cyl bore tolernaces - follow up to mashed push rods

[clinter36]

This motor is a Shibaura N844LT (also Perkins 404). If you read my previous thread this rebuild started with bent push rods.

The head and crank are off to the machine shop and the block is completely stripped. Everything inside this engine with the exception of the push rods has been found in relatively decent shape. The pistons themselves are pretty scored up, but it does not seem to have hurt the cylinder walls.

I spent the weekend measuring the cylinder bore diameter and I would like opinions on if the block should be bored oversize or if I should just hone it and call it good.

The manufacture specs are: 84mm - 84.019mm with the "repair value at 84.2mm (That is .008" over factory spec)

below is my measurements if you care to read all of them, but the generalization is that the cylinders are .001-.002" oversize inline with the crank and .002 - .0025" oversize across the block perpendicular. (That is .0025-.0508mm) so I fall outside of factory specs, but inside of the repair value. Do I hone it..which I have to assume will take at least another .001" off, or do I have it bored .020" by the machine shop? My concern with machining it, is who is to say the machine shop will get it perfect, and the obvious extra cost.



If I read about modern auto engines, they seem to be playing with much higher out of round tolerances something like .00015. I hate to say it but I am not sure this engine is deserving of that kind of precision.

My second questions is there is a little bit of aluminum stuck on the side wall of the cylinders from the piston. I am not sure if I should hone it away, or if the hone will jump a little over that causing an odd wear pattern. Or if I should try to get the aluminum off before honing.. and how I might do that.

thanks
clint

 

[blee03]

As for the measurements and cylinder specs, I’m not exactly sure so I’ll pass on that one but you should definitely post up a few pics of the cylinder walls if you can. Are there any scratches in the cylinder walls at all? How is the cross hatching?

You should be able to use Muriatic acid to remove the the aluminum from the cylinder walls, but definitely do some research on this first

 

[ericm979]

Are oversize pistons available? And not too much more expensive than the stock bore size you have to buy anyhow? Is this rebuilt just so you can sell it, or do you plan to use it for a while?

 

[rScotty]

I haven't built an engine in years; back when I had my gas engine automotive/motorcycle repair shop we did at most maybe one a week. Your existing clearances look good to me - but I don't know diesels as well. Those numbers you give are what we hoped to find when taking an engine apart for some other reason than piston/bore failure.

In either case when we opened an engine and it measured your numbers at my shop we would give it a light hone - if it didn't already have one - clean up the pistons, gap a set of new rings, and reassemble with every expectation that part would of it would run like new. As for the out-of-round, we usually saw that in any used bore. It is a typical result of the rocking forces that the piston puts on the cylinder wall. Pistons aren't made perfectly round and for that matter I doubt they are round after they heat up and expand - especially if they have slipper skirts. I'd always figured part of piston shape was to compensate for shape of a normal cylinder with slight wear - but I don't know that to be true.

We didn't do what I've just describe because it was the best possible mechanical repair, but because it was good enough for our customers and an engine re-ringed in that way always seemed to last long enough that we didn't see the come back with that as a problem.

As you say, if you do go oversize you are taking a slight chance on whether the bores really are better after coming back from the machine shop. But generally they are. I always found that a good automotive machine shop with an experienced machinist would do remarkably accurate boring. It's what they do - or at least that's the quality of work they did some decades ago. Do you have a machine shop you trust? That's critical.

I don't know what to say about the smeared aluminum. I've seen it myself - but it always meant the piston had to be replaced anyway and that meant a re-bore was no real extra expense. The aluminum obviously cannot be allowed to stay. I hope someone else will chime in. I suppose one option would be a light honing with the expectation that would remove the aluminum, if it didn't you are not in any worse case than you are now. I might do that on my own machine, but would not recommend it on a customer's.

In the end, it's that smeared aluminum that is throwing us a curve. We don't know what is under there. Given that - and the fact that the block has to be mounted and & squared up by the machinist to anyway if he is going to check & flatten the head gasket surface - and the additional fact that you apparently are going to have to be buying new pistons anyway .....well, I think I'd bore to match the new pistons. What is the additional cost? ten to twenty a hole? Not enough to matter.....
I WOULD NOT bore until I had the new pistons in hand.
Luck,
rScotty

 

[blee03]

I WOULD NOT bore until I had the new pistons in hand.
Luck,
rScotty

Great point here! If you do go .020 over, the pistons need to be measured first. Afterwards the cylinders can bored to match them with correct piston to cylinder wall clearance

 

[TractorGuy]

Not sure what to make of your measurements or how you obtained them. Are you using a dial bore gauge? How worn ARE the pistons? Are you planning to replace them regardless?

My feeling is if the pistons were scratched up a little but still round and in spec I would allow up to .003" clearance for a wear factor and ring it. Any time I have just done rings and bearings I prefer to use a ball hone and just hone enough to break the glaze. No need to remove material or try to square and out of round bore with a hone.

Another option is if you can find .010" over rings you can hand file the end gaps for a better seal.

If it's worth the investment to you then boring oversize with new pistons would be the way to go for a longer lasting rebuild.

No matter how you go you still need to determine why the valves made contact and what needs to be corrected so it doesn't happen again.

 

[dodge man]

My thought is if you getting new pistons and can get oversized, get it bored. A good shop should be able to improve it.

 

[denchen]

I`m not quite getting this. The repair value you say is 84.2 mm, or in other words 84.2000mm. You have wear of 84.0508mm. On a 84mm bore you have wear of less than .05 of a mm. I would just lightly hone and fit new pistons if the old ones are damaged.

 

[clinter36]

I`m not quite getting this. The repair value you say is 84.2 mm, or in other words 84.2000mm. You have wear of 84.0508mm. On a 84mm bore you have wear of less than .05 of a mm. I would just lightly hone and fit new pistons if the old ones are damaged.

Yes, this was my first gut reaction as well, but then I start thinking about it, and as has been pointed out... what is the additional cost in the grand scheme of things to have it done to the best degree possible? The answer is, its about 500-700 for the block work and I also have to do something about the cam bearings (which the machine shop will deal with, but nothing is free). That's not insignificant, but its also getting the block to the machine shop, 4 weeks lead time, repainting it when it comes back, etc etc.

I have a machinist straight edge and I have checked the deck on the block and I can't find any gaps with a .001" feeler gauge.

Thanks for the great feed back from everyone. My intention is to put a grapple on this machine and use it for a year or two cleaning up about 20 huge pine trees that have fallen and were felled at my rental property. Then I will probably sell it. I have a decent size Kubota tractor for general purpose stuff. If I end up keeping it, I think I would be hard pressed to put 20-30 hrs a year on it. I have owned my Kubota for more than 10 years and I think I put around 500-600 hrs on it.

But regardless of how I use the machine or what I end up doing with it. Since I am going through the trouble of rebuilding it, then I am going to do so with the expectation of getting 3-4k hours out of this engine again. And its not just the cost of the block work. I own a machine shop and I know how difficult it can be to hit high tolerances - all it takes is a little tool chatter and you are screwed. And I don't know anything about the auto machine shop I am using, except that he was recommended by a car engine place because they wont touch diesels.

I am leaning toward trying something to get the aluminum off and honing it... I feel like the devil you know....

clint

 

[jaydee325]

I've used a rigid hone to get taper and out of round within service limits. Pistons were then taken to a machine shop and a light knurl put on them so they can be fitted to the bores. Oversize rings were then filed to achieve proper end gap.

This certainly does not result in a "rebuilt" engine, but can get you going in good shape and at considerable cost savings to custom machine work. My engine worked well for several years until I sold the rig.