Scored Crankshaft Question (Yanmar)

[Wotname]

Can the engine rebuild experts tell me if this crankshaft is toast or just OK?

YSE8 crankshaft big end journal. Yammer service manual states if the wear on the journal is 5 thou or more, it should be replaced. This one is 4 thou undersize but it is the scoring that concerns me (bright rings in the photos). I haven't been able to measure the depth of the scores but as undersize bearings aren't available, measuring the depth of the scoring seems pointless anyway. I'm guessing they are a couple of thou deep.

Should I toss the crank (and find a better one) or just give it a new set of bearing shells and accept that they will wear faster?

Or some other fix???

I'm not really keen to go down the "rebuilding and regrinding" path as presumably this is not a cheap process.

[Sailmonkey]

Does the scoring catch your fingernail? Or is it more a polished scratch?

My inclination would be to slap it back together and set a throttle stop about 80%, and save up for a new engine that has parts available.

[Wotname]

Originally Posted by Sailmonkey

Does the scoring catch your fingernail? Or is it more a polished scratch?

My inclination would be to slap it back together and set a throttle stop about 80%, and save up for a new engine that has parts available.

Yep, catches the fingernail if dragged across it but nicely smooth if you run your nail along it. The worst ones line up nicely with the oil gallery hole in the journal so presumably a product of hard stuff in the oil.

In a perverse way, I suppose it adds to the bearing surface area

[jimbunyard]

Polish the journal with 400 grit or so, or pay someone to do it (lightly!) and run it. What was the oil pressure before you took it down?

[Captsteve53]

Actually having a 4 cyl crankshaft ground was around $100 so not a big outlay at all and a usual engine mod every day, Under shells are normally available in sizes from a few thou up to 40 thou depending on the type of engine /manufacturer, the machine shop should double check all sizes and the mains,

From what you have indicated that particular journal needs a grind especially as it's down on diameter as well, they should grind all the journals to the exact same dimension, then your engine will be as good as new

[Wotname]

Originally Posted by jimbunyard

Polish the journal with 400 grit or so, or pay someone to do it (lightly!) and run it. What was the oil pressure before you took it down?

Thanks for the input but I don't know anything about it's past history (i.e. oil pressure etc). I didn't properly explain, this is an engine that has been made available to me see if it is good enough to rebuild. If it is, the deal will be done and some money will change hands. How much will be dependant on how many new bits have to go into it

It had no compression to speak of but now the head is off, I know why. The exhaust valve and seat was badly pitted. Otherwise the head is looking good. Although I haven't measured the piston yet, it is looking pretty reasonable. The liner has some corrosion pitting on the lower side near the head which apparently is common to the YSx series due to being a horizontal cylinder. But such parts are available. It is the crank which will be a show stopper.

If it had anymore wear (i.e. >5 thou), it becomes a no go item and if it wasn't scored (and <5 thou wear), it would be good.

However it seems to be on the borderline (4 thou wear and scoring) so hence my questions; especially as I am not an expert on engine rebuilding, rather just had DIY practice over the years.

If the consensus is that the big end journal is "doable" as it, then I will continue the tear down to see the mains.

[Wotname]

Originally Posted by Captsteve53

Actually having a 4 cyl crankshaft ground was around $100 so not a big outlay at all and a usual engine mod every day, Under shells are normally available in sizes from a few thou up to 40 thou depending on the type of engine /manufacturer, the machine shop should double check all sizes and the mains,

From what you have indicated that particular journal needs a grind especially as it's down on diameter as well, they should grind all the journals to the exact same dimension, then your engine will be as good as new

The good news is that the YSE8 is a single cylinder so only one journal to grind!
The bad news is there is no undersized bearings available for this vintage engine - at least according to all the Yanmar experts in Aust.

I will also try for a secondhand crank that is still sota good

[model 10]

Toss it. It's likely to have a hardned surface which is why they don't offer oversize bearings. Even if you reground it and could find oversize bearings that would fit, it would need to be re-Tufftrided or whtever process they use. That would ad several hundred to the cost. Welding or chrome plating back to standard are options too.
Used sounds like the cheapest.

[jimbunyard]

Don't know what your economics here are, but if the journal is round, the minor scratching won't matter, and may mostly polish off.

Don't know if they're actually available, but p/n 104200-23360 is listed as '.50 bearing' for the YSE8 engine...I assume the '.50' is mm, which translates to just under .020", which would certainly remove the relatively minor scoring.

What did the bottom half of the crank bearings look like?

The good news is that on a one cylinder engine you only need one (or one set) of everything.

The bad news is that, its an old Yanmar, and Yanmar thinks their parts are worth their weight in gold; the rod brg above goes for 40.00 USD...

[redsky49]

Find a good machinist who will have sympathy for you. They can do miracles. I think building the journal back up and regrinding is the proper way to go.

Check also for axial runout, thrust, and balance. All things a machinist will do, along with checking for cracks, oil passages, etc.

I would never reassemble an engine with a known defect as severe as that.

As for parts prices, look at Kubota or whatever engine block Yanmar used back then.

This is one time where you shouldn't settle for "good enough".

[Wotname]

Originally Posted by jimbunyard

Don't know what your economics here are, but if the journal is round, the minor scratching won't matter, and may mostly polish off.

Don't know if they're actually available, but p/n 104200-23360 is listed as '.50 bearing' for the YSE8 engine...I assume the '.50' is mm, which translates to just under .020", which would certainly remove the relatively minor scoring.

What did the bottom half of the crank bearings look like?

The good news is that on a one cylinder engine you only need one (or one set) of everything.

The bad news is that, its an old Yanmar, and Yanmar thinks their parts are worth their weight in gold; the rod brg above goes for 40.00 USD...

A photo tomorrow (maybe).
Bottom side is also scored and top (piston) side seems to worn into the shell backing.

[Wotname]

Originally Posted by redsky49

Find a good machinist who will have sympathy for you. They can do miracles. I think building the journal back up and regrinding is the proper way to go.

Check also for axial runout, thrust, and balance. All things a machinist will do, along with checking for cracks, oil passages, etc.

I would never reassemble an engine with a known defect as severe as that.

As for parts prices, look at Kubota or whatever engine block Yanmar used back then.

This is one time where you shouldn't settle for "good enough".

Thank for you input.

Yammer actually made the engine rather than used someone else's block. I understand it was originally some horizontally opposed two cylinder British stationary engine. Yanmar acquired (or were given) the rights to it (and probably the factory machinery) sometime after WWII. They loped off one cylinder and made a marine engine out of it - or so the story that I heard goes).

[Wotname]

Originally Posted by jimbunyard

Don't know what your economics here are, but if the journal is round, the minor scratching won't matter, and may mostly polish off.

..........

I guess one of my concerns is that if I remove the scoring, the journal will certainly be >5 thou undersize which is the cut off point in the service manual for replacement.

And yes, the journal is still round - at least inside a thou.

[four winds]

I guess it's my training, but I tend to trust the manual. And the manufacturers engineers and tech writers that produced it.

You have to draw the line somewhere, and they chose 5 thou. You're at 4, it's good to go for a while longer. Probably a long while considering how long it took to get where it is.

You state the score marks are you're concern. I figured that was located at the oil hole. I've hardly ever seen a crank that wasn't scored there. It's presence will not increase wear on the new bearing. Those marks rarely go away with light polishing and routinely cranks are returned to service with them present.

I've only personally rebuilt a couple dozen engines, of all types. And helped on dozens more. This is my opinion based on that experience.

[four winds]

I wouldnt attempt to remove the scoring. The engine will never know it's there.

Only you will have to work at forgetting about it.