It's actually -10 hose with 1/2" fittings. My engine builder recommended it because he knows how I drive and I tend to push things pretty hard. I'll beat on it really hard and check temperatures all over the heads and block and it's super even and never too hot. A lot of people struggle with crossover systems this seems to be the ticket.

Thanks man! good info, I really appreciate it! I did grind, drill & tap my manifolds. I taped it to 1/4 pipe as to run -4 hose & fittings last night. I can always go bigger.

The webbing removed was to the top of the tapped hole.

you don,t want or need a large diameter hose,all you are doing is releasing air that gets trapped in the rear of the block.this is only needed on siamese blocks.

Would you agree -4 is sufficient? What would be the harm in a larger hose size? ThisIsLivin seems to be getting real good #s with -10. Problem I find is information on this subject varys so much & there seems to be vast amount of varying information. Some say nothing is needed. Some say -4 is all you need. Some go larger. Then where to vent it to is yet another topic of discussion. Into the front crossover, overboard, into the exhaust.. I guess its possible I may not need to vent this at all, but i guess id rather air on the side of caution. Im open to all input.

After spending $12k on a major rebuild and reading all the horror stories of crossover cooling systems, I wasn't going to take any chances. I even took a die grinder to my Hardin stainless water pump to improve flow, and carefully radius and polish surfaces the impellor rides on. I even opened up the water outlet in my headers to improve water flow. My engine builder does mostly high performance marine engines and he recommended the -10 line based on his experience. Our intent was to not only vent air/steam but to encourage flow to even out cylinder head temp. He's been doing this a long time and I trust him. I'm a fanatic for testing and measuring and the numbers I see are as close to perfect as anyone can expect. The amount of water coming out of my exhaust at idle is significantly greater than I see on any other boat so I know I am getting good flow. I will say this, when the water temps are in the low 50's or lower, my thermostat doesn't open, the 3 relief holes provide enough flow to keep the engine cool even while cruising at 60. I should mention too that the air temps were also in the low 50's.

It obvious your getting good results. It also sounds like you have the cooling system dialed in good. If works, dont fix it. lol Did you drill extra holes in your thermostat?
Curious, what are you running for a cam? You say you have higher water flow out the exhaust & no reversion?.. Thats something else thats always on my mind. Im cursed with a brain that wont shut off. lol

I am using a Stewart racing high flow thermostat that came with 3 3/16" holes. My cam is a custom from Comp Cams, duration at .050" is 242/252 and lift is .623/.612. If you look at the picture, the water is injected into the exhaust right where the rubber hose is clamped so there is some distance between the runners and water entry point. The downward angle helps as well. I have a real lopey idle that I have to use a lot of timing tricks to keep running at 800rpm. Without the mufflers it sounds incredible. First time I started it in our harbor the echo sounded like thunder.

Im using Hardin marine cross over kits with their thermostats. They tell you to drill them if needed. Your cam is pretty heavy on duration, for what looks to be a wet exhaust. Im not sure im looking or seeing what your talking about as to where the water is injected into the exhaust, coupler? Cant see the port side completely. My cam is deff milder that yours. Its from Crane. I had a rough experience with comp, long story. my duration is 226*in / 234*ex @ .050 & 587in 610ex lift w/ 112* LSA. Crane tells me I wont have reversion with this grind in my 502's but I swear its right on the verge. What "timing tricks" are you doing? Sounds interesting. Im running MSD digital 6M-3L. I like it because I can dial my rev limit and retard my timing at start up. Works good with the curve options of the distributor.
I bet yours sounds great with that cam, love to hear it.

With a crossover you definitely need to drill vent holes in your thermostat. It helps it react faster as the engine heats up. You could be over heating your back cylinders before the hot water gets to the thermostat because there isn't any flow through the engine. I had an MSD 6M-2 ignition before I switched to the Daytona Sensors CD-1. The CD-1 is programmable and will do idle speed control with timing. With my big cam and the wide temperature variation I see it was tough to keep the idle below 1000rpm in neutral and keep it running in gear.on a carbureted motor. With the old ignition and big cam I had the distributor locked at 34 degrees to help the idle. With the CD-1 I can do start retard and full programmable timing. After fussing with start retard modules and mechanical advance, this is so much better. I wanted to make it more drivable for my wife and I so we could enjoy it more. The only thing I will miss is that I could start my engine within 1 compression stroke with the MSD. Found that out the hard way. I was out on the lake anchored listening to tunes and of course had both batteries on. When we decided to leave, I only had enough juice to pop the motor 1 compression stroke. So I carefully pumped the gas (no choke) and hit the key as quickly as I could because I knew if I held it to long there wouldn't be enough voltage to fire the ignition. This was a trick I learned when I was going to college and I couldn't afford a new battery for my car. Sure enough my baby started right up. Needless to say I replaced the batteries before the next outing.

Forgot to reply to your exhaust question. The water is injected at the end of the taper internally. There is an inner and out wall and the inner stops right at the end of the taper with about a .060" gap all the way around. So there is almost 16" between the end of the runners and where water is injected. The exhaust can get hot but you not enough to burn you.