Max cylinder bore.

[mike jesse]

Anyone have an idea of how big a stock cylinder can safely be bored to?

I noticed that when the M8 first took off there were lots of 114's being stepped up to 120's.

Now the new normal is aftermarket cylinders and 128 c.i.

[Jamie Long]

Anyone have an idea of how big a stock cylinder can safely be bored to?

I noticed that when the M8 first took off there were lots of 114's being stepped up to 120's.

Now the new normal is aftermarket cylinders and 128 c.i.

max bore size on 114/117 cylinders is 4.180", the 107 cylinder can be bored to 4.125"
Article on bore & displacement can be found here> https://university.fuelmotousa.com/article/h-d-milwaukee-8-engine-big-bore-displacement-info/

[Nastytls]

"Stock" cylinders aren't being bored to 128". I've only seen aftermarket cylinders are up to 126" and the HD SE cylinders are 128". Haven't seen a stock 107" that bored to more than 117". Not sure why anyone would do that anymore thought, when you can get a lot more CI with aftermarket cylinders w/ thicker liners.

[Don D]

114/117 to 124, 4.185. I have done many. Leaves .078 per side. Going to sleeved cylinders can hug the case bore and go bigger safely, 4.28".

[fbn ent]

I was just told by a builder/friend that no pistons are made for M8s. He said HD says if you need a bore, you buy new cylinders.....?

[Don D]

There are many pistons offered. I prefer Mahle and KB

[cheech]

I was just told by a builder/friend that no pistons are made for M8s. He said HD says if you need a bore, you buy new cylinders.....?

I assume you're referring to OEM offering oversize?
No need to worry, as stated above the aftermarket has that covered.

[fbn ent]

Thanks guys, I passed the info on to him.

[BigT]

114/117 to 124, 4.185. I have done many. Leaves .078 per side. Going to sleeved cylinders can hug the case bore and go bigger safely, 4.28".

Did you mean 120", 4.185?

[Don D]

Depends on stroke, 120/124 4.375/4.5

[Hillside Motorcycle]

We commonly bore the 107"s to 117", using a CP piston. 4.125".
Just completed another one Tuesday, using an S&S .475.

[To The Max]

I had my 107 bored to 117 and it split the spigot on both cylinders in 3000 miles i was very lucky not to have lost the bottom end as the front cylinder spigot nearly fell off. and that was from when i heard the noise at 80mph to when i turned the motor off. Sorry would never do that again. Max

[Don D]

What pistons did you use?

[Nastytls]

I had my 107 bored to 117 and it split the spigot on both cylinders in 3000 miles i was very lucky not to have lost the bottom end as the front cylinder spigot nearly fell off. and that was from when i heard the noise at 80mph to when i turned the motor off. Sorry would never do that again. Max

There are cut away pictures out there, of M8 cylinders bored for 117" and they are practically paper thin at the top of the spigot. Apparently the design of the M8 cylinders is quite different from the TC, so it's not the same as boring a TC for a 107", it's much worse. Knowing this, combined with the fact that new aftermarket cylinders are cheap, I cant see any reason to bore the stock ones.

[To The Max]

What pistons did you use?

C.P  . Max

[Don D]

CP bullet series slipper skirts, their shelf piston, are not offered by us. We use exclusively Mahle and KB (warehouse distributor) and occasionally CP full skirt custom pistons. I have good reasons for this.

[RTMike]

Don do you have any TC 107 Mahle pistons in stock

[Don D]

Sorry no. I am not a WD for them and buy from Suburban HD

[TorQuePimp]

I had my 107 bored to 117 and it split the spigot on both cylinders in 3000 miles i was very lucky not to have lost the bottom end as the front cylinder spigot nearly fell off. and that was from when i heard the noise at 80mph to when i turned the motor off. Sorry would never do that again. Max

There are cut away pictures out there, of M8 cylinders bored for 117" and they are practically paper thin at the top of the spigot. Apparently the design of the M8 cylinders is quite different from the TC, so it's not the same as boring a TC for a 107", it's much worse. Knowing this, combined with the fact that new aftermarket cylinders are cheap, I cant see any reason to bore the stock ones.

Also makes the m8 cylinders much easier and less time consuming to sleeve
Having .100 thick cylinders made out of better iron 👍

[Don D]

I have been having them sleeved with ductile iron. The end product is much more stable than HD or S&S cylinders bored for the larger bores. I have 2 143s in planning stages now.