AMX69PHATTY
Maybe try a Tool Rental place for a 3-Point Bore Gauge ?
Ever heard of Enco ? www.use-enco.com 1-800-873-3626
They have a Dial Bore Gauge for $60 - #612-5920 <-- $40 on SALE !
and a 4" to 5" Outside Micrometer for $40 - #600-0025 <-- backordered
or a Set of Outside Mic's for 0" to 6" in a wooden case for $70 - #600-0006. <-- $61 on SALE !
They're not fancy schmancy but they'd work.
I shouldn't have looked, I might have to get a Bore Gauge myself for $40 ...
Bore Gauge
http://www.use-enco.com/CGI/INSRIT?PMAKA=612-5920
Mic Set
http://www.use-enco.com/CGI/INSRIT?PMAKA=600-0006
Holeshot
If you want to get a quick idea of whatup, get a few feeler gauges (.002"~.009", long ones if possible) and your best piston & poke the piston & gauge into the best cylinder (should be no ridge yet), repeat with ever increasing gauge size until she-a-no-fit. As you already know, the piston is a little bigger towards the skirt.
At least you will know who needs the knots on his head sooner rather than later!!!!
1980_Cj7
I was thinking about using the feeler gauges. I wondered if the width of them and the curvature of the cylinder and bore might cause a false reading though.
I just thought of another question though, at what point on the piston do you measure the clearance? The pistons are tapered .0038 top to bottom, so where you measure could make a big difference. Do you allow the specified clearance at the biggest point of the piston?
1980_Cj7
Got the other head off and checked the deck height with the feeler gauge before pulling the pistons. The deck height is .020!
AMX69PHATTY
Is what you're saying is the Deck Clearance is .020 ?
that top of the piston is .020 down in the hole when at Top Dead Center ?
That is called Deck Clearance.
Deck Hieght is the distance from the Crank Centerline to the block head surface.
On the 401 the stock Deck Hieght is 9.208".
This can be reduced by machining the block but is not normally done.
Just sharing information.
I've been told that on small block chev's that it's not uncommon to find
that the Block Deck is not truely square to the Crank Center Line
resulting in Deck Clearance differences in the bores from one end to the other
which yields slightly different compression ratios in each cylinder.
In that case they mill the block deck to equalize the deck clearance,
and this slightly reduces the stock deck hieght.
I have not heard that this "problem" exists with AMC V-8's since they're not that cheap chevy stuff :?:
To calculate deck clearance one takes
1/2 the crank stroke + Connecting Rod Center Distance + Piston Compression Hieght
and subtracts that from the block deck hieght.
Piston Compression distance is from the wrist pin centerline to the piston top.
In your case:
1.84 Stroke Throw + 5.858 Rod + 1.490 Piston Comp Distance = 9.188
9.208 - 9.188 = .020 Deck Clearance
Stock 401 Piston have a 1.508 Comp Distance, so say 1.510
If your pistons have the stock 1.510 Comp Distance then they would stick OUT of the Block .020 !
Are you saying they stick out of the block .020 ??????
Man, if that's the case, a head gasket is .045 thick, so the pistons are only .025 from the Head !
They say the minimum suggested piston to head clearance is .045 to .040 !
hmmmmm :-|
82Waggy
I'm using KB354 pistons with a compression height of 1.51. These would have yielded a zero deck clearance had the block not required milling from stock.
My 401 block required milling .007 off the deck to square it up, which would have left the KB pistons sticking out about .007 but I lost a few thousands of rod length and crank stroke to reconditioning so my pistons only stick out of the bore about .002- .003 , yielding a quench height of approximately .042 - .043 with an .045 head gasket - which is right in there.
Stock 401 spec'd a piston to deck clearance of .002 (down in the hole), but this would have varied several thousandths from bore to bore on my out of square stock block.
The Speed Pro & TRW pistons are .005 shorter than the KB.
Not sure how you would get .020 piston to deck clearance with any of these pistons.
