M10 Rebuild Problems

[vaborn 3]

Hello everyone, been reading a lot on here lately but just got around to making an account. Tons of info on here and I am glad to live in the age of the internet. 

I recently obtained a 310-8 from my dad and have been working on fixing it up to use as my primary lawn mower. The M10 that was on it was worn out and needed to be rebuilt. I have rebuilt a K series engine before with good success and have a decent amount of experience working on small engines. This one is kicking my butt though.

The bore was out of spec so I over bored it on my mill. This was my first time doing an over bore and I had planned to go 0.010 over but I had some tooling issues (ahmm user error…) and had to go to 0.030 over to correct my mistake. I stopped at 0.003 shy of final dimension and honed the rest of the way until the bore met spec and finished with the same grit of stone that I used on the last rebuild. I put in a 0.030 over piston and rings. I checked all specs and found them to be within what the manual states. The crank journal was out of spec so I found a new used one that checked out. It’s got new valves (they were lapped), new connecting rod (yes, the oil hole is turned towards the camshaft), new main bearings, new carb, and all new seals. Crankshaft end play was checked per procedure in manual and found within spec. Same with camshaft end play. This engine does not have balance gears. I really thought I was going all out with this engine and it would basically be new when done.

 

So here is the problem. It ran great for the first half hour or so just as I was hoping it would. I checked the oil and noticed it was already black but assumed the rings had not seated yet. I changed the oil and mowed with it for about an hour. I started noticing it was knocking and kept getting worse. At first it only did it when not under load but towards the end it was doing it all the time. I know these engines are infamous for having a rattle but this just kept getting worse.The oil was black again which I have not had an issue with on my last rebuild. It smokes when throttling up but otherwise the exhaust looks clear. I decided to tear it back apart and see what was going on.

The connecting rod is much looser than it was when I installed it so i’m pretty sure that’s where the knock is coming from. The bolts were still tight and I had torqued them to the proper spec so i don’t think they are loosening. I haven’t made it back to the shop to actually measure the connecting rod yet to see how out of spec it is. My question is this. Does this sound like an issue with the rings not seating? My theory is that all the junk from combustion was getting in the oil resulting in wear on the crank journal but would that happen that fast? I used aftermarket parts so it that the issue? Did I do something wrong?

I’m pretty frustrated because I put a fair amount of money into this thing and I seem to be back where I started.

Sorry for the long post. Any help is appreciated. Thanks!

[oliver2-44 9,787]

I admire you grit for machining it yourself on a mill.

It's easy for us to armchair questions so please don't feel we're shooting you down.    

What type of hone did you use?  Honing out .003 with a conventional spring hone has a lot of risk of inducing a taper or out of round condition.  Did you check the bore at several heights to verify it was true? 

Back at reassembly did you check you ring end gaps and also plastigate the rod to the crank?

Did you have an air cleaner on it and a good seal between the carb and block?

iI the carb throttle shaft tight in the body where excess air or grit can't get in there.

What oil were you using for break-in.  Most recommendations are for a straight 30 wt, not a multi wt and  not a synthetic oil for  break-in.

[squonk 41,171]

Did you check the parts before installation? Aftermarket parts can be way off. How close are you to Richmond?  @richmondred01

[vaborn 3]

It wasn’t so much that I had any grit it was the fact that there are only a 1 or 2 engine shops within two counties that still do over boring. And they are backed way up. A friend of mine dropped a kohler off at one of them over a year ago and still hasn’t seen it. So I decided this was my only choice if I wanted it done anytime soon. But I did know I was taking a risk. And please don’t be afraid to armchair this situation. I realize I probably made mistakes along the way and am looking to learn.

 

I did use a spring hone. I know it wasn’t the best option but once again I was having trouble finding someone with a honing cabinet that would do it within a reasonable amount of time. I did check the bore after it came off the mill and it was well within spec for out of round and taper. I didn’t check it in multiple spots after honing which was dumb but I guess I just assumed it would still be good. I did check the ring end gap at multiple spots in the cylinder and it was within spec. I did just pull the piston and recheck the bore with actual micrometers and it is within spec. Max taper from top to bottom of 0.0012 measured multiple times. Not ideal for newly bored block but I doubt it’s causing the issue. Out of round is even less than that. 

 

I did find one major problem that is most likely causing my issue. I double and triple checked the placement of the ring gaps when installing the piston making sure that they were rotated 120 deg from each other per manual. But when I just pulled it the compression rings were perfectly lined up! I’m sure that was dumping all sorts of nastiness into the crankcase. Do rings ever rotate while the engine is running? If so, how do I prevent that from happening? I don’t remember spinning the piston after installing it so I doubt that could have caused it but who knows… 

 

It has a brand new carb and gasket so that should be good. I didn’t check the crank with plastigage so it is possible that was already out of spec. I just assumed with the crank journal being in spec and a new connecting rod that it should be ok but I’m learning what assuming does…

 

I did check the piston to bore clearance and it was good. 

