Substitute for Dial Bore Guage

According to the manual for my 2002 Grand with the 4.0L engine, you need to use a dial bore gauge to determine piston size if replacing a piston or pistons. Getting ready to remove and tear down my 4.0L due to tapping or clacking type noise around cylinders 4, 5 or 6. Thinking maybe piston skirt but not sure. A dial bore gauge is one tool I don't have and being retired and on fixed income cash doesn't come as easy as it used to. Is there another way I can determine piston size if I do indeed need to replace one or more?
Thanks,
Bill

check your local parts store to see if they rent/loan specialty tools.
maybe your local machine shop will let you borrow one if you agree to buy some of your parts thru him ?

The block usually has paint marks on the adjacent cylinder for indication what stock piston is fitted... A B C or D. Typically along the right side.

The aftermarket does not offer selective fit pistons, so if you need to bore the block, no need to worry about it.

They won't rent a dial bore gauge (probably don't even sell them) and even if they did I wouldn't trust it. You can try a snap gauge, but they are a pain in the ass and can give inconsistent readings in inexperienced hands. If you were local I would let you borrow mine.

As digger said there may be paint on the passenger side of the block as to the piston size if they used a non standard size. For some reason I remember stamped numbers being present as well, but I'm a bit hazy on that fact. However, this is a moot point as the aftermarket doesn't have small size increments like that and I doubt the dealer even has the pistons in small sizes like that.

Bill, I recommend you take the block to a machine shop and have them bore the block. This will true the bores and they will put a nice cross hatch on it to set the rings. All bores will be one size and they will tell you what the OS is. Then you either have them source pistons or get your own. Not sure if 4.0 wrist pins are pressed on, but if they are they can do that also.

-Ron

You can measure the pistons with a micrometer. Much more common than a dial bore gage. Of course this won't tell you how worn the bores are. A dial bore gage by itself will not give you size. You at least need a micrometer or some other setting master to set zero.

You can use a telescoping "snap" gage and a micrometer, but this all takes a little "feel" to get used to.

WB9YZU said:

Bill, I recommend you take the block to a machine shop and have them bore the block. This will true the bores and they will put a nice cross hatch on it to set the rings. All bores will be one size and they will tell you what the OS is. Then you either have them source pistons or get your own. Not sure if 4.0 wrist pins are pressed on, but if they are they can do that also.

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Thats backwards. First you decide on the over bore. Purchase pistons THEN bore and hone to the piston + desired clearance. Any machine shop that will bore and hone a block w/o the pistons should be avoided.

winterbeater said:

You at least need a micrometer or some other setting master to set zero.

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Forgot to mention that little part to the OP.

You can use a telescoping "snap" gage and a micrometer, but this all takes a little "feel" to get used to.

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And he will still meed a mic.

I use a digital caliper for this kind of work. Something like this $20 Harbor Freight model is good enough. You can reset them to zero and then expand them out to the edges of the bore pretty easily and this will give you the bore size. Simply wiggle it around a bit until it gives the max reading.

http://www.harborfreight.com/6-inch-digital-caliper-47257.html

The one challenge you will likely have is that if the engine has a lot of miles on it, there will be carbon ridge at the top of the cylinder that will distort your readings. O'Reilly's or Autozone will lend you a ridge reamer for free but don't trust this method unless you can remove that ridge first.

HTH
Todd

No way will a cheap digital caliper get the resolution and accuracy you need for this type of work. Plus you can't measure the bore with it w/o a dial bore gauge.

Talyn said:

No way will a cheap digital caliper get the resolution and accuracy you need for this type of work. Plus you can't measure the bore with it w/o a dial bore gauge.

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You might be surprised. I have used this method many times to check machine shop work and it is always correct. I have used it on bore sizes (simply set the outside caliper set to the top of the bore), piston sizes and even crank work. I will admit that my caliper is of a high quality but Pistons will be standard sizes. All you need to do to match a piston is to ensure that the replacement is the same so you are looking for 0.010, 0.020, 0.030 or 0.040 over. A cheap digital caliper is more than accurate enough to tell you how much over stock the existing pistons are.