For reference, stock 401 deck height is 9.208, rod is 5.858, stroke is 3.68, piston ch is 1.508. After machining mine came out at 9.201, 5.855, 3.66, 1.51, give or take a thousandth here or there.
1980_Cj7
I called it the wrong thing. What I am saying is the pistons are down in the holes .020 at TDC. I thought that dimension was supposed to be "0". I mic'd the head gasket and it is .045. We have Ross Racing forged pistons that are supposed to give 9.8 to 1 with 58cc heads. I don't have the piston spec/inspection sheet in front of me, but I'll check the compression height later.
Update: on the passenger side, only number 8 piston is scuffed, and not nearly as bad as the ones on the driver's side. And, it is scuffed on the opposite side from them, the lower side (exhaust manifold side) of the piston. It is about like number 5 in the pics above, but again, on the opposite side of the piston.
We had a second independent machine shop do some measurements, and they are also saying the clearances look OK. They also said the blackness on the tops of the pistons was normal.
Heard back from Ross, and they don't know what caused this. We are really stumped here. No way we are buying more pistons and putting this back together not knowing what caused it and correcting it. Ross mentioned maybe it could be cylinder wash from running too rich. They said that should show up on the lower side (exhaust manifold side) of the cylinders though. Our tail pipe is all black inside but our plugs look good. Fuzz saw them first hand and agreed. I can post a picture of the plugs later. We were running an Edelbrock Performer 750 carb set up at the baseline settings.
I keep going back to the timing issue and how the engine didn't want to turn over until we got the base timing to around 20 degrees and wondering if that plays into any of this. Guess we'll have to pull the timing cover and see if we can see anything awry with the timing set.
Any ideas?
Goose
Well Since you added the part about the #8 scuffing on the bottom, I am even more inclined to believe you have some sort of clearance/cold sieze or maybe offset pins in reversedgoing on.. This is going to sound redundant but if you visualize the direction the engin assembly rotates you will see that with offset piston pins..(If they were reversed) you would be thrusting (stop snickering pat) against the top of the cyl wall on one bank and the bottom on the other bank.. (I would try to explain it butin trying to visualize it my brain locked up and refuses to reboot.) ..at any rate Im betting it is a mechanical issue and not timing/mixture related. You would need something way way out of wack.
1980_Cj7
These pistons DO have offset pins. It seems like they could only be installed one way, due to the eyebrow cuts for the valves, but I'll have to study them again and see if I can figure anything out. I don't know if there are right hand and left hand ones. I don't recall seeing anything in the instructions about having to install them a certain way, but I'm sure going to be looking close at all that tonite. I'd love to find out that they were installed wrong and that is what caused this. Not knowing why this happened is the worst part of it.
Should the pins be offset to the same side on both banks, or should they be different due to the different forces due to the rotation of the crank?
The pistons are engraved "A" thru "H". They didn't seem to be installed in any particular order though.
1980_Cj7
Just checked the compression height on the Ross forged pistons. It is 1.493, so, the .020 down the hole that I measured with the feeler gauge checks out. Calculating it mathmatically, I get .017.
1980_Cj7
1980_Cj7 wroteThese pistons DO have offset pins. It seems like they could only be installed one way, due to the eyebrow cuts for the valves, but I'll have to study them again and see if I can figure anything out. I don't know if there are right hand and left hand ones. I don't recall seeing anything in the instructions about having to install them a certain way, but I'm sure going to be looking close at all that tonite. I'd love to find out that they were installed wrong and that is what caused this. Not knowing why this happened is the worst part of it.
Should the pins be offset to the same side on both banks, or should they be different due to the different forces due to the rotation of the crank?
The pistons are engraved "A" thru "H". They didn't seem to be installed in any particular order though.
Well, after looking closely at the pistons and doing some crude measuring, it didn't appear that the pins are offset. The only reason I said they were earlier is that the guy who built the engine said they were. To be sure, I just called Ross and gave the technician our job card number and asked if these pistons had offset pins. He said they DID NOT.
Next I asked him if it mattered where they were installed other than the valve cuts being on the top (eyebrow orientation as opposed to smile orientation). He said NO.
