Installing the Pistons and Rods

- I have new Silvolite 8.5-1 compression pistons with 5.565" rods. With the small chamber heads, I should get somewhere around 9.3-1 static compression ratio. I have to measure the head chamber volume later. The #1 piston installation went very smoothly until #2 piston went in. I turned the crank and there was interference with #1 piston skirt and the crank's front counterbalance at bottom dead center (BDC). Oh Oh!


Interference between piston skirt (aluminum at bottom) and black counterbalance

Well after checking and rechecking everything and consulting many experts including Silvolite who says this has never happened before. I decided that the interference wasn't that great and I could file the counterbalance and a bit of the piston skirt for 0.0625" clearance.

I turned the block so that the filings would not fall into the block and just in case I put a good coat of 10-30 oil. The oil catches the filings so that I could wipe them out with a clean cloth. After about an hour of careful fine filing, I had the clearances that I needed and minimal metal losses to both the crank and the piston. Not enough to affect the balancing. I basically had to round off the sharp corner of the counterweight and remove about 0.035" of metal off the piston skirt.


Counterweight bright line is where I filed it for clearance

When eventually I started up the engine, there was a loud heavy slapping sound coming from the bottom end. So I reluctantly had to pull the engine and take a look. Piston #1's wristpin has started to work its way out. There's about 1/2" or more showing. These are press fit pistons and this shouldn't be happening. Thankfully, the cylinder wall isn't scored as the wristpin still had about 1/4" to go before touching it.

I've torn the engine apart and checked all 8 pistons/wristpins/rods. Only cylinder #1 was loose. Even after 2 years, my machine shop still stood by its work and had even offered to re-sleeve the block if the cylinder is bad. They replaced the rod and it is now nice and tight like it should be. They were surprised that they didn't catch it at the shop. They had this great attitude about let's fix it and get it right.


Wristpin sliding out to the left - should be flush with piston skirt edge

- Unfortunately, they didn't size the big end of the new rod right. They were off center by 0.0125" which means that the rod bearing stuck out 0.025" on one side. So they replaced the rod again.

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Copyright May 2011 Eugene Blanchard