The Stovebolt Site Forums The Shop Area Engine Shop Check out the ring gap on this 235

Hi there

got the engine stripped down this weekend finally, the mains and rod bearings look really good, and aside from a heafty ridge on the cylinders the walls looked to be wearing well. After pulling the pistons there were three with stuck rings (to be expected after sitting 25 years) so i soaked them in solvent to free them up, and ended up breaking one of the rings. My original intention was to just inspect and reseal the engine, but I'm thinking I wont be so lucky now. A new set of rings for sure will be needed. The pistons are stamped .030 so this has been rebuilt once before, and I tested a couple old rings in the cylinders and the gap is over 3/16". I cant seem to find my bore measuring tool (may have loaned it years ago) so I am going to see if i can get one and take some measurements.

Without getting to far ahead of myself, what is the maximum a 235 can safely be bored? I was hoping to do a light honing and throw some new rings at it but my gut is thinking it may be beyond judging from the gap pictured below.

Thanks
Jeremy

Did you check the gap at various places in the bore? That would give you an idea about how much taper there is and whether you should bore it.
The amount of gap shown can't be good. You'll have a fair amount of blowby thru there.

Check the tech tips page for my procedure for checking cylinder taper with a used piston ring and a feeler gauge- - - -no inside micrometer needed. Square up a piston ring pulled up snug against the ring ridge, and measure the gap by stacking up feeler gauge blades until they fill the gap. Add up the blade thicknesses and record the total reading. Now push the ring down near the bottom of the ring travel with a piston and measure the gap again- - - -it should be considerably smaller. Calculator time- - - -subtract the bottom reading from the top measurement. Divide the difference by 3.14 (Pi). The result is the cylinder taper in thousandths of an inch. If it's over .005", you probably need to rebore the cylinders. A stovebolt block can handle a 60 over bore with no problem, and 80 is a bit marginal, but lots of them are running around that way. At 2000 RPM, a ring in a taper-worn cylinder is flexing 60 times a second trying to keep up with the changing bore size from top to bottom. That makes rings lose their tension in a hurry. Boring eliminates the taper, and practically all of the ring flex.
Jerry

Thanks Jerry

I was actually reading a previous post of yours for measuring taper that way, I will do that tonight. If the taper is good, would my next step be to measure the bore and determine if the .030 pistons and new rings would work? I will need a bit more info on how to size the rings properly, but i may be overthinking this a bit.

Kevin, it must have produced a lot of blow by with that gap, unfortunately I have not seen this one run before, but if the oil on the underside of the engine and tranny was an indication I would say it did.

It's simpler to measure the pistons- - - -a 3 9/16" bore .030" oversize will be 3.5925" The pistons should be a couple of thousandths smaller than that dimension, as the clearance is built into the piston skirt. Measure the piston at the height of the wrist pin, and 90 degrees away from the pin boss. Pistons aren't round- - - -they get smaller as the measurement moves away from that 90 degree area, and the piston crown where the rings fit is smaller than the skirt by several thousandths. If the engine has some mileage on it since the last rebuild, it might be necessary to run a knurl on the skirt in a couple of places to get the clearance back within specs. I like to run somewhere around .0025"-.003" clearance on a cast piston. As the piston heats up, the clearance tightens up a little, and the cam-ground shape gets round because the Invar strut across the wrist pin area won't let the skirt expand much.

I was able to get some time in the shop this weekend and used Jerry's method (Thanks Jerry) of measuring ring gap to determine taper. They are all between .006 and .007 for taper. I was able to borrow a bore gauge and the cylinders are out of round by .0005 to .001 when measuring in three spots top and three spots bottom. Unfortunately my digital calipers stopped working (murphys law) so i will be on the hunt for a micrometer to get an actual piston, and bore measurement. Against everyone's advice and knowledge I am considering just throwing a set of rings at this engine, as the intention of this truck is not to be a daily driver, and at best it will only get 500 miles a year. But that decision wont be made until i get an actual bore and piston measurement.

When I was cleaning the carbon out of the piston lands the second land had a octagon paper thin ring behind it, pictured below. Does anyone know what this is, and if it comes with a new ring set?

Thanks
Jeremy

Here is a picture of an old style oil ring from a GMC 270 I recently broke (the ring not the 270). A ring manufacturer told me this style of ring is no longer available after they made an oil ring set to replace this broken one.

This oil ring was a series of three nested rings.

Whether or not that ring is incorporated into a new set will depend on the manufacturer of the new rings. DO NOT add it if the replacement ring set doesn't include a part like that. You're right on the borderline of needing a rebore job, so honing and re-ringing the engine is an option, especially on a vehicle that won't see a lot of use. I'd strongly recommend knurling the piston skirts to get the clearance right, if you can find a machine shop that still dies that kind of work. At one time, when an engine would routinely get 2-3 ring jobs before getting bored for bigger pistons, piston knurling was simply part of a ring job- - - -all the good quality shops did it. If you were on the south side of the border, I'd suggest that you box up the pistons and rods and send them to me. I can knurl piston skirts on my lathe by chucking up the piston and turning the machine by hand against the knurling tool. Any machinist worthy of the name can do the same thing, but finding one willing to think outside the box like that is virtually impossible these days.

