Piston Ring End Gap

[outdoors4evr]

I noticed my 184 was starting to "breathe" quite a bit through the dipstick (crankcase breather) so I decided to dig into it over the Christmas Holiday. When I pulled each piston, I found some of the ring gaps were only offset by 90 degrees rather than the recommended 180 degrees. The rings were not tight in the piston and could be rotated by hand. There isn't really a way to keep the rings from walking so I don't think there is a good way to solve this issue other than putting the rings back in properly and hoping they don't move or if they do move, at least move together.

I then popped each ring out of the piston and put it in the bore 1/2 way down and measured the end gap. Each ring gap measured between .020" and .025" (20 - 25 thousandths of an inch) I didn't find a specification on this in the service manual, but did find a minimum gap specification of .006" (6 thousandths of an inch) in the paperwork that came with the new rings.

The rings I removed were .020" over so I purchased .030" over rings and filed each them to fit the suggested minimum in the new rings paperwork. I don't have a lot of hours on this tractor since last bored (300 hours) so I just measured the cylinders which are still round and have no ridge at the top. I didn't bother to hone as there is no scoring or issues to address.

Any thoughts on ring end gap minimum and maximum tolerances??? Did I make these too tight for the aluminum dome top (pop up) pistons?
I'll break it in gently and change the oil again before the mowing season. It does run well again and the dipstick is no longer being fogged with oil.

[Eugene]

.007 minimum. .017 maximum. Probably for cast iron pistons. Aluminum pistons may have a slightly different ring end gap range.

Not sure why an engine with 300 hours since rebuild would have a ring end gap of .020 to .025.

[Mike H]

end gap is critical . in general too much ya get blow by too little ya start breaking pistons as the engine warms up.
the rings need to move freely so they don't break the skirting off of the pistons.

Mike

[outdoors4evr]

Not sure why an engine with 300 hours since rebuild would have a ring end gap of .020 to .025.

Simple: The new guy putting it together the first time (me) didn't know any better. Don't put .020 over rings in a .030 over bore.

[Eugene]

Simple: The new guy putting it together the first time (me) didn't know any better.

Since you have the pistons out. One at a time, Plastigage a main bearing/journal, then reinstall the bearing cap. Then move on to next main bearing. During piston assembly, Plasitgage each rod bearing/journal.

[BigBill]

What's the piston clearances? Is there any ridge at the top of the cylinder? Any honing stone cross hatch? If all is ok go next size up.

[outdoors4evr]

Pistons are .020 over size, cylinder was bored and cross hatched about 300 hours ago. No ridge, not enough hours on it since last bored to create a ridge. I didn't bother to cross hatch it again, block material is precious.

[Slim140]

Simple: The new guy putting it together the first time (me) didn't know any better. Don't put .020 over rings in a .030 over bore.[/quote]

Pistons are .020 over size, cylinder was bored and cross hatched about 300 hours ago. No ridge, not enough hours on it since last bored to create a ridge. I didn't bother to cross hatch it again, block material is precious.

Are these two quotes the same engine? Was your engine bored to .030" over and you have .020" over pistons?

[outdoors4evr]

These are the same engine. The previous owner did the bore "just before" I purchased so I don't know the specifics of what dimensions were bored. My first venture into the engine was due to a broken crankshaft and exposed .020 steel flat top pistons (not the right ones for the 184). I replaced the pistons with proper .020 over aluminum pop-up pistons with new .020 rings and cross hatched the cylinders. The .020 rings fit with a .025 end gap, which breathed through the oil filter breather. I ran it like this for a few years, but not liking the breather fogging, I ordered new rings. The .030 didn't quite fit, so I filed them to fit by hand leaving a .008 ring gap.
Eventually, she'll be bored to .030, but I'll probably not buy new pistons until she is .040. The smaller piston isn't a big issue until the piston is allowed to rock in the cylinder. This shows up as scuff marks on the piston skirt.

[mastercraft]

so you're gonna bore it eventually to .030 but not put .030 pistons in it?????????????????

[outdoors4evr]

Right. The difference in .020 rings and .030 rings is additional length. The rings don't get wider or thicker, they get longer (larger circumference). As the cylinder bore wears larger, the rings wear and expand causing the ring gap to increase. Eventually, the gap between the piston and the cylinder wall gets large enough to allow the piston to rock wearing the piston skirt and the cylinder wall.
Most people replace the pistons every other size, but mine were replaced with flat top pistons by the previous owner. I replaced the flat top pistons with the correct aluminum pop-up pistons that were .020 oversize.