COLLAPSED PISTON

82Waggy

Good quench results in a more specific combustion event and therefore allows you to back off on timing advance, thereby reducing negative action on the up-stroke of the piston. .040 - .050 piston to head clearance is a good quench distance. I'd say .004 P:W clearance for a forged piston is too tight for a bore that was not honed with a torque plate. Any distortion at all (out of round) from torquing the heads and you would be below minimum clearance for a forged piston. Step on it before the engine is completely warmed up and you will get what you have.

AMX69PHATTY

I believe the .045 value is for Piston to Head clearance, not Piston Deck Clearance. Standard AMC Head Gaskets, Like Fel-Pro Blue Perma-Torques, are .045 thick compressed. So when the heads are torqued down, if the pistons come up flush with the Block Deck, that is .000 Deck Clearance, then the gaskets provide the .045 Piston to Head Clearance. So if your pistons are .045 down in the hole at TDC from the Block Deck you've got .090 Piston to Head. There's a whole long post here somewhere that talks about quench. What I got out of it was if the pistons get real close to the head, but .045 is the safe minimum, then it creates a shock wave in the cylinder that promotes combustion and prevents detonation. The .045 min. is to prevent a collision due to exspansion, stretch, or slop. Do you have a copy of the Performance American Style Book ? Did you ask the Engine Builder what his opinion was of .006 to .009 Piston to Bore clearance ? Did fuzz401 mention what the Piston to Bore Clearance was in his 401 motor with Ross Pistons ? Has he got the same Pistons as yours ? Sorry so many questions, considering getting Forged Pistons myself. Happy Labor Day Weekend :?:

1980_Cj7

Fuzz has flat top pistons. Completely flat and flush on the top, no dish, with some big reliefs cut in for the valves. Ours have a .145 deep dish in the top. Fuzz's compression ration is 12 to 1 though and he has to run straight racing fuel and $11 per gallon. I'll ask him what his clearances are.

Fuzz's

Ours

It does specifically say, IN UPPERCASE in the Ross instructions, that if a torque plate is not used, additional clearance above and beyond the .004 should be provided. It doesn't say how much additional though. All I know is, in addtion to all the time and work, we are now facing the expense of a new gasket set and a new set of expensive pistons, even if the machine shop cleans up the cylinders for nothing.

What I can't understand though is how the piston wear is right in the middle of the piston, top to bottom, when the bottoms are bigger than the tops. Why isn't the biggest part wearing first? How is the middle hitting and not the bottom? This will be easier to visualize when I post some pics later.

1980_Cj7

Here's the pics as promised. The first pic is No. 1 on the left and No. 3 on the right. The second pic is No. 5 on the left and No. 7 on the right. All showing the top (intake side) of the pistons.

[/img]

82Waggy

The bore would not be out of round with the heads removed, only when they are torqued down. This is the reason for using the plate, it distorts the cylinders while honing so that they will be round when the head is installed. .004 is the minimum wall clearance for a forged piston when everything is perfect. That there is variation in the damage on each piston is no surprise as each cylinder will not necessarily distort the same amount from head torque or heat. As a previous poster mentioned .045 is the typical target quench for an AMC and is obtained by establishing a zero piston to deck clearance and using an .045 thick head gasket. This usually requires a custom piston with the proper compression height (pin center to top). The required compression height is determined after rod resizing, block decking, and crank turning - or, if you use a piston that is close, maybe you can deck the block a few extra thousandths to obtain a zero piston to deck clearance. If your piston to deck clearance is really .045 and you are using an .045 head gasket, the resulting .090 quench distance is too much and will result in uneven combustion. Make sure you are measuring from the deck surface to the high edge of the piston that encircles the dish. You are not getting the compression ratio you think with this combination of .090 distance and dish pistons - closer to 9:1.

fuzz401

I think it was around .007 clearence my pistons were made by ross for a AMC motor they are flat top with 2 -6cc valve pockets bore 4.165 stk 3.680 rod lg 5.858 bk hight 1.493 rings 5/64 - 5/64 - 3/13 they were ordered by the machine shop that did the motor the first time thinking of a 10:1 cr they did not look at the heads very good ended up with 12:1 cr instead my deck height is 0 my head gaskets are the HP by ROL but I will say watch your rod caps were the rod bolt nut sit's the new ARP nuts are bigger then the factory and the factory seat is not always flat thus the ARP nuts will only hit a very small surface on the rod cap

Goose

Also the reason for the wear being in the middle of the piston and not at the largest point, is that is inline with the wrist pin.. which is where the forces are being transferred.(if you push on the top or bottom the place where it will or can flex or bend. so it makes sensethat it would thrust the middle of the skirt into the wall.. after all what was happening is the slug was sticking in the bore..(Might be you got off lucky.. It could have cracked a piston or really siezed in the bore and snapped a rod..). You know I might catch some flack for this but here goes..I have always built engines on the loose side of tolerances (except bearing clearances).for one simple reason. they last. they are forgiving of abuse and while some out there are able to control the machining process to a few thou. When you are paying someone else to do the work, you dont always get what you specify. (OR in this case get someone to read the damn instructions cause I bet two more thou clearance and this wouldn't be an issue). Good luck and Damn the bad luck!

