Thread: Scratched crankshaft question

Unless you want to take it to a specialist crank grinder, I think what you are doing is correct and it will be tedious, I have done this myself to save the expense, if I remmember I think I finished it down to 800

China is in the ballpark for how fine a finish you really want. The most important thing is to have no high spots, the lows from the scratches wont be a problem as nothing can contact these and they will simply fill with oil. The attitude should only be to clean up the high bits rather than to try to clean up the whole journal, otherwise you will end up with to much bearing clearance.

My thoughts....get the crank ground professionally. If it goes wrong , it will go wrong big time.....

I dont want to sound like a smarty, but if the motor is in really good condition, why has it been pulled down ?
I also agree with China, get rid of the high spots and it should be ok.

Originally Posted by Stevenp

I dont want to sound like a smarty, but if the motor is in really good condition, why has it been pulled down ?
I also agree with China, get rid of the high spots and it should be ok.

I bought it on Ebay as part of a very good Honda generator. It was sold as "needs a new motor". But when I took the motor off the trolley I noticed from it's general condition that it hadn't done much work. According to the person who sold it to me, the motor was low on oil and someone started it and ... well you know what happened.

My theory is: it's been sitting around for a while. In the carby, for instance, the needle was stuck in the seat, so my guess is that someone tried to start it after it had been sitting around for a few years, and the dried out and stuck together bearings grabbed. It wouldn't have helped that the needle was stuck, as the motor would have roared into life at full throttle.

Anyway, I had a spare el-cheapo motor, but the generator is so well put together with throttle and idle control in sync with the generator that I want to restore it to original. Thanks for the responses.

The crankshaft grinder folks use some sophisticated cylindrical grinders accurate to 0.0001". That degree of accuracy is important for concentricity of the crank and for bearing life. You will get much better results if you have it done professionally, then buy an undersized bearing set for the rebuild. (Source the bearings first).

If undersize bearings are not available, the rebuild shop can have the journals re-chromed then ground to nominal dimension at extra expense.

You can damage a crank beyond economical repair with ad hoc sanding, no matter how even you think you are doing the job. Its really easy to introduce ovality into the crank, and that will require an even deeper grind to repair.

The relatively inexpensive crank regrind is the foundation of a quality rebuild.

Over the years I have attempted many shortcuts and employed much wishful thinking-all with varying degrees of failure.

Cheers-
Greg

Mmmm, I was a jet engine and recip engine mechanic for 13 years, just remove the high spots from the edge of the gouge and it will be fine. If you dont remove enough you will know soon enough because the crank will seize as it slowly skims the bearing and builds up the oil space with bearing slivers. Once youve knocked the edge of itll be fine so long as it doesnt have excessive clearance to start with, use some plasti gauge to check this local auto shops have it for like a couple of bucks a length. If you decide to polish the gouge out completeley then you really need to use some emery cloth, this is basically sandpaper on a cloth roll. Just use a back and forth motion until its gone. Run your fingernails over the surface afterwards and if they dont catch any where youll be fine, if they catch at any point then you gotta remove it. It takes about 2 minutes with emery cloth to knock out a scratch from a crank. Have done this on numerous occasions
Enjoy now
Steve

Hi Simso...

Just to clarify matters for me, are you saying that emery cloth is an approved repair for aero engines? If so, can you point me to an online source to verify this? I've seen aero cranks scrapped for invisible defects, I can't imagine knocking the burrs off a scratch being in a repair manual because leaving a stress riser in place is never a good idea.

Greg (Veteran of thirteen jet and recip engine failures over the last 37 years, and not so bored that I crave another one.)

Umm....... given that we are talking about a relatively slow revving 4 stroke 10hp engine..... I agree with the jet guy above. Rub off the lumps and run it.

Where not talking about a swiss watch here.

Originally Posted by Fossil

Umm....... given that we are talking about a relatively slow revving 4 stroke 10hp engine..... I agree with the jet guy above. Rub off the lumps and run it.

Where not talking about a swiss watch here.

That's how I feel. Thanks Fossil. I've seen my brother (fitter and turner) sand the crank of a diesel motor in the way I'm doing on this Honda motor, and it ran for years until we sold the truck. I would have phoned him for advice but we have this weird contest going where I pretend I know as much as he does about mechanics (I know about 5% of what he does so this is a constant battle)

Economics have to come into this. The generator cost $300, the bearings $70. If the bearings fail then I've lost $70 and I install the other engine.

Thanks for your input greg, the original post was from a guy who has already started hand polishing a small honda motor, the purpose of the board is to help if possible, clearly your view is simple take it to a specialist and get it ground. Fantastic idea but if we all took things to specialists to do then no one would do know anything but there specific trained job. Im assuming you do metal work, as you post in this board section, if so why arent you taking everything to a sheety or welder for repairs or alterations. Why are you attempting them yourself. Okay back on topic, yes mate I have hand sanded with emery cloth numerous blemishes on engine cranks, "I am a specialist in this field" this is part of what I did for a living. Online reference is not possible, if you are a mechanic as you profess you will know that all aircraft manufacturers manuals are hard copys and controlled by authorised maintanence companys, some companys have allowed for electronic copies but once again only to certified maintanence organistaions.If you take your car crank in to get reground and it is in toleraance, watch and I guarantee they will use emery cloth to clean the journals up and not regrind them. As far as tolerances on an aircraft engine go, they are no different to tolerances on a car engine. Invisible defects, thats awesome, in the last 6 years I have been working in the aircraft industry as a NDT technician, fully certified by CASA, and Ive never heard of an invisible defect. Maybe invisible to you but never invisible to me. Stress raisers exist everywhere, what happens when you damage the skin of an aircraft do you scrap the aircraft or replace the skin, or do you do the standard aircraft practice of 20:1 blend ratio. Me thinks mate you should be more knowledgeable about a topic prior to criticising someone who is offering advise. That being said enjoy
Steve

I find it rather strange that it was started with low oil level. As far as I know most of these Honda units have a built in safety that will not allow the motor to start if the oil level is low.

Would make a thorough check of all oilways and the oil pump while you have it in bits.

Originally Posted by Termite

I find it rather strange that it was started with low oil level. As far as I know most of these Honda units have a built in safety that will not allow the motor to start if the oil level is low.

Would make a thorough check of all oilways and the oil pump while you have it in bits.

It's pre-oil alert.

As it happens the bearing shells they gave me are thicker (or maybe the wrong ones) anyway, so I'll have to have the crank ground ....

And the oil alert is a major pita.
Oil gets a little dirty and motor won't run.