expand at a far greater range than the pin will. So in use the clearance will only grow.
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Re: Piston to pin clearance question

Post by PLohr »

Great information, guys.

Does the amount of piston to pin clearance change if the piston has forced oiling?
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