VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 1938 Piston Fit

Finally getting warm enough to think about some engine assembly!

In the Chevrolet Shop Manual it shows a procedure of using feeler gauges at right angles to the piston pin to check the piston to bore clearance of the cast iron pistons.

My 38 US manual shows that a .002 feeler should allow the piston to be lightly pushed into the bore and that a .003 feeler should lock the piston in the bore.

My 38 Canadian manual says that a 1/2" wide .0015 feeler being pulled with a fish weighing scale should take close to a 25lb pull to remove the feeler from alongside the piston. I found another version of this at chevytrucks.org that recommended 7-18lbs pull on the fish weighing scale.

I've tried both of these methods on my freshly bored/honed engine and both seem loose. Room temp is about 68F.

Are either of these methods very useful these days or should I just go with the machinists measurements???

Thanks

They really arn't that fussy as long as its not to tight. Also there isn't much you can do about at this point.
What do you get with a .003" feeler?
Actually you will not get any cold piston slap with as much as .004 or .005.
Years ago when these engines were overhauled the good shops used a roller type piston skirt expander. The roller clamped onto the thin part of the skirt and buldged it out a bit. I have one of these creatures.
At 68 Deg. and both materials being the same (cast iron) you will be same.
Give the .003" a try.(and use the US manual method)

I would be interested to know what dimension your cylinders were bored to and the size of your pistons (+20, +30, etc.). Sounds like you have a new set of rings, could you put one in the cylinder, push it down stright with an inverted piston and measure the gap in the open part of the ring? At what thickness feeler gauge does the piston "lock" in the cylinder? These could be useful numbers.

Brian

I trust the machinist using modern tools, but I'm trying it this way for the amusement value and just to make sure something isn't totally out of wack and to try the original process. I measured this at -25 for kicks over the winter and the clearance was IMMENSE - .006 feelers would fit loosely.

I just checked all pistons and .002 feelers allow the piston and rod to slide slowly down the cylinder just from the weight of the assemblies. With .003 feelers, the piston can be pushed down the cylinder with medium pressure and feels snug, but not "locked". All cylinders feel the same. It wouldn't take much more to lock the pistons. This method is a bit suspect as it assumes all feelers are going to have the same chaacteristic when bent in an arc, which they won't. **This just prompted me to try another set of feelers and guess what? The .002 fit very snugly and the .003 would not even allow the pistons to be pushed in. This sounds more like what the manual describes so now I'm satisfied.

Bigbth, my cylinders are bored to +30. I could do what you suggest with a new ring but it would be more a characteristic of the ring manufacturer's spec than a cylinder measurement. The manufacturer makes the ring to their own spec to fit a slight range of variance in bore size, and the gap has to be hand filed by myself to get the proper gap. So depending on quality, I would think the gaps could be pretty random out of the box.

Sounds as if everthing is OK.

The rings are what seal the bore and generate the pressure. The gap on the ring is very important to the way the engine will operate but it does sound like you are pretty close.

Brian

When measuring the ring end gap the rule of thumb that I was taught is to allow .003 to .004 per inch bore diameter (3" bore = .009 to .012 gap) with ring positioned as described in earlier posting.
Tony

Tim,
Original shop manual would assume cast iron pistons. You may be using aluminum pistons which could require more expansion room. Or your machinist may be only familiar with appropriate tolerances for aluminum, even if you are using cast iron, and bored the cylinders accordingly.
In any case, I would trust Gene's advice and proceed with confidence. Good luck.

Mike

Thanks all. Ring gap will be a while still as the temp is still too cold outside overnight in the garage to paint the block. Thus my puttering with feeler gauges...

I knew he was using cast iron pistons.

I was wondering what would happen if you measured the piston diameter with a 3.5 inch vernier capable of .oo1 inches and then used an inside micrometer with same capability to measure the diameter of the bore? I am going through the same drill on my 38 Chevy 216 rebuild, but have not bored yet. I have about .005 inches oversize and plan on just honing without boring over and will use the original C.I. pistons.

