Got it now after what OldMike said. I was basing my thoughts on my KTM experiences where vent pipes fed into the airbox only come from the crank case, not the heads. KTMs have an SAS system which draws clean air form the airbox and feeds it via a valve into the stock exhaust to help combust unburnt fuel. Causes problems with de-cat aftermarket exhaust systems but the valves are easily blanked off with plates and then it's only a case of deleting the system from the EMU with a retune.
I'd pictured a similar set-up for the Harley but as it's my first one it's a learning curve. I'll use external vents for the intakes and a Trask cover for the crankcase. Hopefully then I'll get away with a transparent Trask Assault breather and it will stay clean.

Just a thought, why do the top ends on Harleys need venting? Is it because the crankcase pressurises the heads via the pushrod tubes? If that is possible, or is it only a question of piston blow-by? Sorry if it's a stupid question but I'm trying to build up a clearer picture of Harley mechanicals as I've only owned dohc metrics.

 

This is from the M8 engine thread from Trask- The CheckM8 is 50 state compliant. External Breather kits are not 50 state. Here is some more info on the CheckM8 for you guys
The Harley-Davidson M8 engine has faced notable challenges, including excessive oil consumption and blow-by issues. However, Trask Performance has introduced an innovative solution with their CheckM8 vented transmission top cover. This remarkable product addresses these problems effectively through a patent-pending design.

The key feature of the CheckM8 vented transmission top cover is the incorporation of a one-way Boyesen valve. This valve plays a crucial role in bleeding off excess crankcase pressure while retaining oil and air mist within the cam chest. By doing so, it enables the oil pump to operate more efficiently, leading to a significant reduction in oil exiting from the head breathers. This not only mitigates oil-related concerns but also results in improved engine performance by facilitating smoother revving.

Key attributes of the Trask Performance CheckM8 vented transmission top cover:

1. **Innovative Design**: The patent-pending design of this product is specifically engineered to reduce or even eliminate oil discharge from the head breathers of your M8 engine.

2. **Quality Craftsmanship**: Crafted from high-quality billet aluminum, this top cover is built to last. Its durable construction ensures long-lasting performance and reliability.

3. **Easy Installation**: Replacing the stock cover is a breeze, making the installation process straightforward and hassle-free. This switch not only improves functionality but also enhances the aesthetics of your Harley-Davidson engine with a clean and polished appearance.

4. **Proven Performance**: The CheckM8 vented transmission top cover has been rigorously tested and proven on Trask Performance race bikes. Its effectiveness in addressing oil-related issues has been demonstrated in high-performance scenarios.

5. **Made in the U.S.A.**: Trask Performance takes pride in manufacturing this top-quality product in the United States, ensuring it meets the highest standards of craftsmanship and quality control

 

There is a very good crank case pressure explanation that A1 Cycle did, I can't link to it, but you should be able to find it.

 

I installed the Trask M8 transmission breather on my CVO, I have very little oil mist going into my intake. For me- works well. I’ll be putting one in my Heritage soon.

 

Didn't Steve Cole say new M8 VVT's vent thru the crankcase as they should?

 

I did it on my 79 SP, 85 LeMans IV and my 1997 Cali 1100i. As well as on my 89, 2002 and now my 2019 ElectraGlides.

 

The CVO 121 VVT models vent through a new transmission top cover with a built in oil separator. The hose from the top cover routes to the back of the air cleaner.