Holeshot

Ditto building engines on the loose side, especially the bores. I think .009" for my 4.195 engines is where I like it (for drag race & nitrous).

Nasty looking pistons!!!

Get yourself some Muriatic acid to eat the aluminum off the cylinder walls, THEN Sunnen hone them enough to get .009" clearance with the new forged pistons, or don't open up the bore & use KB hypereutectic pistons.

HotRod Mag Norm Brandes AMC builds...

http://www.hotrod.com/searchresults/?scope=t&sitename=hotrod&terms=norm+brandes&go.x=14&go.y=11

"Indy closed up the rear-chamber wall between the valves to yield superior quench. The KB piston (right, compared to the old Speed-Pro replacement forging, left) has a step that keeps the mixture out of the chamber's dead area, while the large flat dish area uses the whole chamber for a good burn. The result is a tumble-and-swirl-promoting 0.030-inch quench."

New KB AMC Pistons:
PN Engine CH Volume
KB354 401 1.51 inch -30.5cc dish
KB392 401 1.51 inch -17.3cc dish
KB394 390 1.56 inch -28.0cc dish

KB Performance Pistons/United Engine & Machine Co.
Carson City, NV
702/887-2971
800/560-4814
www.kb-silvolite.com

Here's another question for the builder, were these file-fit rings???
What clearance did he file them to???
Tight ring gap causes too much heat & the pistons grow too much, when the top ring grows enough to but, off comes the top of the piston!!!

1980_Cj7

Goose wrote
Also the reason for the wear being in the middle of the piston and not at the largest point, is that is inline with the wrist pin.. which is where the forces are being transferred.(if you push on the top or bottom the place where it will or can flex or bend. so it makes sensethat it would thrust the middle of the skirt into the wall.. after all what was happening is the slug was sticking in the bore..(Might be you got off lucky.. It could have cracked a piston or really siezed in the bore and snapped a rod..).

That makes sense.

And yes, no matter how bad your luck is, things can always be worse. I'm just happy it doesn't look like the block is lost. 401 blocks are a lot harder to come by than pistons.

Hmmmm, I just thought of something. Fuzz was driving our Jeep when the knocking started. Maybe we can sue him for a new engine. I hear he's loaded anyhow. :mrgreen:

Goose

I heard he was loaded too.. but I don't think they were talking money...(anybody who drives a Short wheelbase jeep thats that fast.. has to have some fortification somehow :P) :shock:

1980_Cj7

Goose wrote
I heard he was loaded too.. but I don't think they were talking money...(anybody who drives a Short wheelbase jeep thats that fast.. has to have some fortification somehow 😛) :shock:

Nah, no fortification, the hardest think he drinks is Turkey Hill ice tea. The reason he drives a short wheelbase jeep that fast is his rear view mirror. It says "Objects in mirror are dumber than they appear". 111!!!

I think things are finally starting to fall into place. I got to thinking, and pulled out the Ross inspection sheet. It gives laser measurements for each piston at the top, middle, and bottom of the skirt.

Cylinder #1, piston #D2, diameters from Ross inspection sheet: 4.1864 / 4.1902 / 4.1902 least damage

Cylinder #3, piston #A2, diameters from Ross inspection sheet: 4.1866 / 4.1904 / 4.1904 second worst damage

Cylinder #5, piston #B2, diameters from Ross inspection sheet: 4.1864 / 4.1902 / 4.1902 third worst damage

Cylinder #7, piston #F2, diameters from Ross inspection sheet: 4.1865 / 4.1903 / 4.1903 worst damage

The pattern being that the smaller pistons, which would have had more clearance, assuming the bores are all the same size, exhibited less damage.

82Waggy

Thus the reason that bores are usually finish honed and clearanced to match each piston to a corresponding bore. I'd be holding your machinest accountable for too tight a bore clearance and maybe not even matching the bores to a corresponding piston. Anyway, I think it is now clear what went wrong. If you are going to spend the money on forged pistons again, clean up the bores, determine the diameter and compression height you need to establish a zero deck clearance - and the dish size necessary to establish about a 9.25:1 compression ratio, and then get a custom piston made by JE or Venolia to suit these dimensions. And use a torque plate to finish hone. PS: I used a custom JE forged piston for my 390 build that were matched made to fit the bores with the proper clearances and compression height. Used the KB 354 pistons for my 401 that we match honed and decked the block to fit - both zero decked + or - a thousandth or two - and then used an .045 head gasket.

