Author Topic: Stihl 461 piston  (Read 35 times)

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Offline Cut4fun

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Stihl 461 piston
« on: June 26, 2021, 08:59:53 am »
Just got to save these pics and explanation on new saws pistons.  Have seen it before and wondered why.

Question was - 461R New out of box and never been started or fueled yet outside of the factory. Why?

Answer - Trevor Saville
All of these pistons are washed with Tin from the manufacturer (Mahle, KS, Stihl BR/DL, Gilardoni) to help with break-in/cold seizures. The bright area at the bottom is not slop it indicates the max clearance Spec'd for the part. The edge of the skirt is being shined up from all the loose Tin coming off the piston plus the iron from the rings. It also indicates there's a draft angle for piston crown growth. The human eye will pick up on something like that even though its clearnce is .0005 and perfectly fine. As you get past about the 5th tank. Pull the cylinder again you'll see the pattern grow and change as it breaks into the cylinder and washes all the tin off. Run it.
Where you want to worry is when you pull the cylinder on one with dirty ingestion and it's peened the skirt edge plus has gouges that can be felt by touch. That .005 will start the downward spiral of slop and eventually 🔥.

More added - Trevor Saville
 I found the right person to ask years back. Remember when Baileys sold the Wiseco pistons they would grow in the cylinder and cold sieze? Too tight on clearance and no coating to help break-in.They were like little marshmellows if you didn't properly warm them. There has to be cold clearance or you would have to preheat the cylinder with a torch to get it free enough to pull over. An engineer at Mahle told me the F1 motors are preheated hours in advance of a race to even be able to start them as the pistons are clearanced so tight below operation temperature they are literally stuck in the cylinder bore.

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