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8N Engine Knocking

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Richard Z.

06-19-2000 14:16:21




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I recently performed an engine rebuild on my '48. After about 10 hours time I have noticed a few things. After warming up, the oil pressure drops from 35 to 20 at full throttle. Upon hard throttle acceleration, I hear knocking coming from pan area. Under load (mower) at full throttle the knocking is constant. I would like to get a few opinions, but I am suspecting that the crank bearings may be the culprit. Is there a specific torque that the main and rod bearings should be tightened to ? I just tightened them what I felt was enough (afraid to strip) and put in the cotter pins. Any help would be greatly appreciated.

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Richard Z.

06-19-2000 22:24:46




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 Re: 8N Engine Knocking in reply to Richard Z., 06-19-2000 14:16:21  
Thanks all, I am opening her up to see. Checking bearings, clearances, alignment, and will re-torque to the specs provided by Stump.



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STUMP PULLER

06-19-2000 15:59:05




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 Re: 8N Engine Knocking in reply to Richard Z., 06-19-2000 14:16:21  
FO-4 MANUAL SAYS 35-40 FTLBS FOR RODS. 90-100 FT LBS FOR MAINS. GOOD LUCK.



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David

06-19-2000 14:43:02




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 Re: 8N Engine Knocking in reply to Richard Z., 06-19-2000 14:16:21  
Yes, the main and rod bearings must be torqued to spec. I don't know them off the top of my head but an IT manual has all that info. and more, including acceptable bearing clearances. If you replaced bearings, did you properly size them? The drop in oil pressure could be normal or it could be bad or improperly sized bearings. I think a knock under load is many times a rod bearing problem. I'm sure other more mechanic types will have more specific feed-back, but you really should consult a shop manual if you haven't already. Good Luck.

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Richard Z.

06-19-2000 14:49:00




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 Re: Re: 8N Engine Knocking in reply to David, 06-19-2000 14:43:02  
David,

I put in standard sized bearings, that were the same size as those taken out. The old bearings had virtually no wear. I don't have a shop manual, I only have the owners manual which is limited in such information. Thanks...



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Timk

06-20-2000 22:30:37




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 Re: Re: Re: 8N Engine Knocking in reply to Richard Z., 06-19-2000 14:49:00  
Why aren't you spending $25 and getting yourself an FO-4 manual from your local NH dealer. It would solve a lot of your conserns since this probably won't be the only time you work on your N.



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tlak

06-19-2000 15:27:36




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 Re: Re: Re: 8N Engine Knocking in reply to Richard Z., 06-19-2000 14:49:00  
Get some plastigage and a torque wrench. Put the gage in and torque to specs and remove cap and measure the width of the flattened gage. this will tell you if you need oversized bearings. You may just torque the caps and get by. For my 601 rods torque to 45-50lbs and mains 110-115ft-lbs. You need to look up your engine but this shows you how varied and how much pressure you need to torque. Heres a site for plastigage ,you can get it at your local auto store.

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