Just to check the technique be sure to use your inside mic to measure the reading you get from the vernier, that is put the inside mic into the open vernier and see if the both read 3.500 inches.
Mike S of MD

I have seen it done that way but believe using a feeler strip as per the shop manual is more accurate.
A cylinder wall micrometer would be most accurate for measuring the walls.

Mike
To hone without boring is full of dangers, a bit like opening a barrel of deadly snakes. You will need a very good machine to hone all cylinders straight without taper.
Tony

Tony,
Thanks for the caution about honing w/o boring, but I was not sure how scared I should be? I only have .005 oversize cylinder diameters and cannot even "feel" a ridge on top and my pistons came right out without even a hint of a hangup on the ridge (since it doesn't seem to be there).

I did this on a 1948 216 Chevy block and only burned a quart every 1000 miles after the re-build, but the block had .010 oversize cylinders from wear and I had to cut the ridge off to get the pistons out. I used a not-so-good honer (a sand paper style) that time, but felt good about the cylinder walls when the pistons were ready to go in.

This time I have purchased (ebay) a real honer (Lisle) with 2 sets of stones (course and fine) and will set the block on my (newly acquired, ebay) drill press and use the slow speed to turn the hone stones and use the up and down action of the drill press handle to synchronize the spin and plunge for the 60 degree cross hatch.

Yes, I anticipate some small taper, but I will make sure it is within the allowable for rebuilds. I anticipate removing about .001 with honing and will be happy to have finished cylinders with only .006 oversize. I hope to only burn a quart every 2000 miles with this method.

Please let me know what you had in mind about the snakes that I do not see at my heals? I am sure I have not thought of everything and do appreciate your comments in more detail. That will be kindly appreciated.

Mike S

Mike
[/b][/u]DONT do it.[b][u]
Any tapered bore will break the rings and with alloy pistons can also break the ring lands out of the piston, believe me I HAVE been there and put the block in the metal recycle bin.
Always bore out to just undersize then finish hone to correct clearance and save yourself a headache, heartache and worst of all a pocketache.
Tony
PS
If the bore is already at maximum oversize it can be resleeved back to standard and go again

Back when the 216 was common we did hundreds of overhaul jobs on them. Usually between 40,000 and 80,000 miles. Its seldom that they had more than .006" wear (taper) and would accept a re-ring job with no problems. Due to the iron pistons they never broke ring lands. What was done though is the pistons were expanded using a roller devise designed for that purpose. The piston skirt was "rolled out" slightly till there was a very slight drag in the least worn area of the wall.
Thats the beauty of a 216, the pistons arn't fussy about their fit and the rods can always be "re-adjusted" to fit a slightly worn crank journal and still give satisfactory service.
I still have a piston expander in my collection.

The tolerances for taper in the cylinder bore are quite large when considering the entire stroke.

Taper Specification: Kooper’s Engineering Handbook of Piston Rings = .0005 / inch of stroke with max of .001 in any one inch ~.0018 total.

My cylinders taper: .0030, .0015, .0011, .0015, .0021, .0017 inches. As you can see, there is only one cylinder (#1) that is really over, and not by much.

As far as out-of-round: Out of Round Specification: Kooper’s Engineering Handbook of Piston Rings = .001 / inch of Diameter =.0035 total. My values are: .0024, .0015, .0012, .0013, .0010, .0008, and all are within specs for new engines!!

As far as oversize bore is concerned (not considering taper or out of round), the specs are even more tolerant, suggesting .002 to .003 on new engines and allowing much more before corrective action.

Hope this is interesting on motor rebuilds. By the way, I got inside mics on ebay for almost nothing to make the above measurements. Chevrolet allowable specs are close to those quotesd above by Kopper's Engineering Handbook by a famous ring manufacturer of olden days here in Baltimore.

Mike S


If I expand the pistons they will be larger and un-usable for a rebuild if I ever have to sleeve to original 3.500 inch bore again (hope not but do want to save pistons).

I appreciate all these good comments and Gene's usual generosity in his thoughts, words and offers.