Advice on Tapping Mishap

First, welcome to the group Richard (@rici1 ) !
You have already seen above how useful the people here can be.

Second, no worries about where you post. You are among (new) friends here.
No one will verbally ream you out for your grammar, spelling, country of origin of your machines, or your request for help.
Worst case is that one of our helpful moderators would move it to a new category and tell you exactly why.

I personally would like to understand root-cause of this trouble before I made any recommendations.
Note this is NOT to point fault, but to help guide the way forward.

Was the original thread mis-identified? (perhaps it was metric, but an imperial tap was run thru)
Was the thread identified properly, but the tap mis-labeled?
Was the tap proper, but just too loose for the fit?

I am a fan of Helicoils, but that's likely only because that is the solution most easily available to me, and they have saved my butt a few times.

Brian
 
Just get an epoxy thread repair kit. Mike
Not on a marine engine! Just like on an airplane engine, but wetter.
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.
Concur; it might be worth checking. Metric may fit without retapping.

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Fist option is locate hardware that matches the tap.

Drill and tap a "test block" that you can carry to a well stocked supply house and find a bolt that properly fits the test block.

Also take original sample.

Have supply house provide new size same style and length as sample.

Easy fix.

Sent from my SM-G781V using Tapatalk
 
I agree with the above comments that you need to nail down how oversize the hole is and what type of thread you are dealing with. Lets assume the threads are shot. Helicoils are an option but require better access and removal of more metal. Since you have access to the back side right now, you could go up one bolt size (SAE or metric) and retap the holes. That does not burn any bridges and you could go to helicoil later in the unlikely event of a failure.
Like a lot of the crap I see, I am betting someone put SAE bolts in a metric hole or the reverse! Some of these threads look so similar it is easy to mix them up.
 
5/16 has a nominal diameter of 7.93mm. M8 bolts are, well, roughly 8.0mm diameter. If that tap is measuring 8.3mm, then it's junk. I got a bum set from Harbor Freight once, but those were undersized.

I am not a fan of Helicoils; seen too many fail. Yeah yeah, I know, "if installed correctly" and all that. I much preferred Timeserts. More expensive, but a stronger permanent repair.

What you do will partially depend on the situation. Are the bolts merely supporting the pan, or is the pan load bearing. IE engine mounts or components attached to the pan. The next size up is M9, an extremely rare size. After that is 3/8. Both would require drilling out the block and the pan flange.
 
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If you are considering upsizing the bolts to the next size you also need to consider whether there is clearance on the sump for the larger bolt's head size. Sometimes that clearance is tight to begin with.
 
I re-read your original post more carefully. 34 holes! Yikes!
If you ran your tap through all those holes you better find out exactly what the tap is as a starting point. How did you measure the diameter? I assume it is a 4 flute tap? (a three flute is hard to measure)
 
Thanks again for all your replies, and thanks Brino.

I did check the bolts with a thread gauge and cross-checked with the workshop manual - they are definitely 5/16 UNC.

The tap does have the correct TPI and the label is 5/16 18. It is a 4 flute tap and I checked with a Vernier, major diameter 8.3mm. The fact that it also has 'NF' stamped onto the tap is indicative of the poor quality of manufacture I think. I'll take it to a workshop this week and see if they have stock to match, otherwise it looks like it'll be inserts I guess. Point taken about going to the next size up - the clearance on the pan with 5/16 is tight enough, but fortunately I am just supporting the weight of the pan and oil. I'll look into timeserts, thanks for the headsup.

Cheers,

Richard
 
Hey guys...general question to the peanut gallery. How much adjustment is available on a standard thread cutting die? Could you open one up to get an 8.3ish mm OD screw with a 5/16 die? I think you can see where I am going with this.
 
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