Deck clearance variance?

Post by **SilverXJ** » March 15th, 2009, 4:02 pm

I checked the deck clearance on the pistons today... results were a bit disturbing. After I found TDC, I measured the left and right side of the piston and rocked the piston until the two were equal. Recorded that number. Then I measure at the front and back of the piston above the piston pin, and recorded those numbers. I used a dial indicator on one of the bridge type stands.

Take a look at my results:
Cylinder #; Front; Rear; Left&Right
1; .004; .004; .0032
2; .0045; .001; .0025
3; -.0005; -.002; -.0012 (this is actually above the deck)
4; .0015; .001; 0
5; .005; .004; .004
6; .002; .0075; .004

From the highest to the lowest spot I have a variance of .0095. Using the .044" gasket (my measurement, 505performance says .0425) I would have a quench of .051" to .042", again taking the highest and lowest measurements.

Is this normal? Is this variance within allowances?

Also, the machinist noted that when he was resizing the rods for the ARP rod bolts that some rods needed to have more material removed form the big end than others to resize them, indicating different length rods.

I just want to get this damn thing running again!

Post by **Mgardiner1** » March 15th, 2009, 4:06 pm

I had a general trend of .007 from front to back (#1 = .010, #6 = .003) and it was a gradual incline. I attributed this to the deck not being perfectly true to the crank journals when he machined the deck.

Post by **SilverXJ** » March 15th, 2009, 4:32 pm

My trend appears to be a U shape from front to back

Post by **Mgardiner1** » March 15th, 2009, 5:17 pm

yes, it does appear that way.

Kinda hard for me to explain how that happened.... The resizing of the rods.... sure would be a hell of a coincidence that the required resizing would have happened EXACTLY in that order.

Maybe the block was flexed by how the machinist clamped it to the table of his mill?

Post by **SilverXJ** » March 15th, 2009, 6:19 pm

But then would the deck not be flat and can be checked with a straight edge?

I would just throw it together.. however that piston above deck makes me nervous with such a tight quench.

Post by **cherokee** » March 15th, 2009, 7:00 pm

If the deck checks straight, have you considered changing piston/rod assemblies with certain holes?

Post by **SilverXJ** » March 15th, 2009, 7:12 pm

That is an idea, however if the deck is straight, then swapping the pistons/rods around would show either an issue with the crank (if the deck height doesn't change) or piston/rod length (if the different deck heights move). I'm not sure about the pistons being moved to different bores though. I would be its the rods that are causing this as that is something that the machinist mentioned that he saw. Also, he checked the mains and they were straight too.

Post by **TurboTom** » March 15th, 2009, 7:37 pm

Was the crank machined? I would guess the rod/piston combo

Post by **hatem61** » March 16th, 2009, 1:37 am

SiverXJ, you may consider re-measuring using one conrod/piston assembly in all 6 bores, this would clear a crankshaft or a block deck doubt.

Post by **dwg86** » March 16th, 2009, 3:49 am

SilverXJ wrote:I checked the deck clearance on the pistons today... results were a bit disturbing. After I found TDC, I measured the left and right side of the piston and rocked the piston until the two were equal. Recorded that number. Then I measure at the front and back of the piston above the piston pin, and recorded those numbers. I used a dial indicator on one of the bridge type stands.

Take a look at my results:
Cylinder #; Front; Rear; Left&Right
1; .004; .004; .0032
2; .0045; .001; .0025
3; -.0005; -.002; -.0012 (this is actually above the deck)
4; .0015; .001; 0
5; .005; .004; .004
6; .002; .0075; .004

From the highest to the lowest spot I have a variance of .0095. Using the .044" gasket (my measurement, 505performance says .0425) I would have a quench of .051" to .042", again taking the highest and lowest measurements.

Is this normal? Is this variance within allowances?

Also, the machinist noted that when he was resizing the rods for the ARP rod bolts that some rods needed to have more material removed form the big end than others to resize them, indicating different length rods.

I just want to get this damn thing running again!

Looking at your numbers, my biggest concern would be the difference from front to back. That would indicate the big end was resized wrong (crooked) or the rod is bent. The side to side could be from the piston rocking on the pin, this would be normal.

When I built my amc 401 I had the same issue with all the pistons on one bank of cylinders. They all were down .004 in the front and all .008 up in the back. The shop said the rods were bent. I think they machined them crooked. How could they all be bent exactly the same and only on one side? Plus the 401 uses forged connecting rods. Kind of hard to bend a forged connecting rod, that was in a stock engine. My argument was they were suppossed to check them for straightness before they resized them (it was in there shop recipt..."check rods for staightness and resize"). You can't resize a connecting to many times. The center to center distance gets too short. I had to buy another set of rods.

I would call the machine shop and talk to them.

Post by **SilverXJ** » March 16th, 2009, 4:30 am

I didn't like the front/rear difference either. How critical is this?

Post by **dwg86** » March 16th, 2009, 5:04 am

I would bring a piston to TDC, and then push down on it to make sure its sitting square on the bearing. It doesn't take much of a difference at the lower end of the rod to show up at the top of the piston 6 inches away. Think of it like this, if you took 2 lines starting at the same point and made them 1/8 inch apart at 1 inch out, at 1 ft they would be further apart than 1/8 inch. So looking at you numbers if you have a piston that is off by .003(about the thickness of a sheet of paper) at TDC, the difference at the rod bearing would be less than that. It doesn't sound too bad. The biggest problem is when the cylider fires, the piston is going to want to run straight in the bore. This will cause the rod bearing to run on one side of the journal. I don't think the top of the piston being .003 out, front to back, will cause any issues, or wipe out a bearing. Now with my 401 build a total difference of .012 I thought was too much.

Post by **SilverXJ** » March 16th, 2009, 5:46 am

TurboTom wrote:Was the crank machined? I would guess the rod/piston combo

Yes, it was turned 10/10

Post by **SilverXJ** » March 16th, 2009, 6:04 am

I definitely need to back and check #2 and #6. When I push down, I should aim for dead center of the pin? Also, should I even worry about leveling the piston side to side (left and right)?

Post by **dwg86** » March 16th, 2009, 6:18 am

SilverXJ wrote:I definitely need to back and check #2 and #6. When I push down, I should aim for dead center of the pin? Also, should I even worry about leveling the piston side to side (left and right)?

Try to get it as close as you can. I'm curious, did the rods come from one engine, or were they a mixed set?

Deck clearance variance?

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