I grow grapes with a 4 arm Kniffen system. My system is somewhat similar to yours. The first thought I had is that you need at least another post! Two posts spaced 30' apart is a long distance. I have no doubt the wire will be strong enough if 30' long to support the weight but the problem will be keeping the wire tight. My posts are roughly 10' apart. This allows me to keep the sections of wire short and use a turnbuckle on each end. Easier to keep a shorter length of wire tight than a 30' long one! With many small sections of wire, if one section breaks, they are independent so the whole row won't fall down, just the broken section.

I used either 10 or 12 guage wire and it worked just fine. I too used 8' posts sunk 2' deep with concrete poured around them. If I did it over I would use 10' posts and cut them tall enough to have the top wire at 6' from ground. The reason is at 5.5' tall you only have 2.5' of space between the bottom and top wires. I find that the vines on the top wire shade out the bottom wire a lot. My yield is much higher on the top wire because of this.

My posts are old telephone poles as they were free. For hardware I used some large screws with a round eye on the end. These were put in after drilling a pilot hole first on the posts. Get them big enough to support the heavy weight of the grape crop! The first set was real wimpy and a few pulled out due to the weight. I replaced them with some real hefty ones. Since your posts are 4" I would use some that are perhaps 2.5" long but have some width to them. Or as you suggest the lag bolts going all the way thru the post would be really strong.

I hope you put a deadman on the end of your posts? Or at least a soil anchor? I learned the hard way when I skipped that and my end posts started leaning towards the interior as the weight pulled them in over time. I had to dig them out and correct the issue.

Not sure what size turnbuckles I have but several different sizes. I just avoided the really little ones as I figured the bigger ones would be easier to tighten up. They come in so many sizes. I would guess I used some that are roughly 8'-10" in length and they work just fine.

spartanapples - Thanks for your advice.

I bought another pole today as you suggested and looked at 2.5" screws which I'll probably get too.

I hadn't thought of larger turnbuckles being easier to tighten up - I'll be sure to get hefty ones.

I read that an umbrella Kniffen might be better than a 4 arm Kniffen as shading by leaves would be less of problem.

Have you ever tried or considered an umbrella Kniffen system?

Thanks again.

Just for fun I did try the umbrella kniffen on one of my grapevines last year. It worked fine. Lots of smaller canes to work with but the fruit crop was good.

With my 4 arm kniffen the growth rate by late summer is so heavy that I normally start pruning off a lot of the branch tips on new growth. I do not need canes that are 12' long. I also start removing some of the leaves by each cluster of fruit to get more sunlight to the fruit and also to be able to get my insecticide onto each cluster. I seem to have a huge issue with grape berry moth the past few years. If I do not do some leaf removal later in the season, it is hard to spray and have the insecticide get thru the dense canopy and onto the fruit. Grape berry moth is a menace in my area! Usually 2 generations but two years ago we had a 3rd generation late in the season that really damaged my fruit since by that time I had stopped spraying thinking there was no further need to. The wasp damage seems to get more severe each year too as the grapes start to ripen. I have to put out wasp traps to control them or they destroy my grape crop in short order. I grew grapes as a teenager many years ago and never recall having wasp issues like I do presently.

Thanks again. I'll be on the lookout
for moths, wasps and other insects. My vines are now tied. Some are
still just a single trunk heading towards the top wire, one has four
arms and two others only have three arms this year! I bought some
Monterey Fruit Tree Spray Plus (active ingredients: pyrethrins and
70% neem oil) and Monterey Garden Insect Spray (Spinosad ). Don't
know if those will be enough. As a beginner with many
options/opinions it was like pulling them out of a hat.

Hopefully as time goes on I'll be
able to make some intelligent contributions to the forum rather than just asking
questions.