Another Big Bore Question

[drf255]

It seems that big bore kits produce less power than their smaller bore OEM counterparts.

However, increasing displacement always seems like a good idea.

My understanding has come to be that the AM "big bore" kits have poor port timing and smaller cross sectional transfer area secondary's to the larger bore. Then there's the squish issues, where the tolerances of the AM P&C are just sloppier.

My question is this, with proper machining and porting/welding, has anyone made a big bore kit outperform a factory jug?

My specific project will be either an 044 or an 046 with a 54mm jug. If I can get a P&C cheap enough to not mind the likely destruction.

I plan on welding up the transfers on the outside to permit making them deeper, porting and cutting the base and band.

 

[Duke Thieroff]

Big Bore kits are "ok" in my book, but they suffer from some issues typical of AM stuff, depending on the manufacturer.

They aren't an engineered part, so very little development goes into them other than making the diameter larger. Sometimes there are some shortcomings with the castings (porous) and plating as a result of the casting.

Most AM kits are designed after engineered OEM parts with some shortcomings to save on manufacturing costs.

A big bore kit could be good if the port timing is conducive to machine work, porting etc.

 

[drf255]

If I buy one, it will be from you Duke.

I know I'll get something with good quality that was inspected (by someone who knows their stuff) before it was sold.

Your decision to not sell certain brands because of their quality has always impressed me.

 

[weimedog]

To answer your question, it doesn't take much to push one of those 52mm big bores to Stock levels. Can't comment on longevity over years as I have no experience with AM top ends past a year or so. I do have a pile of "OEM" cylinders that ran their normal service life and with a little glaze breaking can run another few years! My 52mm AM builds matches a typical stock 372 with more torque, it will pull a longer bar if asked. And that before doing any "porting". Just a simple muffler mod and a base gasket delete. The value of the muffler mod is questionable. The base gasket delete probably is what made the most difference as the "squish" was ridiculous.

This video has raw data... featuring A slightly "Tweaked" OEM 365/372 vs. a bunch of AM saws. AND a pure "Stock" 372 top end build inside a Jonsered 2166 wrapper too for reference. Will give you a realistic idea of what to expect.



To those who took time to look..... I did many more cuts, only showed a few. Each saw had several more cuts than was shown. The times I posted were compiled from all those cuts. To the original theme of this thread..... the most "Stock" build was also the slowest. And to the Stihl/Husqvarna rivalry, the Pure Stock MS460 was stronger than the "Stock" with base gasket 372/Jonsered build. AND I can tell you, much to my surprise, that MS460 was easily as smooth as any of the 372's and vibrated less than both Aftermarket 52mm top end builds. Both the "52MM BB's" were built without a base gasket to bump compression. The value of those simple mods is shown with the "Standard (for me) 372 Build" which produced one of the two best times on the day. It has a gutted muffler and no base gasket. Also has an Aftermarket meteor piston, although I doubt that made any difference. There actually are "trends" you can see in this video that translate to real life .....if you are hearing what is happening.

 

[Outlaw5.0]

I have only used the Gen 1 and Gen 2 046/460 big bore from Baileys. They both had good and bad points, the Gen 2 free ported stock due to the shortening of the piston skirts(the exhaust port floor was raised and piston skirts shortened). I have not used or seen the Highway 046/460 big bore so I can't comment on them.

 

[Stihl 041S]

An OEM 046 top end is a good BB kit for an 044
As an 066 is for the 064.

 

[blsnelling]

"Just say no" to BB kits. You'll get farther with a OEM cylinder.

 

[drf255]

"Just say no" to BB kits. You'll get farther with a OEM cylinder.

Have you experimented with them?

I wish I knew someone with a Dyno. My plan is to go 54mm on an 046. Effectively making it 85cc.

I plan on welding up the jug over the transfers and hogging them out to OEM depth.

 

[drf255]

My interest is modifications through changing factory geometry. I just welded up an turned down a 039 piston for a member here. Added 2.5 grams of 4047 alloy to this Huztl piston. Weight should be a wash, as they're generally much lighter than meteor or OEM.




Sent from my iPad using Tapatalk

 

[weimedog]

Have you experimented with them?

I wish I knew someone with a Dyno. My plan is to go 54mm on an 046. Effectively making it 85cc.

I plan on welding up the jug over the transfers and hogging them out to OEM depth.

Looks like we are in the same state, and I plan on building a Dyno this year. On the objectives list. Now just time.

 

[drf255]

W

Looks like we are in the same state, and I plan on building a Dyno this year. On the objectives list. Now just time.

What town?