Stripped threads re-threading size questions

[jbsdf]

Thank you all for all of that, I got more help than I bargained for!

Engine is a Bukh 10 single cylinder 10 hp diesel installed on a ocean cruising sailing yacht 30' in length. I can't tell you how important it is to get this right as your whole world and your survival depends on having that engine working when you exit the trade winds and have to come into port.
There are people who sail engine-less but that is getting more and more rare as time goes on definitely something I'm not qualified for. I had to sail into port one time and almost took out a couple million dollars of yachts. Luckily the engine would still start for short periods and I fired it up just in time to slam into reverse and rev hard, and we purred to a graceful stop.

dutch mfg so likely metric probably M6 as suggested. I do have original bolts they are in a sad state of saltwater decay though but they can still be measured. I will get pics and measures as soon as I can make it out to the boat.
I will say this though the original bolts are of a finer thread than the ones I measured. The ones I measured were incorrectly installed and are of a more coarse thread, so they literally cut their own coarse "threads" into the fine threads in the block. This actually held up for uninstalling and re-installing the bolts something like 4 times but when the head gasket started leaking and I had to get my hands really dirty and when I needed it most, they failed.

M7x.75 comes out to 0.2440945" maybe enough to actually shave the threads and not enlarge the hole too much. What do you guys think?

Heli-coil seems like a good idea but as it was said this is a thermostat housing and a watertight installation is critical (and were talking highly corrosive sea water here) as it was correctly said by Red96 making sure that this doesn't leak is the only goal we have here I would be suspect of the inserts creating a gap in where the thermostat housing and the cap and the cast iron extrusion that is the thermostat housing and making this install useless if it is not absolutely watertight and will last the corrosive environment of the sea.

What I worry about is if the Heli-coil install didn't work out that there would be enough metal left to do anything else. What do you guys think? can a Heli-coil install be made to successfully keep the water jacket sealed?

Also Red96 you are in my neck of the woods yacht is in Everett. Would you be interested in checking out the situation here? PM me if you are interested!

True story, I actually used to work in machine shops doing full CAD, CAM, G-code, Setup machining, jigs, vice jaws, everything necessary to hand it off to an unskilled laborer. Unfortunately I never got much traditional machining experience as I jumped straight into the deep end as a teenager. I decided even after going to college and obtaining a certificate in basic manufacturing and, a CNC machinist certificate of proficiency, because of this the high stress of my jobs without traditional machining experience warranted a career change, or at least a life style change haha but that doesn't mean I can't use some of my barely grasped skills when repairing my engine.

 

[Jake M]

Heli-coil seems like a good idea but as it was said this is a thermostat housing and a watertight installation is critical (and were talking highly corrosive sea water here) as it was correctly said by Red96 making sure that this doesn't leak is the only goal we have here I would be suspect of the inserts creating a gap in where the thermostat housing and the cap and the cast iron extrusion that is the thermostat housing and making this install useless if it is not absolutely watertight and will last the corrosive environment of the sea.

What I worry about is if the Heli-coil install didn't work out that there would be enough metal left to do anything else. What do you guys think? can a Heli-coil install be made to successfully keep the water jacket sealed?

I don't know that engine, so googled pictures. (Whatever that's worth.....) It looks like there's either some options, and/or the search is mixing in other models, so your pictures will still be valuable. And we've got to figure out if these are blind holes or through holes, but that's just details to work out the available leak path(s) and address that. No big deal either way. Overall though the engine side (with the threaded holes) seems to be the same. All of the things I see have one thing in common. You're not joking about not a lot of metal to remove from the beginning, forget about what may or may not be missing from the salt water environment. Looks like good, easy access though, as long as the rest of the boat isn't in the way.

I still say the best answer will come with your pictures, but thus far Heli-coils tick all of your boxes. Contrary to internet notions, how big or heavy an insert is does not indicate how well it performs, or that it is right for every application. Most of them are stronger than the original threads if the installation goes even close to going well, and stronger than the bolt in most cases. And being fair, the failure here wasn't a badly designed joint, it wasn't inadequate threads, it wasn't a material failure. A M6X1.0 heli-coil repair vs a M7X1.0 "upsize" will be such a trivial difference that you really don't even care, as the error from hand drilling and hand tapping (Very acceptable error, and probably no more error than what's in the original, mass production part) will bury the difference between the two. It's really that trivial. So with an insert you get a reinforced thread, stainless at that, which won't wear or degrade with normal service. Like I said, I still want to see pictures before I give my "final answer", but thus far, heli coils sound like the most practal, most robust solution, won't require modifying the water neck, and with future servicing will still be able to order and interchange parts right out of the manual. Sure, it's not an every day service point, but it's a thermostat housing. It is a periodic service point. You'll be there again one day.

