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Onan Bf/ms Rebuild Dilemma


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#1 dthomp17 ONLINE  

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Posted August 10, 2012 - 12:44 PM

I have a 16hp onan BF/MS that has been burning a significant amount of oil. It seems to run OK but fouls plugs regularly. I finally decided to pull the heads and see what it looked like. There's a lot of oil in the cylinders but I think it can be rebuilt, The problem is that parts are very hard to find and VERY expensive. I haven’t been able to tear it down completely yet because I can’t get the flywheel off and can’t get one of the set screws out of the drive pulley. The cylinder bore specifications for this engine are very tight at 3.125 + 0.0005 inches. My cylinders are not scored and look pretty good but the measurements are 3.128 “ which is out of spec, but not too bad. I’ve looked for 0.005” oversized piston rings but can’t find any (0.010” oversized rings are available at almost twice the cost of standard rings). Assuming the crankshaft and cam are OK, I can do a basic rebuild with new standard rings, valves, and valve guides for about $500 -$600 dollars, or I can get it bored to 3.250 “ with new pistons, rings, valves and valve guides for $800 - $1000 dollars. If other issues are found after complete teardown, they would increase the cost. The best approach would probably be to re-bore but is it worth the cost? Another obvious issue I've noticed is that one of the cylinder block threads that connect the exhaust pipe are in very bad shape and not sure how this could be repaired.

At this point, I’m thinking I should consider a re-power with either a Briggs, Tecumseh, or Kohler if I can find a good used one. There’s a good looking Honda GX390 on Criagslist for $250 but that would be less power (13HP). It would probably still do anything I needed it to do. Any thoughts?????

#2 John@Reliable OFFLINE  

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Posted August 10, 2012 - 01:02 PM

I think it's worth rebuilding,considering if done right good for another 30 years, hard to hurt a Onan. Try to stay away from Cummings for parts, I have found that searspartsdirect.com of all places still have good prices and available on many part #'s. Also some Onan only groups with a lot of great information too.

P.S. breather and reed valves can cause a lot of oil burning too.

Edited by John@Reliable, August 10, 2012 - 01:04 PM.

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#3 PappyH OFFLINE  

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Posted August 10, 2012 - 01:32 PM

I am in the same boat. I have rounded up almost all parts I need to rebuild mine but I still need bearings and gaskets. It's really not a rebuild as I will be using alot of used parts. I have decided to give up on the Onan and find a good used B/S or Kohler since this is a working tractor and I will be spending about the same money. Most parts are still available for B/S and Kohler at a reasonable cost.

#4 olcowhand OFFLINE  

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Posted August 10, 2012 - 01:36 PM

Go to Onanparts.com. He can fix you up. click here
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#5 Guest_rat88_*

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Posted August 10, 2012 - 01:47 PM

I looked into rebuilding a 16hp twin with a busted rod. after getting started on the teardown, I found that it required special tools to get the flywheel off. more than just a flywheel puller, but I dont remember the details. ended up buying a runner for $200 and sold the tractor

#6 olcowhand OFFLINE  

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Posted August 10, 2012 - 01:52 PM

A simple "crows foot" puller removes the flywheel, but the problem is pulling the crank timing gear off. I had to make my own puller to do this, as a bought one is pricey. If crank bearings aren't worn (check with dial indicator), then you won't need to remove the crank gear or crank as long as the journals are within spec also.

#7 dthomp17 OFFLINE  

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Posted August 10, 2012 - 02:21 PM

I've been using a "crow's foot" puller on the flywheel with no luck. I've squirted penitrating oil on it and let it soak, I've put just about all the pressure I can put on it without breaking the puller or the puller bolts. I've tapped the puller screw with a hammer hoping to break the flywheel loose and so far nothing. Also, one of the drive pulley set screws is stuck and won't come loose. I think I have wallowed out the head of the screw a little trying to break it loose with an allen wrench. I don't think you can drill it out without a special drill bit because I assume it's hardened steel.

Any suggestions on the damaged exhaust threads in the engine block?

#8 dthomp17 ONLINE  

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Posted August 10, 2012 - 02:30 PM

Another thing, based on my cylinder measurements and the bore specifications, If I choose to rebuild, should I only consider boreing with new pistons/rings etc?

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Posted August 10, 2012 - 02:38 PM

the onans (atleast the 1 I had) had a place by the port that looked like it should or could have been drilled and tapped for a flange mount pipe, similar to the briggs style.

#10 olcowhand ONLINE  

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Posted August 10, 2012 - 02:51 PM

I've used engines with that much or more wear and gotten years of service from them, but I would advise boring for the longrun. As to the flywheel, it takes a very tightened puller & a good whack with a heavy hammer to pop them loose. When they pop loose, you'll swear something broke. Just be sure the puller center bolt doesn't try to push into the flywheel retaining bolt hole. As to the bad threads, you could always drill & tap to the next size, then install a reducing stud w/nut (3/8 to 5/16) instead of using a bolt. HERE is where to find this reducing stud. lock two 5/16 nuts onto the top portion, then using the top nut, tighten the 3/8 portion into the newly tapped hole. Be sure to red Loctite the lower portion into the block.
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#11 dthomp17 OFFLINE  

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Posted August 10, 2012 - 06:11 PM

Do you have any suggestions for the stuck set screw in the pulley?

#12 olcowhand OFFLINE  

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Posted August 10, 2012 - 06:14 PM

Do you have any suggestions for the stuck set screw in the pulley?


I have drilled them out with a quality drill bit before. Drill a smaller pilot hole 1st, then one large enough to just clean out to the threads. I wouldn't try drilling a small hole and attempting an easy out tool. If you break one of them off in there, you sure can't drill it out!
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#13 Guest_rat88_*

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Posted August 10, 2012 - 09:18 PM

if the stuck set screw is the one on the key, you might be able to pull it off with a puller. Not my first choice , but I have had to resort to that a few times. Too much force could snap the pulley

#14 olcowhand ONLINE  

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Posted August 10, 2012 - 09:49 PM

If this is on a Sears Suburban or similar, those pulleys are wide, and a real beast to come off, even with the set screw out.

#15 dthomp17 OFFLINE  

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Posted August 11, 2012 - 06:16 PM

The stuck set screw is on the shaft, not on the key. I thought about trying to pull it without removing the screw but decided against it. Figured I'd break something for sure if I did.

I looked at my exhaust port again and it has no provisions for studs or any place that you could drill and tap a place for a stud. It only has a hole with standard pipe threads that are pretty mangled.. You could possibly clean up the threads a little with a pipe thread tap but not sure it would be good enough to hold the exhaust pipe tight. I took a picture of the mangled exhaust port hole but can't remember how to upload it.

Edited by dthomp17, August 11, 2012 - 06:17 PM.





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