Im wanting to do a BB this winter and cant decide on the 103 or the 106 or 107. I intend to have the heads worked but dont want to have crank work done. I like the idea of the lighter cast pistons but the biggest I can find are 3-7/8" (103). On the other hand, I think while Im doing it, I might as well go to a 107 but again, I cant run cast pistons. What are the pros and cons to forged pistons and cast pistons?

 

pros - forged is gonna take detonation alot more then a cast... Witjh the relative low rpm not sure how much piston weight is gonna matter compared to a engine that revs 18k+

cons - forged cost prob a lot more (atleast in the car world)

 

cast, imaging pouring fluid metal into a cast- you take it out clean it up- 'piston'

Forged- similar process but under pressure- there are no voids, inconsistencies in density, metallic structure more stable.
it is possible that more 'pure' alloy will be used in this process.

so stronger, more solid, may weigh more, but higher strength allows more weigh relieving.
possibly better quality control material and machining- just because it is a premium cost product

for your build find a system that works with your bike ( weight) gearing, riding style, wishes ( MPG/ HP/Torque/ reliability and service life etc) and will not require re-engineering everything else to match it.

Mike

 

If you could find the proper size, a Hypereutectic piston would be the best compromise. They have a higher Silicon content and expand much less than cast or forged pistons do. That being said, you can fit them with tighter piston to wall clearances for a better seal. Google it, they explain it in a lot more detail than I can.

 

Unless you're getting really exotic ( High compression ratios ) there is no need for a forged piston. There have been major advances in quality over the past several years. Another factor to consider is noise. Forged psitons can be / are very noisy but are stronger ... the question is how much strength do you need?

 

Fuel Moto has their 107 BB kits forged piston to cylinder wall clearances set at .0025, which is pretty tight for an (almost) 4 inch bore, air cooled engine. I expected some problems with scuffing (their pistons do not have the skirt coated like the factory piston does) but after 6000+ miles on the new build haven't notice anything unusual, yet. With the tighter clearances I haven't noticed any piston slap when cold. I haven't questioned how they can set them so tight and not run into problems, all I know is it has worked well for me.

 

Comparisons I've read make a good argument for both types due to the huge technological advances made in cast piston manufacturing. My builder preferred I use the forged pistons, and it's pretty hard for me to tell Kenny his business...

 

There are millions of both cast and forged pistons out there, giving good service. You will need a local shop to bore your pistons, so go with what he recommends or supplies. Much more important to get a matched set, than to worry about something that doesn't matter!

 

For long term reliability,go cast.Forged pistons expand at a slower rate than the cast cylinders.You'll need to warm the bike a little longer and take it easy on the mtor till it's up tp operating temperature.If not you get piston slap,scuffing,leaky rings and much shorter service life.

 

Does anyone know where to get cast pistons for a 106 or 107?