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Poor mans rebuild - Hone/pistons/rings


schaefer72

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hi - thinking of doing a poor mans rebuild on my guttless 20 hp onan - i have tried adjusting everything there is but it still seems to lack power and has bad exhaust - wondering how much help it might be to hone the cylinders and replace the piston and rings only - to help add more compression and reduce oil burning - how many others have done this and do you think it would help - it could be a fairly easy 1 or 2 day project - i see nos onan parts all the time i could get ahold of - i have a 16 hp briggs on my 7016 the prev. owner did this and it seems to run great ? - your thoughts ? what are the thoughts of the engine experts out there ? - thanks alot
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I have re-ringed my B & S Model 19 several times. Did the valves also last time. Nothing ventured, nothing gained.
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Jason, I've had mixed results doing the (poor man rebuild), it just depends on how much cylinder wear your engine has. If you are going to the expense of new pistons you might want to have the cylinders checked for wear and being out of round. Also might want to check the valves, JMO...:) I'm currently working on a Briggs 19D and am trying to keep the cost down as much as possible, piston, rings, grind valves, new gaskets and seals.... Good luck,:)
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I agree with Mike, if you are going to get new pistons, I would have the cylinders measured. If you have them rebored, it shouldn't increase your cost more than $50 to $75. I'm not real familiar with Onans but every one I have overhauled has required a rebore. Pat
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I have done rings only, several times. It is usually because of stuck rings, a rusted cylinder, or overheating taking the temper (springiness) out of the rings. I've always had good luck. As I have said in previous posts, you need to know what you are looking for when you take the engine apart, so that you check for other problems. I usually replace only the necessary parts, and sometimes play the odds on wear to keep the rebuilds reasonable. I wouldn't do this if I was rebuilding one for someone else.
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I would re-ring it to see if it helped, it never hurts. Heck most guys that ride dirt bikes hard rebuild the top end every few weeks to ensure good power. If it doesn't work, the try a rebuild. It is alot cheaper to try and fix something with a few bucks then going all out and finding it could have been a simple solution. -BradW
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  • 2 weeks later...
Jason: While I agree with Sam, it is in my nature to do the poor man's rebuild, for all of the reasons given above. I would do it, but would rebore if I needed to got to new pistons.] However, this brings up a question I wanted to pose to our engine experts: Some of my friends run Lycoming-powered airboats up on the ice, and one advocated getting new rings that were just one size (say, .010) oversize, and then cutting down the endgap of the rings slightly, using either a wheel, file, sandpaper, etc..., until they would just fit into the bore with the factory end gap spec. given for the correct sized rings. They say this helps "fill out" or seal against the bore which may be honed slighlty oversize, but is not yet big enough to require a rebore/new pistons. Has anybody heard of this? If not, what are the opinions of this practice? Good luck with the rebuild. Peter
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Peter, First you need to see what kind of ridge is in the cylinder. This is usually indicative of the actual cylinder wear. If there is little or no ridge, then use stock rings. If there is a significant ridge, but you don't want to overbore, the use the re-gapped over size rings. I wouldn't recommend more than .010 oversize rings without a bore job. This is done to replace the ring compression which is lost due to cylinder wear. Be careful with this technique because there is less ring inside the piston groove which will mean more wear within that groove and can also result in broken rings. Many such techniques are used in race engines, but these guys are not looking for long life, they are looking for ultimate performance. Been there, done that. Also there is no need to replace the piston, which is a high dollar item, unless it is bored oversize or the ring groves are worn significantly out of spec.
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Stoneheartfarm
I've had both good and bad luck with re-ring jobs. I think it's worth a try, but alot depends on how good your eye is and how lucky you get. (ie, no major block/crank problems.) Steve
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If the cylinder is within specs after the honing then reboring is a waste of metal (cylinder and coin). At least that's my 2-cents.
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Well, I have a 18 hp Onan that needs a rebuild and all I hear is that the parts are very expensive, so If you've got a line on cheap parts buy all you can. MHO
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I would just try rings and hone it down first. If you are gona put pistons in ya better get the block checked for wear. We ringed and honed a tractor a few years ago and it runs great It use to suck more oil then gas but after the overhall it hardly burns any oil. The reason we honed it insted of putting sleves in it was because sleavs were not avable for it. BigSix talked about regroveing the pistons we did that on this tractor also because we would of had to have new pistons custom made in Calforna. We had Arnolds regrove our pistons and you used the standerd ring set and added spacers to make up for the oversizeing.
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A friend of mine had an Onan rebuilt by an automotice machine shop a few years back. They actually fitted it with automotive pistons and rings, because of the cost and difficulty of finding replacement Onan pistons. Runs great. Best that he could find out, was that Onan had designed that engine as a discardable engine and built no rebuild parts for it. According to the engine owner, he was told that Onan did many of their engines that way, to keep down the cost of building and storing extra parts. He says that if it is difficult to locate the part numbers on the engine, because they are not located in a very obvious place, that engine is probably one of the throw away types.
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on my jobsite we have several welders powered by the onan engines. i have discovered several problems with trying to overhaul them. they love to gall valves and you will break at least half of the head bolts attempting removal. i agree with the above comment that they were produced as a throw away engine. a complete overhaul kit , all parts included is about 1500 for one of ours, a reman engine is about 1200. figure that one out. just thought id get in on this one!! later good people!!
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