 

[wachuko]

Google search: “Great engines, the Rolls Royce ( so they say) of marine diesels. Really solid and sound engineering. Points to watch are thermostats and exhaust elbows, otherwise if you look after it, your Bukh will look after you.”

 

[OTmachine]

Some copper crush washers at the heads of the thermostat bolts should stop any bypass coolant leakage around the heli-coils.

 

[Flyinfool]

A normal straight thread is not leak proof so the leakage of a Heli-Coil will not be an issue. All Heli-Coil brand inserts are made of Stainless steel. I am not sure of other brands. If worried about leakage past the threads a small amount of silicone on the threads will kill any chance of a leak and also does a good job of locking the bolt while keeping it removable. I have never seen a properly installed Heli-Coil fail. Where I work we installed literally thousands of them.

 

[Chipper5783]

Helicoils are a pretty good option. I have had them fail (generally from abuse, someone is careless about threading the bolt in).

You said you were a bit shy on available wall thickness. Depending on the size relationship, you may be able to gain a little bit of your housing wall. I have had excellent results using a homemade style double threaded sleeve - similar to a keensert. Start with a fine thread bolt (metric or imperial doesn’t matter - that you have a tap for), with a minor diameter slightly larger than the major diameter of the hole you are repairing. In you case for the M6 thread, an M7x0.75 if you can get it (a pretty rare item), next choice would be 5/16-24 (even 5/16-18 would work). Then drill and tap this bolt to your M6 thread (you have access to a lathe?). Drill and tap your housing. Thread in the sleeve with Loctite - cut it off flush.

You end up with a fairly thin double threaded sleeve. There is no wire that could get unwound.

 

[OTmachine]

A normal straight thread is not leak proof so the leakage of a Heli-Coil will not be an issue. All Heli-Coil brand inserts are made of Stainless steel. I am not sure of other brands. If worried about leakage past the threads a small amount of silicone on the threads will kill any chance of a leak and also does a good job of locking the bolt while keeping it removable. I have never seen a properly installed Heli-Coil fail. Where I work we installed literally thousands of them.

Been there, done that, multiple times. Hard to get a good seal between the tapped hole and the heli-coil.

 

[Flyinfool]

Helicoils are a pretty good option. I have had them fail (generally from abuse, someone is careless about threading the bolt in).

You said you were a bit shy on available wall thickness. Depending on the size relationship, you may be able to gain a little bit of your housing wall. I have had excellent results using a homemade style double threaded sleeve - similar to a keensert. Start with a fine thread bolt (metric or imperial doesn’t matter - that you have a tap for), with a minor diameter slightly larger than the major diameter of the hole you are repairing. In you case for the M6 thread, an M7x0.75 if you can get it (a pretty rare item), next choice would be 5/16-24 (even 5/16-18 would work). Then drill and tap this bolt to your M6 thread (you have access to a lathe?). Drill and tap your housing. Thread in the sleeve with Loctite - cut it off flush.

You end up with a fairly thin double threaded sleeve. There is no wire that could get unwound.

While Heli-coils do work great, IF you do not already have the tap and install tool, these can be very expensive to purchase.

Can you reach the backside of the the hole you are repairing? If so a good blob of grease will help to catch all the chips from drilling and tapping and heli-coil drive tang removal, so that they are not floating around in your cooling system looking for the worst possible place to come to rest. Like the thermostat or impeller.

I have also used a similar method to Chipper where wall thickness was an issue. Except that I first drill and tap the hole for the filler bolt in the part being repaired. Tapping it to just a couple of turns short of full thread depth so that the repair bolt will jam tight at the very bottom of the threads.

Insert the whole bolt into the new tapped hole using either red loctite or JB Weld epoxy. The JB Weld is preferred. Jam the bolt tight against the few imperfect threads that you left at the bottom of the hole. Let that set up for 24 hours.

