Advice on Tapping Mishap

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rici1

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Good Day,

I hope you don't mind me posting here, but I am looking for advise on a mishap I have had whilst cleaning out some threads.

I am not an engineer by trade, I'm just starting out as a hobby, and am rebuilding a marine diesel. The threads in the block for the sump bolts were contaminated and the bolts themselves in poor condition, so I decided to replace the bolts, new sprung washers, and run a tap through the threads to clean out the corrosion and detritus. The block is cast iron, the thread 5/16" UNC, and the bolts class 2A, grade 2, self colour. Length of engagement 0.6". The weight of the sump and oil is about 140lbs and the manual specifies max 17 ft-lbs of torque on each of the 34 bolts.

I purchased a set of UNC/UNF taps, ran the first one, checked the bolt and then ran the tap through the rest. I'm kicking myself now for not checking thoroughly enough, being in a rush, for when I came to bolt the sump back on I noticed excess play in the bolts. Checking the tap, I see a major diameter of 8.3mm and a minor diameter of 6.7mm. It's even marked "5/16 NF 18" which makes no sense (the fine tap is marked "5/16 NF 24" which is a bit better I suppose).

I am worried about stripping the internal thread when reassembling and also about the bolts coming loose through lack of friction and loss of thread engagement.

The way I see it I can either reassemble to specified torque using sprung washers and perhaps locktite 242 or I can use an insert such as Helicoil.

The problem with the second option is difficult access, the engine is pretty much assembled so to drill the holes it would have to be a hand drill. Then I'm not sure about the amount of material to actually drill into, I am guessing for a 5/16" Helicoil I would have to enlarge the hole by about 0.1" and as you can see from the picture they already are up against the fillet of the block.

Can anyone advise on the best approach to this mess?

Much appreciated,

Richard
 

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Being a marine engine fix it right and right now.
If you have to pull the sump then so be it.
If you are checking the holes with a new grade 8 bolt and it is loose then it is no good and needs to be repaired.
The heli coils need to be inserted from the flange side.
 
Thanks.

I did consider getting oversize bolts made up, but as the tap is a non standard size I was thinking it might be difficult to measure and then produce bolts with the precision required?

The sump is off already and the engine free standing in a garage, but I would still have to use a hand drill to prepare for helicoils (unless I strip back to bare block) which I was quite nervous about - hence consideration of thread lock instead.

But I agree that the inserts are the better option.
 
It just depends on your equipment available and comfort level. Making oversized bolts at 18 tpi is pretty rudimentary if you have a lathe and mill, and I would personally prefer this over inserts. Otherwise, you do what you can do.
 
One reason replacement bolts mangle block threads is folk sometimes do not carefully check if the new bolts are a tiny bit too long, or if there is some junk in the hole, causing the bolt to hit bottom before the head pulls onto the sump cover properly, including counting gasket compression.

Going to a larger diameter bolt is a possible fix that has to be done with care, ensuring there is enough casting to take it, not to end up too thin, or breaking through anywhere on the inside. The better way is to fix the messed up one using a Helicoil insert. The insert needs a special size tap drill, but the end result is a great fix, forever. I think it's a better way than drilling out to a larger bolt, but it's a bit more expensive. Caution: Even the Helicoil fix still needs the sufficient metal around it, just like the oversize bolt.
 
Just get an epoxy thread repair kit. Mike
 
Helicoil or timeserts are the correct fix for this. Helicoils should require less drilling than the timeserts, and from the photo provided it looks like there is plenty of metal to allow for proper installation of them.

When deciding how to repair this you need to consider how difficult a repair will be if the repair fails while the engine is installed. Now is the time to spend the time and effort to do it correctly. If the engine is on a stand and can not be rotated then there is actually less to worry about with contaminating the engine with drilling swarf, gravity is working in your favor and all of the swarf will drop away from the engine.

Sent from my Pixel 5 using Tapatalk
 
rici1 said:
excess play in the bolts.
Click to expand...

It is not clear to me what the original thread pitch is and what is the actual thread pitch of the tap you used. Did the tap cut much metal or just crud? If the marking on the tap does not make sense then what does it actually measure?
It might be possible to tap the internal threads to an 8 mm pitch (either 1.0 mm or 1.25mm) in a 24 or 18 tpi hole. This is not ideal but considering that it is not a high torque application it should provide enough thread engagement to do the job.
 
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