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Motor rebuild advice...

Mon Apr 02, 2012 1:22 pm

Before I disassembled the cub, the engine ran ok, but had been run a lot with a bad hydro pump seal and an overfilled oil pan. it smoked some and seemed a little shy on power but I figured that was from being overfilled and causing buildup on the valve train. anyway after taking it apart I found it had the original bearings (4-51) and pistons and valves. I'm replacing the guides and bearings and rings at least. The upper cyl wall measures .010" over and rings are obviously bad, but the cyl wall looks great other that wore at the top. Down in the cyl it measures just about .0035" over stock. Pistons look great and are cast iron. id rather not drop another $200 plus for oversize pistons if I can help it. any advice? The valve job itself should help tremendously because it really needed it.

Re: Motor rebuild advice...

Mon Apr 02, 2012 2:21 pm

I know you probably don't want to hear this, but my opinion is that .010" is way too much. The tolerances for the original cylinders is 2.625 - 2.627". If you were .001 or .002 over that, I'd be inclined to guess you'd probably be ok for normal running around chores and parades and such with new standard rings.
The top of the cylinder wears because of the heat, as does the top of the piston, which is probably out of tolerance as well. So with .010" off the cylinders and I would bet some off the pistons, your issue is compounded, and it all ads up to a trip to the machinist.

Re: Motor rebuild advice...

Tue Apr 03, 2012 7:42 am

I'm with Raymond, that's a lot of cylindar wall taper, along with excessive piston skirt clearance. The only way you'll have a satisfactory rebuild is to have it bored.

Re: Motor rebuild advice...

Tue Apr 03, 2012 8:06 am

You can get custom oversize rings!?! Dusty B

Re: Motor rebuild advice...

Tue Apr 03, 2012 11:03 am

The tolerances for the original cylinders is 2.625 - 2.627".
that is the new engine dimension tolerance. the amount of allowable wear is .007 beyond that, if i remember the spec correctly. But at .010 over, the engine is out of spec regardless, and should be bored to whatever size cleans up the block. You cannot fit oversize rings in the engine because as the piston reaches the bottom of its travel, the wear decreases to nearly nothing, causing the engine to be too tight.

as an alternative, finding a replacement block in your area might prove to be a less expensive proposition.

Re: Motor rebuild advice...

Tue Apr 03, 2012 12:50 pm

I ran into the same problem this past winter. I ended up putting double the amount in my motor than planned. I spent the big money and had a machine shop go completely through my motor boring it out and fixing everything else that needed. I am glad I did it, money well spent. Hope to have it running next month.


Re: Motor rebuild advice...

Tue Apr 03, 2012 2:45 pm

You need to decide if you want to invest the money you've already committed to or throw it away. You can do the repairs that you are planning and will still have a worn out engine or invest another couple hundred dollars and have a "new" engine.

Note the "Clearance in Cylinder Bore"in the manual below:



Like Jason said, I too competely through my engine and I am glad I did, it runs like new and never gives me any issues. Money well spent.

Re: Motor rebuild advice...

Tue Apr 03, 2012 11:12 pm

maybe i wasn't very clear, but the .010" wear is on in the top maybe 1/2" of the cyl wall only. The rest is max of .0035". The repair manual above states that the tolerance is .0105" taper top to bottom of "ring travel area" and mine is only .0065" from ring travel area all the way to the top of the block and that taper isn't actually a taper. My machinist friend/neighbor thinks someone used a ridge reamer at some point, but the material removed was basically limited to the area above the ring travel. Pistons actually show hardly any wear at all, well within tolerance. Cub blocks around here are basically non existant and I think I'd rather keep a block that has had not been bored before as compared to one that's .030" or more over already or unknown since I know mine is a great foundation to work with.

There's also an article I ran across by HL on here where he points out that "standard" rings are actually not only fine but meant to be used on motors with up to .010" of wear. Mine actually is just over .009" of wear specifically and again that is in the area where a ridge reamer was used/misused and outside of the ring's travel. The other 95% of the wall area is .002-.0035". That wear took over 60 years and I'm sure quite a few of those it was worked hard. It didn't just all of a sudden become unuseably weak(which it isn't at all) and if the wear had caused so much of a loss of power, the previous owner (also the original) would have noticed and done something about it because he used it for mowing, cultivating, and discing often. It had good oil pressure and the only reason I'm really working on it is because I disassembled it,cleaned it up to paint and changed out seals and gaskets. Before someone says he sold it because it was wore out... he didn't, he passed away and his sister got it.