Cut off the bolt and grind or machine flush to the gasket surface. I often use a spotting drill to to remove the inserted bolt flush without damaging the gasket surface. This also gives a good center for the tap drill to get started. Then drill and tap the M6 thread like you were putting it in for the very first time.

I have done this where there was less than .010 wall between the outside thread minor Dia and the inside thread major dia where this repair piece would be to fragile to make on the lathe or even handle for insertion into the hole. Has held fine for many years. I have even used this where the original hole was drilled and tapped in the wrong location by a half of the hole diameter.

 

[jbsdf]

Google search: “Great engines, the Rolls Royce ( so they say) of marine diesels. Really solid and sound engineering. Points to watch are thermostats and exhaust elbows, otherwise if you look after it, your Bukh will look after you.”

Sounds about right since I am having issues with both. Thermostat is out of service as well. Trying to locate one currently. The engines are built to last. The block is very heavy for such a small engine, the crank has a special kind of counterweight to cut the vibration down. Engine weighs like 350lb flywheel alone weighs 50lb or more. Biggest flywheel for smallest engine anywhere. Also can be hand started in event of lightening strike to the mast which would render starter and all other electrical and electronic devices inoperable. Or if your battery dies while out at sea. It has a decompression lever which opens a valve in order to build up momentum with the heavy flywheel if batteries or starter become inoperable. Unfortunately the hand crank runs into both the mast and the toilet in my install so a custom extended hand crank lever will need to be fabricated for hand starting.

The downside of the engine is that raw sea water is used to cool the engine instead of using a heat-exchanger which newer engines have and can easily be cleaned or replaced. Not impossible to add but definitely a lot of work and money. Some people feed fresh water into the system after running or if it sits for a long time, but a large amount of fresh water is not always available to you at sea when priories are drinking the freshwater.

Also an exhaust manifold stud broke in half when putting that back together creating a small exhaust leak. Only about 1/2" of the stud is left there. Trying to figure out the best way to remove it, but it might have to be drilled out.

My friend and I are thinking perhaps a smaller than original bolt size Heli-coil might be an option to avoid drilling out the holes too large to do anything else with. This way if it the Heli-coil doesn't work out I can still drill out the holes for M8 or possibly M7x0.75 if I can find it. In my case I will have to purchase the entire kit as Heli-coils are new to me. Where I am worried about leakage is between the block where the thermostat housing extrusion bolts onto it. Will the surface be flush after installing the Heli-coil?

 

[OTmachine]

Sounds about right since I am having issues with both. Thermostat is out of service as well. Trying to locate one currently. The engines are built to last. The block is very heavy for such a small engine, the crank has a special kind of counterweight to cut the vibration down. Engine weighs like 350lb flywheel alone weighs 50lb or more. Biggest flywheel for smallest engine anywhere. Also can be hand started in event of lightening strike to the mast which would render starter and all other electrical and electronic devices inoperable. Or if your battery dies while out at sea. It has a decompression lever which opens a valve in order to build up momentum with the heavy flywheel if batteries or starter become inoperable. Unfortunately the hand crank runs into both the mast and the toilet in my install so a custom extended hand crank lever will need to be fabricated for hand starting.

The downside of the engine is that raw sea water is used to cool the engine instead of using a heat-exchanger which newer engines have and can easily be cleaned or replaced. Not impossible to add but definitely a lot of work and money. Some people feed fresh water into the system after running or if it sits for a long time, but a large amount of fresh water is not always available to you at sea when priories are drinking the freshwater.

Also an exhaust manifold stud broke in half when putting that back together creating a small exhaust leak. Only about 1/2" of the stud is left there. Trying to figure out the best way to remove it, but it might have to be drilled out.

My friend and I are thinking perhaps a smaller than original bolt size Heli-coil might be an option to avoid drilling out the holes too large to do anything else with. This way if it the Heli-coil doesn't work out I can still drill out the holes for M8 or possibly M7x0.75 if I can find it. In my case I will have to purchase the entire kit as Heli-coils are new to me. Where I am worried about leakage is between the block where the thermostat housing extrusion bolts onto it. Will the surface be flush after installing the Heli-coil?

Normal heli-coil installation is 1 thread below surface.