Engine problems....The Return!

[simonjrwinter]

ake a look at this

[simonjrwinter]

And this

[simonjrwinter]

Is that normal for an engine with only 400 miles on it?
The story is that I built the engine and did everything correctly, primed the oil pump, ran in the cam etc. engine fired first time and ran faultlessly for 400 miles until the exhaust rocker on no5 cylinder fell off (my fault I 'spose, what a muppet) and I drove the car (gently) 5 miles with the cylinder backfiring through the carb. removed rocker cover, nothing damaged or missing, so re-fitted the rocker. Engine started first time but with knocking (sounded like it was coming from the cam, but cam perfect)
I've now torn the engine down and can find nothing wrong (bearing-wise) apart from this scuffing on this piston.
I can appreciate that this piston was subjected to tremendous pressure and I would hope that's the reason for the damage, but is this what's causing my knock?
Have I stuffed these pistons/bores?
Everything else SEEMS to be nice and straight (H-beam rods, so I would hope they're OK)
any opinions most welcome.
Simon

[blykins]

I don't think that's normal at all. What gets me is the fact that the scars in the cylinder wall go all the way to the deck...the scars on the piston skirt wouldn't have caused that. Looks almost like a ring was broke or bent when the piston was installed. Either that or you got some serious debris in the engine....was your oil clean? Did all the pistons have their spirolox intact?

[simonjrwinter]

I think when I took it apart before, I may have let some debris into that bore, I take full responsibility for that foolish mistake!
The rings on that piston are undamaged, and although scratched, the damage to the bore cannot be felt. I would think (hope) that a light hone would remove them.

[trularin]

Looks like you got something, filing maybe, in the ring land and it did it's deed.

The knocking you heard was probably the wrist pin pivot to the piston skirt to the cylinder wall. Since there was more friction on one side of the piston than the other, the piston would slap in the cylinder.

Measure how deep the cut is and see if you can have it machined out.

Check the ring to land clearance all the way around that piston.

Bad break, sorry you had this happen.

Good thing you found it early.

[blykins]

I think there's just been something caught in there....and it just happened to stick around long enough to make a mess. If you can't feel the scratches with your fingernail, you're probably right...a honing will probably clean them up.

[simonjrwinter]

I'm still baffled as to what my knocking could be?

[blykins]

Did anyone else hear the knocking? You think you were just being too analytical?

Did you check your connecting rod clearances?

[simonjrwinter]

Yep, It was heard by other people (not anyone who could tell me what it was though) The thing I liked about the engine, and the reason I built it with a fairly mild cam, was the nice quiet tickover, but the knock ruined that.
I dremeled (is that a word?) the bottoms of all the bores so the rod bolts have a decent clearance.

[mr0077]

Simon, was that scuffing only on #5 or typical of all bores? It looks exactly like the evidence I found on a motor I built that I forgot to gap the rings on...they were too tight and caused similar damage to all cylinder walls and pistons. In all but one bore the scratches were too light to snag a fingernail on. They cleaned up with an extra 0.005 honing, but it meant replacement pistons, of course...
I don't think that the rocker coming off would be the cause of this damage, but could it have been the final straw if the gap(s) were marginal? Seems that with the rocker off, the cylinder would simply be unable to exhaust on the exhaust stroke, so it would blow back into the intake manifold when the intake valve opened, then suck in a little air/fuel and repeat the cycle, with no appreciable extra loading on the moving parts.

[simonjrwinter]

Thanks Ken,

I did gap the ring to the mfrs recommendations (can't remember what now)

[blykins]

You didn't clearance the block when you assembled the engine? I think you're supposed to have around .060" clearance between the rod bolts and cylinder skirts..

[scottj]

Simon- I agree with Ken that an inoperative exhaust valve didn't cause extra load on that piston. I have seen cylinders and pistons scuffed from fuel washing the oil off the walls. I wonder if the inop exhaust valve was causing that cylinder to wash down?
Too many problems with one hole to be coincidental I think.
Scott

[GUMBALL UK]

Simon,

I hope you have found your problem - although looking at the photos I'm not that sure that it could have worn enough to cause a knock?? could have been some debris in there that caused the damage - hard to tell, but look on the bright side - at least you have found something!! Check the bottoms of the bores on the block and study that rocker assembly for damage - there just could be a small piece missing that has appeared in that bore (that would be the most likely place for it to go if it were the rocker)

GUMBALL UK

P.S. you really should put the cap back on the Hylomar - that stuff gets everywhere!!

[simonjrwinter]

FOUND IT!!!!

[simonjrwinter]

And another.
I've obviously got a major problem here

[simonjrwinter]

This is number three piston, opposite bank to No5!!!
The piston has picked up big time on the cylinder wall, THATS my knock but why.
When the engine wes rebored to plus 30thou, I don't think the shop used deck plates, could this be a factor?
I'm guessing its a heat/oil poblem?
Any advice appreciated

[xlr8or]

My $.02:
Send it to a shop that can properly machine the block to clean it up and do a full blue printing so you know all the parts are fitted correctly and tolerances are within acceptable limits.

Machining and assembly of the short block is best left to the profesionals that do it every day. Let them do thier thing then add all the top end bits yourself.

I hope it all works out for you.

[simonjrwinter]

I've just dug out the receipt for the engine work I had done. It shows they bored the block to give a .0032 clearance to the pistons, is that about right for forged pistons?
Could it be a bit tight when the pistons get hot?