Well, the engine has 800 miles on it. KB hypereutectic (lightweight) pistons, 6in rods, Childs&Albert moly/ductile file to fit rings (top ring gap @ around 0.030). Bored 0.030 over, decked, balanced. Anyway, does anyone have any ideas what would cause this kind of bore wear. Its only at the very top and bottom of the bore, as shown by the pics. My best guess is that the machine shop allowed too much bore clearance for hyper. pistons, and they have been rocking in the bore. Id say the situation is made worse due to the short skirt of the piston. Does anyone have any ideas?

 

Did you measure the bore dia and piston dia when you put it together?

It looks to me like the piston is too tight, not too loose.

It may also be real tight on the pin.

 

Another guess...
Wristpins are offset with many types of pistons, and installing a piston with the pin offset in the incorrect direction causes abnormal wear.

 

The pistons have only two valve reliefs (different sized intake/exhaust), so they can only go in one way.

No I didnt measure the bore or piston diam., maybe I shouldnt trust the machine shop so much huh? The pistons seemed to have a fairly loose pin fit, Im using full-floaters if it matters.

Thanks for the tips guys.

 

If the pins are offset in the pistons you have (and I'm not sure they are) It isn't nessecarily the valve relief that determines where the piston goes. Part of that is determined by the offset in relation to the crank rotation.
In other words, the pistons on the left bank will be offset in the same direction as the ones on the right bank, in relation to crank rotation, but 180* opposite in relation to the valve reliefs.
Pistons come in sets that are made for either the right or left side and you can get the valve reliefs right but the offset wrong by turning the piston upside down and installing it on the opposite side that it was made for.

I'm not good at explaining things like this sometimes. I hope I didn't confuse you.
Maybe someone else could paint a clearer picture.

 

I had the same exact problem with my 383 after just about the same mileage, pulled the heads off to swap them and wham, wanted to cry. the only difference is my motor only showed that wear on the #5 hole... I was loosing oil too, the combustion chamber was all oiled out.

 

File to fit rings.... hhmmmm, Ok, was the engine bored using a plate? Have you pulled a piston to see what it looks like? I would have to agree that it looks like the piston is being forced to move on the pin, either by the pin being too tight, or by the bore not being square/round. Make sure you file those rings correctly and with the correct amount of gap.
The more I look at this, I don't think the bore is round.

 

Did we establish:

If the bores are actually round?

If the bores are tapered?

If the skirt clearance is adequate?

And 0.030" sounds actually a bit loose for a ring gap, but I know the hypers are a little different.

Can you get a feeler in the top shoulder above the top ring land? How thick?

 

Im not sure how to tell if the bores are round or tapered. And Im not sure about the skirt clearance, either. Unfortunately I didnt make these measurements before assembling the engine. I thought the ring gap seemed excessive as well, but KB listed a 0.0075 end gap factor, and I decided it would be better to err a bit on the high side rather than risking the ring ends butting. As far as the feeler gauge, the measurement ranges from 0.005 at the smallest to 0.010 at the widest in places. ???

Something else to note, in the third picture I posted, the piston on the left doesnt seem to be leaving any carbon ridge between where the ring stops and the bore stops. But the one on the right seems normal. There is a bore on each bank that isnt developing a ridge. Seems odd to me.

 

Seems odd to me that...

You have apprx .005 taper..
and you have any ridge at all...

with 800 miles



My guess is you didn't get the machine work done that you paid for.


How reputable was the shop you used?

 

Ive never had work done there before, and no one I know personally has either. Maybe he didnt know what I meant by "torque plate hone".

So... Chances are this engine is scrap, probably not much I can do outside another rebuild.

 

The torque plate honing or lack of, isn't really the question, IMO, I don't think you even got 8 holes machined...unless they were extremly worn when you took it in, but a decent shop should have told you that you'd need to go more on the bore to make it correct.

 

The .030 " on a ring gap is set up like a worn motor. Looks like scuffing from too tight. Also looks like the rings were cutting in. When you file fit, did you take a small stone to deburr the filed edges?

