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Old 03-20-2009, 11:24 AM
V8 Super Beetle's Avatar
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Engine Building Questions

Yesterday I got the part number off the top of the pistons which are still in the shortblock at the moment. 355 sbc.

Piston part # TRW L3022F
.030 over (which I already knew)

I was able to find out the specs on these pistons by doing a search.

L in the part number means they're forged pistons.
Compression height is 1.535.
Pin height is 0.927
Ring sizes are 5/64 5/64 3/16


Questions:

1.) What would you recommend the decks be set to at the machine shop? I'll be using Felpro shim head gaskets with a .015 compressed height. I plan to rebuild this engine again later down the road and want to have room for future rebuilds.

2.) Being that the pistons are forged, which I did not know at the time I purchase these rings; part #SUM-133-139-30, would these stock / street rebuild pistons rings hold up to a moderate 100 shot of N2O OK? If so, should I set the ring gaps to a certain # or will there set gap be fine? Spray would be used VERY rarely. If not, what rings would you recommend? Budget minded please. Summit rings would be fine.

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Old 03-20-2009, 03:46 PM
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Quote:
Originally Posted by V8 Super Beetle
Yesterday I got the part number off the top of the pistons which are still in the shortblock at the moment. 355 sbc.

Piston part # TRW L3022F
.030 over (which I already knew)

I was able to find out the specs on these pistons by doing a search.

L in the part number means they're forged pistons.
Compression height is 1.535.
Pin height is 0.927
Ring sizes are 5/64 5/64 3/16


Questions:

1.) What would you recommend the decks be set to at the machine shop? I'll be using Felpro shim head gaskets with a .015 compressed height. I plan to rebuild this engine again later down the road and want to have room for future rebuilds.

2.) Being that the pistons are forged, which I did not know at the time I purchase these rings; part #SUM-133-139-30, would these stock / street rebuild pistons rings hold up to a moderate 100 shot of N2O OK? If so, should I set the ring gaps to a certain # or will there set gap be fine? Spray would be used VERY rarely. If not, what rings would you recommend? Budget minded please. Summit rings would be fine.
OK, let's get your head wrapped around the basics. When produced at the factory, blocks are cut with a nominal block deck height of 9.025" (centerline of the main bearing bore to the block deck where the head bolts on). This assumes that the block has not been decked by some knot-head before you got it. Adding up your stack of parts will tell you the piston deck height and the gasket to use for a tight squish. 350 crank radius of 1.74" + 5.700" rod length + 1.560" piston compression height = 9.000". Theoretically, this would leave a piston deck height of 0.025", which, together with your 0.015" head gasket would produce a squish of 0.040".

However, don't assume anything. Always measure your clearances before final machining and assembly. You still have the pistons in the short block and that's good. With the motor on a stand, tilt it so that one bank is level. Using a steel rule and a set of feeler gauges, we'll find the actual piston deck height. Standing at the side of the block, span the edge of your rule across the bore about 1/4" from the edge of the bore at either the 3 oclock or 9 oclock position. Slide feeler gauge blades between the bottom of the edge of the steel rule and the piston crown. Checking at noon or 6 oclock will allow the piston to rock on its wrist pin and tilt one way or the other, giving you an erroneous reading. 3 or 9 is directly above the pin. Rotate the crank a few degrees backward and forward and re-check your measurement to make sure the piston is at TDC. If you find a piston deck height of 0.020" to 0.030", you'll be fine with the 0.015" gasket (assuming the block decks are smooth and parallel). Do this measurement on each of the four corners of the block to see if there is a slope to the block front to rear and/or a difference from one bank to the other.

If the piston deck height is less than 0.020", then you will have to use a thicker gasket. Victor Reinz makes a 350 gasket that measures 0.025", part #5746. DO NOT BUILD THE MOTOR WITH A SQUISH TIGHTER THAN 0.035" OR LOOSER THAN 0.050".

You can, of course, use the depth function of your caliper to measure the piston deck height. I just explained the steel rule/feeler gauge method because not everyone has a dial caliper.

I don't know about a 100 shot on cast rings. It's been a long time since I used nitrous, so I'll let someone else weigh in on that.

By the way, when checking the pistons for piston deck height, go ahead and check at 3 and 9 both. If the measurements are different from one side of the piston to the other, it could indicate a bent rod. If you suspect a bent rod, check the rod bearing for that hole. If the bearing is worn more on one side than the other, the rod is bent.

Last edited by techinspector1; 03-20-2009 at 03:55 PM.
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