Curses. I was hoping we were on to something there. Back to the starting line!
Anyhow, here's a pic of the plug that was in cylinder number 7, the one with the worst damage. All the other plugs look the same.
Goose
Darn and here I thought I was becoming a theoretical genius..
That plug looks fine to me.
1980_Cj7
Goose wroteDarn and here I thought I was becoming a theoretical genius..
And we were sure hoping you were!
If we were to look for a lubrication problem as the cause of this, where would we start?
We already verified that the oil holes on the rod caps were in the correct orientation. Oil pump and timing cover are new. Oil pressure was great. Lifter valley mod was done. Block was tanked.
Mudrat
Goose wrote This is going to sound redundant ... You would need something way way out of wack. .. stop snickering pat
Guess adding anything to this wouldn't matter then :? :mrgreen: But I did like the offset pin pushing bit =D>
Goose
YEA it sure sounded erudite and edjamacated didnt it??
Goose
Ya know I was just looking at pyagids animated dojigger there and darn it was easier to visualize what I was talking about.
I don't know where you would look for oiling issues if the squirt holes were lined up and the bearing inserts werent backwards blocking them..unless for some reason you werent getting enough pressure through the crank..then you would think it would have spun a bearing..
Have you thought of finding a Priest and 5 quarts of holy water ?
Blown7
Naw, look at these links
http://www.hastingsmanufacturing.com/Service%20Tips/piston_cylinder_scuffing.htm
http://www.aa1car.com/library/2003/eb40354.htm
http://parts.evinrude.com/OIL%20WEBSITE/carbon.html
I think your problem was a little of everything, not the squirter holes. My Carillo rods don't even have them.
First you had a terrible time of timing, bore size too tight. I don't know how long you spent on engine warmup.
Rehone that block to .006 thousanths clearance, call JE pistons get some good pistons in there. Check the ring clearances. Time the cam correctly.
Then you'll be good to go.
Jeff
1980_Cj7
Thanks Blown7. I have already seen 2 of those links you posted. Guess I HAVE been doing my homework. That first one about the burr at the bottom is interesting. I'm going to check that.
As far as new pistons, what has any of you guys experience been with compression ratios? We want to be able to run pump gas. What would be the highest ratio we can go that will work? Don't care if we have to run 94 octane.
We have a deck height problem with the Ross pistons having a shorter compression height. Are any of the other pistons, JE, KB, etc. closer to stock in that dimension? (I'm going to check their specs myself, but I just wondered if any of you had any good or bad experiences with any of them.)
82Waggy
The KB 354 has a compression height of 1.510, which would make up the .017 difference you have to obtain a zero deck clearance, and they are available in .030-.060 overbore - BUT, these are typically run tighter than a forged piston as they do not expand as much, so you may need to re-bore/hone your block to fit them properly. They have a 28cc dish and will yield somewhere around 9.7:1cr depending on your final bore dimensions. 9.7:1 may be a bit high but is dependant upon your cams intake closing point (dynamic compression ratio). Since you have the heads off, you might consider polishing the chambers to open them up a bit to around 62-65cc and reduce compression ratio to about 9.25:1. KB's site has several calculators you may find useful. These pistons can be ordered from summit at around 403.00. One additional cost downside is that they use smaller wrist pins so you will have to bush your rods. I'm using these in my 401.
If you do not rebore, probably your only alternative is to determine your final bore diameter after clean up and get a custom forged piston made to fit your circumstances - JE or Venolia can make exactly what you need to exacting tolerances so that you can fit the piston with proper wall clearance, pin diameter, dish size, and compression height. Custom pistons typically cost around 800.00 plus. I went this route with stock diameter pressed pins in my 390.
Whatever way you go, take this opportunity to hone your block with a torque plate.
82Waggy
To clarify my previous post on custom pistons, you would have these made to the diameter you need to fit your finished bore with the proper wall clearance - so you would want to have your block finish honed first (all bores the same diameter) and get pistons made to match.