 

I used SAE 30 conventional for the break in. Air filter was in place.

 

I need to get some plastigage but I’m certain the connecting rod is out of spec as it had a decent amount of play just by moving it by hand. So I guess I need to get a new connecting rod. I’m assuming I will need new rings as well? 

 

Thanks for the help guys. All great questions and they gave me the motivation I needed to rip the thing apart again to find some answers. 

 

Squonk, I’m located on the west side of Harrisonburg so about 2 hours from you.

 

 

Edited July 15 by vaborn

[vaborn 3]

So after a quick google search it sounds like it is normal for piston rings to rotate while an engine is running. Maybe they just happened to be lined up when I shut it down last? I guess I’m back to square one on this.

[squonk 41,171]

1 hour ago, vaborn said:

I

 

Squonk, I’m located on the west side of Harrisonburg so about 2 hours from you.

 

 

I am in NY. I tagged @richmondred01 as he rebuilds these engines all the time with quick turn around and maybe can lend some insight. 

[oliver2-44 9,787]

@vaborn I completely understand how hard it is to find a machine shop to work on these engines. I’ve been lookin for 5 years and just recently learned of one 4 hours from me. I’ll probable be making a road-trip soon. 

[RED-Z06 2,221]

My M12S did the same thing...ran perfect about 45 minutes after a hone out, new piston and rings, rod/crank was mint.  Rebuilt it, ran 45 minutes...started to rattle, and smoke slightly, at 2 hours the knock was bad, smoke was apparent.

 

Pulled it apart, rod worn, piston worn, rings worn..oil pitch black.

 

Im guessing that despite my careful but vigorous cleaning of the block...some grit was still there...and it ate everything 

[Rick3478 428]

My first thought was the oil hole in the connecting rod, should be toward the camshaft, doesn't lubricate right if you put it in backwards.  But on re-reading, it seems you covered that.

 

Edited July 19 by Rick3478
Correct misunderstanding

[Rick3478 428]

On 7/15/2024 at 3:58 PM, vaborn said:

I did find one major problem that is most likely causing my issue. I double and triple checked the placement of the ring gaps when installing the piston making sure that they were rotated 120 deg from each other per manual. But when I just pulled it the compression rings were perfectly lined up! I’m sure that was dumping all sorts of nastiness into the crankcase. Do rings ever rotate while the engine is running? If so, how do I prevent that from happening? I don’t remember spinning the piston after installing it so I doubt that could have caused it but who knows…

 

The rings are going to rotate around to wherever they want to run.  I have always suspected the 120 degree assembly thing is probably overkill that just ensures you get an even lube smear when you first start it.

[Fordiesel69 263]

There is a small hole that goes into the breather area.  When honing, grit will end up in here.

 

Never had an issue with one eating itself.  I do change oil a lot at first.  Like after 15 min, then again after an hour, then after a few hours, then switch to your permenant oil.

 

The cast iron is pretty soft on these motors,so they do tend to shred some during break in.

 

Also chinese parts for these K series are usually out of spec to begin with.  You have to machine the engine for the parts instead of the other way around.   

[vaborn 3]

I thought I washed the block pretty thoroughly but I suppose a deposit of grit could have been hiding out somewhere. I think I’m gonna get another set of rings and a new connecting rod for it, re-hone the cylinder, wash it out very thoroughly, wash it out again, maybe do that again a third and fourth time, check all specs, reassemble, and see what happens. Thanks everyone! I’ll keep ya posted.

[mrc 818]

just wanted to let any guys close to me know that westfield auto parts rebuilds/machines these old engines.   i know the owner and all the guys there.  VERY old school parts store with full machine shop out back.  

 

wetsfield auto parts  (carquest affiliated)

8-12 birge ave

westfield mass  01085

413-568-1611

 

hope this helps any of you keep our old iron going

 

regards

mike

 

[vaborn 3]

Just to give an update… The scoring on the cylinder wall was too deep to hone out without going out of spec (or over boring to 0.040) so I decided against that. I found another tractor with an M12 on it that was in good shape and bought that to use instead. I’ll just rob parts off the other one as needed. 
 

I’m pretty bummed that the rebuild didn’t work out but I think I’m gonna just chalk it up to inexperience/lack of proper tooling to complete the job properly.

The only other thing I did think about the other day was that the tractor had been setting for a long time prior to me attempting the rebuild. The fuel tank was empty and just I put a new inline fuel filter filter on and dumped fresh gas in. I didn’t think to wash the tank out to get rid of any varnish deposited inside from the old fuel. I wonder if the new fuel carried the old varnish into the engine and gummed up the rings causing the scoring? I’ve had some small engine mechanics tell me bad gas can cause cylinder wall scoring so I assume this is a possibility. 
 

Regardless of what the actual cause was. I think that if the M12 ever needs rebuilt I’ll track down a professional to do the machine work…

 

Thanks for everyone’s input.