 

Im not following here. The pistons are 0.030 over, wouldnt it have been impossible to assemble the engine if I had an unmachined cylinder? In any case, the bores looked great before I assembled the engine, and there were no gouges or scratches in the origninal bore before the machine work.

Yeah I deburred them after filing.

So, in your opinions, can this engine last through the summer without a catastrophic failure?

 

Just by looking at the pics, which isn't exactly a science....it kinda looks like the piston's in too tight.

I know you don't want to hear this, but I'd check for roundness and exactly what the bore diameter really is like mentioned above.

If the measurments are out of whack, you'll end up yanking the entire engine out to have it re-machined. And I agree with the above - if that's a ridge I'm seeing near the top of the cylinder after 800 miles of run time...there's something wrong.

 

Maybe you accidentally picked up someone elses block at the machine shop... or the shop gave you the wrong block.

 

Don't most pistons have a barrel finish above the piston skirts and below the ring lands?

 

I think your original assessment is correct- she's set too loose. You only need .0020-.0025 piston to bore clearance with those pistons. Way less than a forged piston, similar to a stock cast piston. Piston is rocking back and forth on the thrust side and slamming the top and bottom edges of the piston into the bore as it passes TDC.

KB's require some MASSIVE top ring gaps. File-to-fit rings are the only thing that will work with them. Top ring gap should be no less than .026" on a 4" bore for a street N/A application. For a street nitrous/blower application they should have a minimum .032" gap. Most pre-gapped ring sets will be around .018" on the top ring- way too tight for KBs.

However, if you were running the top ring too tight and butting the ring ends together, believe me, you'd know it. Chunks of the piston above the top ringland would be ripped off, instantly destroying the engine. Ask me how I know.

 

i'm willing to bet that they machined the bores for forged pistons. Did you give the machine shop a finished dimension? Hypers are 2-2.5 thousanths. Forged are over 5 (i believe). The difference is enough to cause problems. I think your ring gap is fine, i have the same pistons, it's accepted that you can go .040" without any loss of power, but you NEED over .026 with the KB's they hold more heat in the combustion chamber. You should also check the pin offset and make sure its right.

 

also, does this appear on both cylinder banks? that may be an indication of the pin offset being wrong.

 

hence the need for fully blueprinting an engine before final assembly. i would never trust any machine shops word on anything. ive seen way too much sloppy work done so id document every clearance before any engine gets put together and fired up.

 

There is really no such thing as "machining for forged pistons". The piston wall clearance is built into the piston diameter, NOT the block bore diameter. If the shop bored the holes to 4.030 and the pistons are .030 over pistons, bore diameter is most likely not the problem.

If you have a KB piston that specifies a 4.030 bore and .0025 bore clearance, the machine shop bores and hones the block for an exact 4.030 diameter. The actual diameter of the piston in this case would be 4.0275 at the factory specified bore clearance of .0025. This is where the clearance comes from.

In certain cases of extreme use, the machine shop may be told by the customer to bore to a different clearance other than what the piston manufacturer specifies, happens, but is not very common (Top Fuel).

 

Also,

Spend $50.00, get a dial bore gauge, and learn how to use it.
Measure everything you get back from the shop.

With the economy the way it is these days, you simply cannot trust expensive parts to machine shops who may or may not hire qualified machinists and who may or may not keep up the maintainence on their equipment.

I once picked up a block from a shop where I arrived a little early. They were honing a block and the Rottler machine was making a funny noise. I looked at the honing head and saw that the stones were almost completely worn away with metal on metal contact. The 18 year old kid let the machine finish and then took the block over to the cleaning tank where he cleaned and wrapped it up!!! I'll bet that the the last 5 or 6 blocks done on that machine had problems. I picked up my .030 over block and took it over to another shop where it became .040 over block.

Checking the shops work is a MUST.

 

You're dead right.
The only real difference comes in with the finish home for the type of ring that you're running.

 

I can fit a 0.010" feeler gauge between the top ringland and bore. (at the top of the bore) That seems excessive regardless of whether the clearance is built into the piston or bore.