Our plan was to at a minimum hone the cylinders to get it more uniform (he has a boring and a honing machine). It's kinda odd to see suggestions that this block is wore out and that a better option would be to buy another block that I know nothing about until I tear into it. The motor wasn't that bad in the first place... the rings are wore out but it still performs acceptably... I can't imagine that a new set of rings in itself won't tighten it up quite a bit and save me a couple hundred bucks that I'd rather put into the main/rod bearings and valves and countless other things like the clutch disc and TO bearing. If money wasn't an issue, I'd stick a fully built yella cub motor in it and have had someone else do all the work... I've always heard your power in these motors is gained and lost in the valve train anyway. My neighbor is retired and has a fully equipped machine shop in his back yard and owns a few cubs (yellow and red), a super c, and a super A and has worked on many of them in the past. If an extra $250 shows up, I'll definately go .030" or .040" because boring it really won't take much longer than just honing it... just a few minutes setting it up on each cylinder and walk away and let it do its thing. Part of me hates to see those original IH pistons go for some tisco oversize pistons too.

Re: Motor rebuild advice...

Tue Apr 03, 2012 11:22 pm

duh... actually, click on the Best of HL Chauvin above and to the right and read 'smoke...new rings...no smoke"... his example was wore .015" over at the top and .005" at the bottom and after rereading it I see that standard rings are even for .010" overbores. Mowing 2 acres of heavy grass with just new rings and no smoke convinces me that it works.

Re: Motor rebuild advice...

Tue Apr 03, 2012 11:45 pm

Ricky, the bore clearance is .0026" between piston and wall obviously so the wall clearance where the rings ride is in tolerance if its at or below .0052". Mine is well within that at max of .0035" in the ring travel area (what the specs refer to)... its actually pretty dang good for 60 years old I think. Cub blocks won't be around forever so I'd like to keep mine as is as long as possible so it'll be still going another 60 years from now plus. I mean, whats the horsepower difference in a .040" over and stock anyway?

Re: Motor rebuild advice...

Wed Apr 04, 2012 6:33 am

Okay, so how much wear do you have at the top of the ring travel area? That should be where your "top of cylinder" measurements were taken. If you don't show a lot of wear there, you can re-ring with satisfactory results. For the sake of ring survival, cylinder taper concerns me a lot more than basic wear.

Re: Motor rebuild advice...

Wed Apr 04, 2012 2:11 pm

there isn't much taper at all in the ring travel area of the cylinders... Not much more than .001" variance from top to bottom in the ring travel/contact area. I guess my concern with replacing bearings and doing a valve job is more bang for the buck and less chance of something really bad happening to my block. Does anyone have any experience with actual performance increases with .030 or .040" overbored compared to stock? I see there are a small and large bore main jet and tube for the carb, small for the early cubs and large for the later. I'm guessing that's to supply more fuel to feed the higher rpms with the newer, higher power c60 motor and that they should be fine to use on an early Fcub and bump up the rpms to get a bit more hp.

Re: Motor rebuild advice...

Thu Feb 28, 2013 7:11 am

Just as an update... I decided to bore this motor .030" over and put in new aluminum pistons. We are taking a cut off the head and turned the crankshaft .010" under and have all new bearings. All engine parts came from hamiltonbobs. If I can scrounge up some more $ soon, ill put a new valve kit in it. Hope to have it together and ready to plant by the end of march. Rest of the tractor is painted and ready for reassembly.

Re: Motor rebuild advice...

Fri Mar 01, 2013 12:04 pm

New valve kit will be in Monday... I guess I'll see what a new cub motor is like before too long. Well hopefully with alum pistons, a larger bore, and a shaved head it'll be a little stouter.

Re: Motor rebuild advice...

Fri Mar 01, 2013 12:59 pm

Make sure you check the clearance of the piston to the head. You are going from flat tops to pop ups and shaving the head. Some have had to pull the head later to deal with the piston hitting the head so better safe than sorry. A little modeling clay on the piston helps to see what is happening and don't forget the gasket. It is nice to have an old one around for clearance checking.
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