ONAN B48M ENGINE KNOCK

Kirk-Tx · Apr 23, 2018

Threw a rod in my 448 case lawn tractor. I took the block to machine shop and he told me what rods & pistons to order. He
reassembled it. I put everything back together and it had a knock. I took it back to the shop, he took it apart and everything
looked good. I checked the govenor & valves and everything appeared right. Took it back to machine shop running & he said "bottom
end" was ok and wouldn't hurt to run. I took it apart and to another mechanic who said everything looked good. Still has a knock.
Any ideas? I don't want to risk using it with a knock.

Ken Combs · Apr 23, 2018

Have you seen it with the heads off since the knock started?

Can you describe the knock? every time it fires or???? Tap/thud ??

if on fact the crank and rods are good, I'd suspect something on top of a piston, dropped nut, washer etc.

What was the part number of the rods/pistons you bought? Standard or o/s?

Lots of missing info here.

Kirk-Tx · Apr 23, 2018

I have had heads off multiple times and nothing obvious wrong. Steady knock as it runs. Cylinder was good enough to be honed and bought standard pistons. Rods were .010 over. I had one lifter I couldn't get out at onset and machine shop was supposed to take it out and polish it. One time when I was checking for causes I noticed it would not spin so I took it back for him to fix and he did. He pressed out valve guide and honed it. I just knew that was the problem but it was not. I don't have part numbers with me now. Thanks

Billy Shafer · Apr 23, 2018

I would go back and check that stuck lifter. That engine is a very simple engine to overhaul.How good is this shop.

Sprint 6 · Apr 23, 2018

Flywheel torqued properly?

kevinstuff · Apr 25, 2018

I think Onan had a service update, to adjust points to .016 , instead of the .021 , in other words the timing may be to far advanced. Also adjust the valves, and you may have stuck
valves

Kirk-Tx · Apr 25, 2018

What is torque specs? I have not put a torque wrench on it, but have had it off and on several times.

Kirk-Tx · Apr 25, 2018

We just had them do an identical engine for my son about a year ago with no problems. All 4 lifters seem to be spinning easily when not under pressure from spring. You are right that is very simple engine. That's what has me confused. I feel like I have had everything checked yet I'm looking at having to tear it down for the third time. Oil pressure is good.

Kirk-Tx · Apr 25, 2018

Ok, we went a little tighter than the .021 (0.020) but not 0.016. I can try that easily. I verified that valves are not stuck.

Billy Shafer · Apr 25, 2018

I will check the PSB warnings and all files I have on that engine.

Billy Shafer · Apr 26, 2018

Nothing in the PSB files for the B48G. PSB365 does say to change point gap for B48G spec C to .016 from .021 for better operation.

WGM · Apr 26, 2018

I?d be checking with Boomer about that. He is the resident Oman expert.

Billy Shafer · Apr 26, 2018

Torgue is 50lbs for that engine.

kevinstuff · May 3, 2018

Might be too much end play on crankshaft?
I once had low mileage Toyota with a bad
engine knock , it ran fine , tore it down ,
It was seized piston pin , beating the piston
to death in the bore, new rod and piston fixed that

ONAN B48M ENGINE KNOCK

Kirk-Tx

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Ken Combs

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Kirk-Tx

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Billy Shafer

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Sprint 6

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kevinstuff

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Kirk-Tx

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Kirk-Tx

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Kirk-Tx

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Billy Shafer

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Billy Shafer

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WGM

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