You can measure piston skirt clearance with a feeler gauge by sliding the piston into the cylinder upside down and inserting the feeler gauge at right angles to the pin. If there's more than .0025"-.003" of clearance on a cold engine, there's probably going to be a little skirt rattle present during warmup. I used to have a set of feeler gauge blades 12" long and a spring scale that we used for piston fitting, but it can be done with a regular-length gauge blade if you're careful.
Jerry

Thanks Guys

Tom, that piece was from the second ring not the oil ring, i am thinking it was something used to apply extra ring pressure?

Jerry, thanks for the suggestion but just so I am clear (bare with me):

1- Insert piston upside down so skirt is just below cylinder ridge?
2- Measure the gap with feeler gauge opposite of pin - pin is going front to back with engine, so I am measuring the driver and passenger side of the cylinder?

I have access to a lathe, but don't dare to attempt the knurl as I am sure I would be buying new pistons after. I will try finding someone local, but i think your right when you say this will be hard to find.

Yes, the widest part of the piston, which establishes the skirt clearance, is 90 degrees from the wrist pin and at the same height as the pin. Pick out a feeler gauge blade (maybe .002" for instance) lay it alongside the piston skirt, and insert the piston and the feeler gauge into the cylinder together. When the clearance is taken up properly, you should need to make a fairly firm tug to slide the feeler gauge out. Back when we did a lot of clearance checking that way, we would shoot for a 3-4 lb. pull on a spring scale to slide the gauge blade out. Just measure one side, as the blade will wedge the piston snug against the cylinder wall on the other side. The connecting rod makes a good handle to hang onto while doing that check.
Jerry

In another post a while back I told a story of honing cylinders and knurling pistons for a V4 Saab (Ford industrial engine). Replacement pistons were available only as a set with connecting rods and bearings. This was more than the car was worth. I honed the cylinders true and had the pistons knurled, then filed them to get proper clearance. The customer a poor artist was very happy. Ten years later another local mechanic called me on the phone to find out what I'd done to the engine. He said it needed rings again. The artist still owned it.

And also get rid of the ridge before honing the bore.

Ed

Thanks guys

I just got a set of micrometers delivered and was able to measure everything. The bore of the cylinders just below the ridge measures 3.5985, and the piston measures 3.5918, this is the average across all 6 with the worst bore being .0005 more. Is this to much clearance to make a running engine by installing new .030 rings? I know I am beyond the limits here, but if this was 70 years ago would a farmer make this work?

Also keep in mind .007 taper

You're above what would be acceptable in a commercial shop with a reputation for doing high quality work to uphold. I'd suggest using a rigid hone (not a spring-loaded glaze breaker, and honing 3-4 thousandths out of the bottom of the cylinder to minimize the taper, and then putting as heavy knurl on the piston skirts to get the skirt clearance close to right. I've honed cylinders .005" or more bigger than the normal size and taken up the difference by expanding the pistons, and the engine was good for 50K miles or more without excessive noise or oil consumption. If you're planning on making the truck a daily driver, do the job right and rebore it, regrind the crankshaft, recondition the connecting rods, etc. For a quick freshen-up on an engine that sees limited use, hone, knurl, and re-ring it.
Jerry

Thanks Jerry

Hone, knurl, and .030 rings it is. Is knurling something to be left to the pros? Or could I carefully chuck the pistons and turn the lathe by hand? On the 235 should I only have the thrust side done or both? And just to confirm the thrust side is the cam side?

Grab the piston with the lathe in a 3-jaw chuck in the area where the rings are installed, and roll the skirt past the knurling tool by hand, slowly increasing pressure on the cross feed to put three pretty deep knurl patterns about 1/2" apart which cover most of the area near the wrist pin- - -one knurl above the pin, another at pin level, and the last one below the pin. Do this on both skirts, and use a micrometer to measure your progress. It's really a pretty simple procedure, and you can increase the diameter of the skirt by 3 or 4 thousandths pretty easily. Perfect Circle uased to sell a bench top tool called a "Nurlizer" to shops without a lathe. The knurling wheels were made to put a stylized "P-C" pattern on the piston skirts.
Jerry

Those little wavy things were called ring expanders. Supposed to help rings seal in bad cyls. Just snake oil.

Thanks Everyone

Jerry, would this knurl do the trick:

https://www.kmstools.com/magnum-pivot-head-knurling-tool-3059

And just so I understand, does the knurl expand the entire piston? or, is the raised portion of the actual knurl what creates the larger diameter? I ask because I want to be sure I am measuring the right location to check progress, do I measure on the knurl, or the smooth surface between the knurl?

Again, i really appreciate the help
Jeremy

Yes, I've got one exactly like that which I use with my Craftsman 12" lathe. The most recent knurl job I did was on some oddball 256 cubic inch GMC pistons that were simply impossible to find replacements for. Measure the knurled part, not the smooth areas between them. The crosshatch pattern the knurling wheels leaves causes that area to be slightly larger diameter than the smooth area. The crosshatch also traps oil and helps lubricate the piston to cylinder wall interface. Don't believe the naysayers who claim a knurl job doesn't last- - - -I have measured several thousandths' difference in diameter on knurled pistons when it was time for another ring job.
Jerry

Your the best Jerry, thanks

Hoping to get this done on the weekend, i will post a pic of either the knurled piston, or one with a broken skirt!

Just go slow and sneak up on the final diameter you want. I'm sure I've built hundreds of engines with knurled pistons, back in the days when mechanics actually fixed things instead of just swapping parts! There are even spring steel "expanders" still available to cut down on piston slap on the old cast iron 216 pistons. I've got a couple of sets of them stashed away somewhere.
Jerry