AMX69PHATTY

Here's a link to the Keith Black SilvoLite United Engine & Machine Co. Web site page that talks about quench.

it mentions something about .090 Piston to Head clearance being undesireable in thier opinion.

http://kb-silvolite.com/article.php?action=read&A_id=39

and here's a second KB SilvoLite web page that discusses Quench and Piston to Head Clearance

http://kb-silvolite.com/article.php?action=read&A_id=35

:idea:

and another about forces on the sides of pistons, neat graphics

http://www.kb-silvolite.com/feature.php?action=read&F_id=25
:-|

Blown7

1980_Cj7 wrote
Goose wrote
I heard he was loaded too.. but I don't think they were talking money...(anybody who drives a Short wheelbase jeep thats that fast.. has to have some fortification somehow 😛) :shock:

Nah, no fortification, the hardest think he drinks is Turkey Hill ice tea. The reason he drives a short wheelbase jeep that fast is his rear view mirror. It says "Objects in mirror are dumber than they appear". 111!!!

I think things are finally starting to fall into place. I got to thinking, and pulled out the Ross inspection sheet. It gives laser measurements for each piston at the top, middle, and bottom of the skirt.

Cylinder #1, piston #D2, diameters from Ross inspection sheet: 4.1864 / 4.1902 / 4.1902 least damage

Cylinder #3, piston #A2, diameters from Ross inspection sheet: 4.1866 / 4.1904 / 4.1904 second worst damage

Cylinder #5, piston #B2, diameters from Ross inspection sheet: 4.1864 / 4.1902 / 4.1902 third worst damage

Cylinder #7, piston #F2, diameters from Ross inspection sheet: 4.1865 / 4.1903 / 4.1903 worst damage

The pattern being that the smaller pistons, which would have had more clearance, assuming the bores are all the same size, exhibited less damage.

One ten thousanth isn't going to make that much difference in the piston size, don't look for your answers there. You have to check the bore diameter verses the actual piston size. ( Don't even use the piston data sheet) A good machinist will use his own micrometer and verify the actual measurements. I run .008 thousanths on forged pistons and nitrous so far not a problem. You DEFINATELY have to let the engine warm up before you go anywhere with forged pistons. I run JE Pistons. I guess thats why I never saw that kind of damage, I do all my own work.
Also I did away with the offset piston pin hole in my custom pistons. another great useless idea compounded in the 1970's era engineering world.

Jeff

fuzz401

I heard he was loaded too.. but I don't think they were talking money...(anybody who drives a Short wheelbase jeep thats that fast.. has to have some fortification somehow 😛)

12.9 that's not that fast :? there cars with short wheel bases too ; they just don't see all the scenery like I do

Nah, no fortification, the hardest think he drinks is Turkey Hill ice tea. The reason he drives a short wheelbase jeep that fast is his rear view mirror. It says "Objects in mirror are dumber than they appear".

it is this that says it all

Goose

Fuzz my friend.. I have gone 12.0 in a Gremlin and I'm willing to bet real money that, it's not nearly as exciting as that Jeep of yours at 12.9.. For one thing the windows were rolled up! Also I like that decal.. I'll have to look for one of those!

fuzz401

Fuzz my friend.. I have gone 12.0 in a Gremlin and I'm willing to bet real money that, it's not nearly as exciting as that Jeep of yours at 12.9..

that my next thing but 11 to 10 in a gremlin :?

12.9 in the jeep is not bad stright as an arrow no weave at all just like driving your car down the road :t:

1980_Cj7

Goose wrote
Fuzz my friend.. I have gone 12.0 in a Gremlin and I'm willing to bet real money that, it's not nearly as exciting as that Jeep of yours at 12.9..
For one thing the windows were rolled up!

And the worst part is, Fuzz does it with both hands tied...

to his steering wheel. He hasn't quite mastered "behind his back" yet.

1980_Cj7

One ten thousanth isn't going to make that much difference in the piston size, don't look for your answers there. You have to check the bore diameter verses the actual piston size. ( Don't even use the piston data sheet) A good machinist will use his own micrometer and verify the actual measurements. I run .008 thousanths on forged pistons and nitrous so far not a problem. You DEFINATELY have to let the engine warm up before you go anywhere with forged pistons. I run JE Pistons. I guess thats why I never saw that kind of damage, I do all my own work.
Also I did away with the offset piston pin hole in my custom pistons. another great useless idea compounded in the 1970's era engineering world.

Jeff

I know actual measurements comparing each piston to the cylinder it is in would be best, but unfortunately I don't have the proper tools to do that. I'm thinking of finding someone (other than the guy who built the engine) who does, and have them measure and record what we have. [/quote]

1980_Cj7

Anybody know anybody in the Harrisburg, PA area that would have a cylinder gauge and big micrometer to measure the pistons that would be willing to do some measuring for us?

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