Technical Piston to wall clearance

So not a question on specs, but tools. I once had a 218 machined, and one piston was tight. I took the block back, they honed it some more.

Okay, so time being at a premium for me these days, say I pop for 30 bucks for 12 inch feeler gauges to check and find a tight bore.
Will a rigid adjustable hone in a hand drill work to get proper clearance?
I realize I can’t be as precise as a shop tool, but for a budget build will it do the job?

ie have the block bored, have my buddy with a rod heater put new pistons on, and take over myself?

 

Sure it'll work. I've done just that many times with no trouble. Just go slow, measure often , use plenty of lube (WD40 works fine, as does ATF) and sneak up on the clearance you want. Back off the tension on the hone for the last dozen strokes or so to make the finish a bit less aggressive and stroke the hone just a bit faster to crosshatch it.

 

Thanks @chicken !

 

A mic & a dial bore gauge or even snap gauges are light years ahead of feeler gauges . Knowing what material the pistons are made of will tell you proper clearance , ring material will tell you what grit stone .
Engines BITD were lucky to get 75k miles out of internals , partly due to oils , partly due to materials & partly due to sloppy clearances .

 

Yes, I recall as a youngster that 100k miles was near unreachable from the big 3.
But dang, they made looking cars!

 

Hi Mike ,

Using a hone at home will work just fine .
It’s a shame that the shop can’t get the clearance correct,,but I guess it all goes back to the operator .
I’ve seen this many times before,,,( I never experienced it myself ),,but I witnessed others have tight bores and have to tear it back down and correct .
Usually only 1 or 2 bores would be tight .

It all goes back to the machinists and reading the mics ,,,,some are better than others .

Tommy

 

Someone here posted this link a few months ago for checking piston to bore clearance with a spring scale: https://pautomotivemechanics.blogspot.com/2011/06/piston-measurement-and-clearance.html

 

One hole a bit tight is one thing, but your comment on having it bored then buying a rigid hone and fitting pistons is another.
I would be hard pressed to finish hone all the bores with a drill and hone.
Opening just one a bit, no problem but all would be hard work.
If the shop bores and hones AND has your pistons they can blueprint and make sure they have the proper clearance.
Theres a chance there is just enough size difference to warrant it.

 

Go .003" per inch of bore, & not greater than .004"
so a 3.125" bore would be .0094" total radial clearance (.0047" per side)

under no circumstances would a feeler gage be appropriate tool. A telescoping gage and an OD mic is ok, but a dial bore gage is most appropriate.

The grit of the stone on the hone is determined by the piston ring material

 

WHOA !! Hate to disagree with another Alabamdamer, but that's wildly wrong recommendation for piston-to-wall clearance. Typically, the recommendation would be approximately .003" total.

What our OP may be experiencing is the machine shop gave him accurate cylinders and the piston supplier gave him random pistons.

What we do is measure each piston at the appropriate skirt location and write the dimension on the top of the piston. Today's better CNC machined pistons will be dead-nuts all the same. Some obsolete engine repops, not so much. In any case, each cylinder is honed to match each piston.

And yes, back in the day when small shops did not have micrometers, the OEM Shop Manual gave a procedure for fitting pistons with a long feeler gauge and a fish scale. The tension required to pull the specified feeler gauge between piston and wall was the best they had to work with.

jack vines

 

No bucks, you can sand it by hand. Won't seal as well, won't last forever.
little bucks, you can hand hone. A guy with tons of experience doing the job and with the tools at hand will get near to or better than a production machine run by a production operator. This is anywhere from a step up from the sandpaper to production to custom work. The time, effort and tools will affect the outcome.
There are machines that do final honing, but again, it's experience, the tools and the time invested.
Worn tools, sloppy setup and either lack of time or knowledge can make these worse than even the hand sanding. It shouldn't, but stuff happens.

 

I think pistons from the feeler-gage/spring scale days often had slotted skirts that were mighty flexible.

https://egge.com/wp-content/uploads/E111_web_3.jpg

https://egge.com/wp-content/uploads/19462_1669.jpg

Like others said, investigating actual piston diameters and cylinder diameters would be time well spent.

And as always, cleaning the cylinder bores until a rubbing with a white oily rag shows no gray.

 

Hold on....off track as usual!! The OP said he's going to buy long feeler gauges to check the clearances of the pistons in the bores. IF he finds a tight one his question was could it be fixed with a rigid hone and drill. Of course it can.
The absolute nonsense of .003-.004" per inch of bore for piston clearance is nuts. That's ring end clearance rule of thumb.
I suggest reading post #1 before responding.

 

Appreciate the information folks. I didn’t know about using a scale to measure drag on the gauges.
As Chicken mentioned, is just as I asked.
Thanks again.