If you are looking to determine the wear on the cylinder walls you need a dial bore gauge because you won't be able to get a caliper into the middle of the sleeve (which is where the bore wears) but if all you are looking for is the current bore size to match a piston, a caliper will do the trick.

tjmotter said:

You might be surprised.

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No, I won't be surprised as it is not that good. The only thing you verified was that the machine shop was in the ballpark. For its price its good. I have one of those cheap calipers. I use it a lot as its cheap and if I drop it I don't care, unlike my $200 Mitutoyo caliper or the $400 set of mics. It should never be use on engine work as you need a good set of mics for that that can get down ten-thousandths of an inch. Measuring .010" or .020" is one thing, but getting down to ten-thousandths of an inch (stock piston to wall clearance is 0.0008 to 0.0015 in) it can not do. I wouldn't even trust it to a thou.

If you had a set of standards you would see how inaccurate those calipers are. For an example the 2" standard measures 2.001" on the calipers, the 3" measures 3.0015". The calipers really can't resolve to .000x", but it has a 5 there. Which means it could be slightly higher or slightly lower

Also, the majority of the cylinder wall wear does not occur in the middle of the wall, but at the top of the ring travel where pressure and piston rock is greatest.

Hey, we agree but the OP is only looking to confirm the size of his current pistons and a cheap caliper can do the job. It won't tell him if he is within tolerance but he didn't ask for that. If he has a broken piston skirt and just wants to buy a replacement, at least he would know how much over it is.

Talyn said:

No, I won't be surprised as it is not that good. The only thing you verified was that the machine shop was in the ballpark. For its price its good. I have one of those cheap calipers. I use it a lot as its cheap and if I drop it I don't care, unlike my $200 Mitutoyo caliper or the $400 set of mics. It should never be use on engine work as you need a good set of mics for that that can get down ten-thousandths of an inch. Measuring .010" or .020" is one thing, but getting down to ten-thousandths of an inch (stock piston to wall clearance is 0.0008 to 0.0015 in) it can not do. I wouldn't even trust it to a thou.

If you had a set of standards you would see how inaccurate those calipers are. For an example the 2" standard measures 2.001" on the calipers, the 3" measures 3.0015". The calipers really can't resolve to .000x", but it has a 5 there. Which means it could be slightly higher or slightly lower

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I don't see any agreeing. The OP is looking to do something he can't accomplish. Its a stock piston unless the engine has been rebuilt and he can't get it the piston in over sizes as small as he needs. If the piston skirt is broken or worn then a new piston may fix that. If its the cylinder wall that is worn an new piston won't help. However he will have no way of knowing which is is until he gets the piston and installs it given his skill set (no offense to OP). The cheap calipers won't be able to tell him is the piston is slightly worn only that it is majorly worn.

I'm a simple man. If he has a broken piston skirt, changing out the affected piston is a simple and cheap solution and $20 spent on a cheap tool to confirm the current size is a reasonable investment. If the pistons aren't broken, he has a wear issue and will need machine work done. Simple. Spending $200 for a specialty tool to understand how big his wear problem is a waste of $200 because the block needs to go into a shop to get fixed anyway

$200 won't cut it for the correct tools. If his skirt is broken he will be able to see it. If its worn he probably won't be able to measure it.

Really, his cheapest solution is to pull the piston in question and bring it to a machine shop for measurement with a mic. Shouldn't be more than $20 to verify the size. The better idea is to pull the engine and bring it to a machine shop for bore wear inspection, but that can get costly and depending on the integrity of the shop they might not give the correct answer.

Personally what I would do if I was that low on funds is to have a machine shop measure the piston and verify that it isn't oversized. Then I would inspect the bore for major wear as far as scoring, ring ridge and crosshatch. If the bore looks good I would drop a new piston in and see what happens. At least in this case I know the piston is good and the shirt won't break off any time soon. It may still make a noise due to too much clearance with the hyper pistons, but at least I know the piston won't end in the pan anytime soon.