Question for the Engineers..

[castaway]

Whilst investigating my other little gearbox snag I noticed that the 'box' cooling jacket has got a stripped thread on the water drain, and was only staying in due to familiarity!

The plug is amazingly accessable and is 3/8in and looks like 24tpi and very very fine thread... Its the same for the other b/box oil drains but they are OK. What I need to know is what taps I should use to cut a new thread in a slightly bigger/courser thread. I don't want to have to drill as I hav'nt got room to get a drill to the job.

Any help would be appreciated, The 3/8 shackle pin that is standing in as temp plug can't become to permanent!

Engine is Perkins 4107 and box is Nicor.

Thanks Nick

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[Avocet]

Need a bit more information. Is the plug 3/8" on the OUTSIDE of it's threads or is that the core diameter (across the roots of the threads)? Also, is it tapered? Many hydraulic fittings plugs and such like are tapered so that they seal better when tightened more.

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[johnsomerhausen]

Instead of tapping the holke, why don't you go to an auto parts shop and get a kit of "thread restorer". It's actually a special king of epoxy that you smear onto the plug after convering it with an "anti-adhesive". You let it cure and you've got a good seal again. I think Loctite is one of the manufacturrers.
john

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[castaway]

Its actually on the outside of the threads, and no, its not tapered.


Thanks Nick

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[TheBoatman]

You have a number of problems here.
1. Forget 3/8" anything if the thread is stripped you cant use 3/8.
2. You could use 10mm because its just slightly bigger than 3/8.
3. When you re-tap the hole some swarf is bound to get in the box.
4. The best solution if it's a die cast ali box would be to put a stainless insert in but that would have to be done by someone that knew what they were doing and had the tools but it would also mean drilling the box which you say is not an option.

I think the best solution would be to get a 10mm "set bolt" (one which has the thread all the way up), put plenty of tapping compound onto the tap(s) and take it slowly winding out the tap at regular intervals to clear the swarf from the tap. You could use grease if you don't have trefelex (tapping compound) the grease will hold the swarf more easily.

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[VicS]

3/8" by 24 tpi is 3/8 UNF. The next size up is 7/16 which is 20 tpi but the recommend pilot hole size is 9.9mm or 0.3898" which is more than the hole you have even with all the thread stripped out.

Next consideration is 7/16 BSF (18 tpi) but again the recommended pilot hole size, 9.7mm or 0.3819", is still more than 3/8" although at 0.3663" the core diameter is less than 3/8".

So now look at going metric. 10mm is probably too close to the original. 11mm requires a 9.5mm or 0.3740" pilot hole (virtually 3/8" ). The pitch is 1.5 mm which is approximately 17 tpi.

7/16" UNC (14 tpi ) requires a 9.4mm or 0.3701" pilot hole which is only slightly less than 3/8".

7/16" Whitworth (14 tpi ) should be cut in a 9.25mm or 0.3642" pilot hole which is rather smaller than the cleaned-out size of the existing hole and would result in an effective thread depth reduced below the recommended 75%

The choice would seem to be between 7/16" UNC, 11mm and possibly 7/16 BSF. Your choice will depend on the availability of taps to you, just how clear the existing hole is and the need to get a new plug machined.
I would expect many workshops will tell you that 7/16" Whitworth is near enough to 7/16" UNC!

Depending on how much clear space you have inside the box you may need a plug tap as well as taper and second cut taps.

The soundest possible advice must be to remove the unit to a workshop and drill out and fit a 7/16 UNF plug but you obviously want to avoid doing that. So take care to line the taps up properly and take your time.

Payne's working party is 4th and 5th Oct so come back and tell us how you got on

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[castaway]

Thanks Vic, and for all the other informed replies. (just what this forum is so good at)

Would you give Glen and Harold my very best regards, I still regard the years I spent at Paynes the very best sailing I have ever had, (but poss just a bit of rose tinted specs there!)

Boatman pointed out that there may be a problem with swarf going into the box, but I hope that as its the water jacket this a should not be a major issue??

All best Nick

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[Avocet]

I wouldn't be too worried if it was my water jaket. Does the system have a pump to circulate the cooling water? If so, maybe strip the pump and look for swarf next time it's out. In the past I've found that smearing plenty of sticky grease on the drill and then the tap catches the vast majority of the swarf anyway.

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[chas]

Not being an engineer, I don't know if I am qualified to enter this. How hot does the water get? Some years ago I had a similar probelm with a water jacket drain at the bottom of an engine and when I took it to a garage, the mechanic fitted a plastic (nylon?) plug which threaded itself in. I was concerned it would melt but he said not and he was right. It was still there when I sold the car.

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