 

good point on the boreing, i forgot all about that. It is important to measure your pistons though before you get the block honed to be sure that the clearance is right. I measured my pistons, and sent one in with the block so they could be certain that the finish hone was right.

BTW, where can you get a decent dial bore gauge for 50 bucks? My friend lends me his fowler but that was close to 200!

 

Finish hone is only for ring break in and not specifically for piston fit.
So a block receiving .030 over pistons would be bored lets say .027 and the last .003 is the finish hone taking it to .030.

 

The ring lands are always smaller than the rest of the piston, especially at the very top. That's not an accurate way of guessing bore clearance.

Incidentally, I just noticed that the wear pattern continues above the top ring.... so whatever it is/was, it can't be the piston itself, since the piston never touches the bore up there during normal operation. That pretty much requires that it be foreign matter in the cylinder, or something that happened during assembly.

So, just out of curiosity, why is a new motor with 800 miles on it being torn apart in the first place?

 

The engine was running very poorly. I hooked up a leakdown tester while trying to diagnose it and found that it was loosing a lot of pressure right out the intake/exh. ports. Looks like the valves arent sealing, but thats another problem altogether...

 

-double post

 

Laiky,

I bought a Dial Bore Gauge from this guy off eBay.

http://cgi.ebay.com/ebaymotors/ws/eB...category=43989

Its Chinese, but, it reads to .0001 accurately....just fine for casual hot-rod work. Also comes with all of the extensions and such.

Like anything else, the skill in using a tool to its potential is usually on the human end, not the tool. I frequently tell this to a golfing buddy who recently purchased a $500.00 Callaway warbird driver, and, whom I consistently outdrive by 40+ yards with my plain old, $75.00 Taylor Made Burner.

Peace.....

 

i agree. But good tools do make a difference. i have been borrowing a friends fowler. It is accurate enough that temp variations make a difference.

 

If the engine was running badly it's entirely possible that you simply washed the rings out. Too much fuel/incomplete combustion and you get liquid fuel washing down the bore, killing the rings and also causing highly accelerated wear.

 

perhaps whatever damaged the valves scuffed the cylinders? Did you ever run this motor without an aircleaner? Maybe you ingested something abrasive that chewed up the valve seats and scuffed the bores? Doesn't seem like its a coincidence that the valves do not seal and the bores are damaged!

 

I have a feelin my engine ingested something to get similar marks to the ones you observed.... a few of the skirts had scratches going up and down... nothing some 1500 and scotch brite didnt take right out, but something was definately traveling on the walls. I am sure you plan to, but you really need to take the whole bottom end apart to tell the whole story... check all your bearings closely for contamination, how did the head gasket look? check your ring lands closely and the condition of the coating on the rings (assuming you were running coated rings).

 

I guess thats possible, but if it were foreign material, its pretty odd that all the cylinders would have a very similar wear, and in the exact same spots.

Hmm. I bet youre right. The inside of the intake was literally wet with gas when I took it off. The oil doesnt seem like it has gas in it though...

 

[QUOTE]Originally posted by formularpm
[B]I guess thats possible, but if it were foreign material, its pretty odd that all the cylinders would have a very similar wear, and in the exact same spots.



If you did injest something like sand or dirt it would be in all the cylinders, at least most of them. I would also imagine that the thrust side of the pistons would see the most significant scuffing. I think the wear might just be in the same places in each cylinder.


You didn't mention, how well did the engine run before you took it apart??was it smooth and powerful or did it run rough and idle pooly, backfire etc.

 

I'm not certain about KB pistons, but the Speed Pro's I have call for a clearance of .001-.0015". That's tight as a .... well, you know.

Look up the clearance requirements for your specific pistons. I'd agree with Damon that the clearance is too large and the piston is rocking excessively.

 

Where are the pics?
I'm in the middle of a teardown and I remembered this thread because I was seeing shiny spots on my cyl walls like the ones that were shown here... til the pics disappeared.

Did you ever figure it out?

 

http://memimage.cardomain.com/member...40_25_full.jpg

http://memimage.cardomain.com/member...40_26_full.jpg

http://memimage.cardomain.com/member...40_27_full.jpg


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