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Slight Onan engine questionnaire

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Old 08-27-2008, 10:45:52 PM
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Lightbulb Slight Onan engine questionnaire

OK. We have a Onan Model P220 and it goes on a whel horse 520-H getting down to the nitty gritty, it was a fine runner as an Onan usually is but then the engine reportedly started to caugh and miss and now it only fired on one cylinder. SO i did all the symptom checks what not and so forth and found by looking through the intake port at the top of the engine that the valve seat came loose. UH OH. Great. So i pull the engine and pull the head thats closest to the tractors operator (note this is a twin cylinder) and pull the intake valve and the seat falls right out with it. Extreemly loose i should say and in the past it appears as if the engine has had that very same trouble (peening marks) well the seat was so peened it could be peened no more so then i tried a trick of welding tiny spots around the seat and making it to were the seat could be pressed in and it would literally jam it's self in for good, which would have worked if part of the intake port nearest the seat was busted out (not by me) So then a new block came into order and so i tore the engine down and just got done refreshing a good but used block (valve seats solidly intact with the engine) so its soon to be re-assyembled as rebuilt. Soon as i get the parts but heres the kicker, while tearing down the old block to use the internal and external parts on the good but used block, the OTHER intake valve seat on cylinder 2 just willingly popped out I was amaized, sorta, but it doesnt appear that, that side of the engine has had ANY problems in the past what so ever, so im wondering did the P220 Onan's have problems with there valve seats or was this engine just one of the unfortunate victoms put through H*ll in it's time? I had to wonderbecouse if they were noted for that....then what if something like that happens to the other replacement block? The seats seem to be solid in this other block but i did peen lightly around them in good pattern for just a safety measure simply becouse its not pleasent for me to rebuild an engine then have to tear it down again. I couldnt have hurt anything. Just help it a bit. Maybee im just rattling on here for nothing, after all it's not my peice of machinery. It's my dads freinds tractor. Im just trying my best to eliminate feauture potential problems. Any insight? I guess i just worry to much.
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Old 08-28-2008, 10:15:19 AM
CJBennett CJBennett is offline
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Default Re: Slight Onan engine questionaire

Don't worry. You'll be fine, and so will your engine. Usally the seats pop out from over heatin , Just don't over heat it,..
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Old 08-28-2008, 11:39:34 AM
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Default Re: Slight Onan engine questionnaire

Thats sorta what i was thinkin. Overheating. Corce the cooling fins apear to be clean so maybee it was run low on oil at one time. I guess i'll mention this to the owner. Check the oil, pay attention to the temp. and dont overheat the engine. ok. well back to the 'ol grind.
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Old 08-28-2008, 12:51:25 PM
Ed Radtke Ed Radtke is offline
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Default Re: Slight Onan engine questionnaire

Next time ,have new oversize seats installed.Its cheep enough.
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Old 08-28-2008, 02:35:43 PM
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Default Re: Slight Onan engine questionnaire

didnt know onan offered oversize seats...ahwell, i got another block cheep enough The old block had part of the intake port busted out nearest the seat so it made the seat cock sideways everytime the valve were to come down.
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Old 12-09-2008, 08:58:36 PM
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OnanParts OnanParts is offline
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Default Re: Slight Onan engine questionnaire

Over the years I have come across several Onan blocks with the loose seat issue. Always an intake seat. I think there was a production run or runs where the seat bore was not deep enough to allow the shoulder of the bore to be peened over with enough metal to "lock" in the seat.

Onan only has one oversize seat available, part # 110-0197-10 and it's only +.010" in most cases by the time you discover a loose seat a +.010" won't be of much help. The other issue is that the seat bore gets hammered deeper and a new oversize seat will sit too low.

Several options: Weld up the seat bore and have it machined back to accept a STD seat, replace the block or use an oversize seat not available from Onan. The stock bore is 1.4" and I know of a source for suitable seats that are 1 1/2" & 1 9/16" OD and thicker as well to take up any depth problems.

I have used these larger seats 8-9 times with no problems as long as they are installed properly with a correctly machined seat bore. If you get stuck in a corner PM me and I will point you in the right direction.
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Old 12-10-2008, 12:05:43 AM
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Default Re: Slight Onan engine questionnaire

Well i thank you but it's already a done deal. I found a block a while ago. Good used one, rebuilt it, put it back on the tractor. Customers happy so far.
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Old 12-11-2008, 04:09:28 PM
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CaseyJoeS. CaseyJoeS. is offline
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Default Re: Slight Onan engine questionnaire

In the case on an intake valve seat falling out, it is either from a design or production defect. It would not be caused by overheating, because the exhaust valve seat would fall out first because it runs so much hotter than the intake seat does. In this case it sounds like it was design problem.
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Old 12-11-2008, 05:54:13 PM
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ListerDiesel ListerDiesel is offline
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Default Re: Slight Onan engine questionnaire

We have seen inserts that have been plated up on the OD to make them tight in the head. Copper plating is usually used, or Chrome if that is available.

It's very much a question of tolerances between the hole in the head or block and the OD of the seat. If you don't need to use dry ice to shrink the seat to fit, you're probably heading for a loose seat sometime.

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Old 12-11-2008, 07:36:15 PM
Patrick Patrick is offline
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Default Re: Slight Onan engine questionnaire

Too true. My neighbour, with a big garage full of classic cars (and beer!), had his MGB head rebuilt to run on unleaded. Hardened seats, etc.
2 months later he got a really bad misfire and unpleasant noises - we thought it had dropped a valve head, as he drives it pretty hard.
Took the head off, it had dropped a valve seat, which then broke up and killed the head, the piston, and both valves in that cylinder.
On asking the rebuilder, who was actually a member of the FER, (Federation of Engine Rebuilders", I think) he said "We just press them in HOT on a COLD head". Well - they're going to fall out then, aren't they.
We had to go to the FER arbitration service to get it fixed for free, and about 6 months later they were knocked off the FER member's list.
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Old 09-21-2016, 05:27:11 PM
doug9694 doug9694 is offline
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Default Re: Slight Onan engine questionnaire

I had a suspicion it may be a valve seat. Seen this on many Briggs! But on the ex. mostly. I will be taking the cooler side off that the owner said has the problem.
Onan p218c 25 years old. Maybe I can bring it back cheap if there is enough metal left. Or talk him into a new engine. I will use the design flaw line if it turns out to be the intake.
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