Not sure but the hole is about 3/16" so not much will be removed to drill/tap. The manual specifies tapered plugs but I think a bolt would be more secure.

The manual shows those drillings to be 7mm, so tap a short distance for 5/16 NC, and use socket head screws that jam in the threads.
If the drillings are smaller, it may be possible to tap for 1/4".

Yea it was a while since I looked at it but when I drilled out the carbon I went up to 3/16 before I hit aluminum(I think). I thought of using socket head screws so the head would go into the passage as a hex won't go in. thanks for the suggestion.

Tony,

By socket head screw, what I had in mind was a set screw that will go completely into the hole.

I don't think there is an aluminum bolt/screw that has a head like that. what I could do is partially tap the hole, run the bolt in until it bottoms and cut it off the correct length so the intake manifold keeps it from backing out.

Steel set screw + red Loctite = Forever

Sounds good. The holes are about 1/4" so 5/16 should be good.

I have been thinking about having my block (cylinders)sonic tested before I spend money on pistons. I measured a few places with a caliper but it's limited to the area at the top of the cylinder at the coolant passages. I measured about .175" which is more than ample and I can't imagine it would vary much but I went from 92mm to 96.5mm so a lot of cylinder was removed. thing is there is no one around here that can do it. Even if I could measure a know thickness and then compare to that. Here's one I found that costs about what it would if I had it done at a machine shop and has good reviews:
https://www.amazon.com/Ultrasonic-Thickness-Gauge-Tester-Calibration/dp/B07VVJT49S/ref=cm_cr_arp_d_product_top?ie=UTF8

Tony,

Note that in the amazon listing, there is a statement that says that the instrument is not suitable for cast iron!

Thanks-I read the description closer and noticed that. I also asked the specific question in the "Customer questions & answers" area but no response yet.

I might try to find a caliper that can reach in farther through the coolant passages. I don't think the walls are so thin as to be super critical. I have read of thrust side walls as thin as .125" on 800hp engines with no problem.
Edit-saw this and gave me an idea. https://www.ebay.com/itm/203488563223?epid=1438622379&hash=item2f60dd1017:g:TZYAAOSw96VgwOrc
I think I can fab some extended jaws and attach them to my caliper.

I see it's been a year since I updated this project even though progress has been made. Biggest update is I discovered my cylinder walls are marginally thin (like .010" in places) and I have decided to start with another block. In retrospect I was fixated on using the 5.6 pistons and the machine shop I used was a "Mom & Pop" shop with no sonic tester and 50 year old equipment. The old block still needed to be honed for custom pistons and re-decked to clean up the improper surface so good time to move on to a replacement block. The plan is to have the replacement block sonic tested and then bored to maintain .175" walls. I calculated a resulting bore of about 93 ish mm resulting in 5.15L displacement. I will still use 5.6 head gaskets even with the smaller bore as the 4.5 gaskets protrude into the 5.6 head combustion chamber and shroud the valves/create hot spots. Pretty sure the 5.6 intake valve will work with a 93mm bore especially with custom pistons. The challenge now is logistics as we now live in a somewhat isolated area with no machine shop. The shop with the best equipment is about 4.5 hrs away.

Tony,

Before going thru the exercise of locating, prepping, and measuring another block(s), it may be worthwhile to go to an engine shop that has modern ultrasound equipment, and have your block mapped. If only one or two cylinders show wall thickness under .100", the thin ones can be sleeved. LA Sleeve has a sleeve specifically for the M117 block.
Is Portland the closest, or is there a shop in the Eugene/Springfield area? It is important that the testing equipment be of a low frequency type that will be accurate in cast iron.

I think it's overall thin. I think I will be money ahead to move on to another block. I don't have a large investment in it other than toting it among 3 states and keeping the bores oiled. I can still use it to mock up other projects. The machine shop I would use is in Portland. Portland Engine Rebuilders – Portland Oregon Automotive Machine Shop
A sleeve might make sense for one cylinder but not 8 when replacement blocks are plentiful and inexpensive.

If there's a reputable machine shop down the road from the source of the block, you could get it checked out before you have it shipped. :)

-CTH