 

Dial bore gauges on ebay for $50 m/l

 

@73RR

Ive
a set of dial bore gauges off eBay from some 20 or so years ago.
I can “make” them work, I’m not sure how shops do it, but I zero the gauge when in a hole (yes I know to measure different locations) then clamp the gauge in my small rubber jawed vice, set my mic and see where the gauge reads then.
Worked for my crank journals and bearings on the 350 I set up, so maybe I just need to get the proper “large jawed” micrometer for pistons?

 

The key word is BUDGET, many engines have been assembled using feelers to check piston to cylinder clearance not perfect but works just fine for a low dollar build. Using a feeler gauge matched to the spec clearance will work. It will be tight. If the clearance is specified 0.003 a 0.002 gauge will go in fairly easy a 0.003 will be tight and a 0.004 should not go at all. Personally I would rather be a little loose than too tight, too tight and you risk seizing. Just my two cents
Dan
PS
I agree that using a dial bore gauge and mics is the best method but not necessary on a budget build. The shop should have measured all the pistons and then matched the bores. Guess I’ve been lucky all the pistons I’ve bought have been very close in diameter.

 

Depends on if you're measuring too .001 or .0001

 

That’s one thing about my dial bore eBay gauge. The read out is only accurate to +\-
.001. I need to find a .0001 for it.

 

Budget36, you're right about getting a piston diameter sized mic for measuring clearances with your dial bore gauge. Measure the piston, use the mic to set the dial bore gauge and zero it, then measure the cylinder to directly read the clearance.
A good set of standards for setting up the mics is really nice too, but there goes the budget! LOL

 

The last 5 sets of pistons I have purchased miked dead on ,,,, (just like Jack said ),,,,modern CNC machined pistons are usually very true to size .

The other variable then is the boring bar,,and the operator running it .

My guys I use now have a CNC boring bar,,,,and a CNC hone .
I tell them the finish bore size,,,,and they put it right on the money , every time !
Kind of pricey,,,,,but top notch results.

Tommy

 

Back in the day, I used my Dads Sunnen hone to finish fit pistons for the racing engines I built. It had straight long stones and then fiber wipers! What astounded me was, how long it took to pull .001 inch of metal out of the cylinder! Only had one set of stones, can’t remember the grit! Too many years, too many beers! I like to double check my pistons with the long feeler gauges, after using either my inside mics or snap gauges! Never got the hang of a “ bore gauge”. Just too old school, I guess.





Bones

 

Well, A lot of the measurement depends on the Piston material as well. forged, Cast, Hypereutectic. All clearance at a different measurement.

 

My last two budget builds worked well.
A 350 Chevy had some ridge - enough to catch a finger nail,I ridge reamed then honed to get a cross hatch. Installed new rings checked gaps, OK ,new standard rings and bearings checked with plasti gage assembled and ran it for about 80,000 miles, the vehicle was toast so I pulled the engine and dissembled, no major problems but needed to go to machine shop for proper machining and new wear parts.
An OT engine got the same treatment almost 60000 miles ago, it has lost some get up and go but is still ok.
My last build had all necessary machine work and new parts all clearances are with in a 0.0001 or 0.00002 of target and well within the published specs, runs excellent also cost 3 or 4 times or more than the other builds.
The first two builds I mentioned are basically stock, the last build is a few stages past stock.
I guess is what I’m saying is: Accuracy and precision is relative to what is expected out of the engine build. Using feeler gages to check and adjust clearance is fine for a DD or slightly modified engine but is maybe not best on a high or max effort engine.
Dan

 

I assure you , you use his measurements, for piston clearance, you are going to have problems! Four inch piston….x .003 inches per inch…..=around .012 inches of clearance……piston slap from Hell!
Believe me I know! Back in the day, when I was a kid, I put a .040 over piston in a .050 bore Cushman Eagle! You talk about making some noise!






Bones

 

....aaahhhh, the days of old Cushman scooters....and avoiding the cops since we were only about 14 at the time.
Great reminder, thanks Bones!

 

Dial bore gage is great for checking if your cylinder is straight. Long feeler gauges have their place. I have custom pistons made and like to check the skirt clearance with both mics and a long feeler.
The JE pistons for my EVO Harley called for .000” @ the bottom of the skirt… yes zero…I had them honed to .0015”.

I have hone (not a glaze breaker) and have honed one cylinder .010” in a car when it swallowed a 10-32 brass nut and scored a cylinder. Since it was .005”of metal I had nothing to loose as pistons were available within 3 grams. It took a while with my 1/2” drill and continuous checking with the dial bore gauge for straight.

 

Cheap import mics and snap gauges are more than up to the task. You can easily get a set of both for under $100. If you're just working for yourself it doesn't really matter how accurate they are because they'll be repeatable (it's basically impossible for both those tools not to be).

I get that this is a budget build but once you have those tools so many things become so much easier.

I like to drop a ring into the cylinder inch by inch and use it to trace circles with a sharpie then I draw vertical lines and number the resulting checkerboard and then use that to take and record measurements.

 

Many an engine has been "bored" this way over the years. It's slow, but it works. I know a guy who did an in-frame on a backhoe this way.

 

Anything worth doing is worth doing well .