Sump plug diameter

mauricejota7604

mauricejota7604

Hero member
Location
Victoria Australia
I am nearing the completion one of my early 3Cs and as usual I have left the wiring until last as its such a fiddly job!
But one other small item I am having a bit of a challenge with is the sump plug thread in the crankcase, I didn't have a sump plug when I bought the bike in pieces and every bolt I have bought will not fit it. If I put my vernier into the hole I get 15.9mm anybody got any ideas? I don't have a pitch gauge ??
 
Buy a thread gauge to start with there cheap and takes out the guess work, or do you want me to loan you one ?

Am thinking it had some form of insert thread repair which has wound out at some time or someone went to do it and never finished. 
 
Maurice,
being an early 3C, does it have the boss for the plug?
or is it threaded literally straight into the crankcase?
16mm can be used in the boss, but better to have a 14mm Timecert, (or BigCert, or even a 16mm Timecert)
pic below is what I do. probably a bit late for that now tho',  don't even have to worry about getting it square to the cases. which is a risk when using a tap or thread repair, because, when the plug is tightened up it wants to be contacting the crush washer equally all around its periphery, which it wont if the thread is pissed.
CLEM
 

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mauricejota7604 said:
I am nearing the completion one of my early 3Cs and as usual I have left the wiring until last as its such a fiddly job!
But one other small item I am having a bit of a challenge with is the sump plug thread in the crankcase, I didn't have a sump plug when I bought the bike in pieces and every bolt I have bought will not fit it. If I put my vernier into the hole I get 15.9mm anybody got any ideas? I don't have a pitch gauge ??
Click to expand...

Should have looked into that earlier Maurice!

Stock size is 14x1.5mm.  When knackered and bored out to M16-something, the tap contacts the side of the gearbox "wall" and usually tilts, resulting in a cock-eyed thread that is difficult to seal properly.  Your inside measurement of almost 16mm points to an 18mm thread, making things all the more difficult!  Unless it was bored and tapped on a machine, chances are the thread is totally askew...

piet
 
Drill size for a M16 1.5 is 14.8mm.............so best guess is that someone has taken out to M16

A lot of Renault, Citroen and such use an M16 x 1.5 sump plug. Perhaps you could at least see if it screws in. If it fits and screws home and seals ( torque up dont swing on the wrench and wring its neck ) then you could leave it like that or for a more pro repair....

big-sert-aluminum-washer-seat-01-433x472.jpg


https://www.mechanicstoolsandbits.com/time-sert/big-sert-oversized-kits/big-sert-spark-plug-kits/big-sert-spark-plug-inserts/big-sert-washer-seat-aluminum-inserts-m14/big-sert-51407a-m14x1-25x16-8-aluminum-washer-seat-spark-plug-insert.html

 
I presume the engine is in the frame if you're at the wiring up stage. Makes ait a bit of a chore if you decide to weld it up and start again.

The things that previous owners do eh? It might be an imperial thread or a big-sert thread. Thread gauges are a little difficult to use on internal threads, especially if you're groping around upside down under the bike. But a 16mm hole gives you a bit of room to wangle the gauge around so it should be possible. If you can work out what thread it is (and if the thread is in good condition) I could turn up an insert to match, and put a 14 x 1.5 internal thread in it for the sump plug.

I like Phil's idea of using a spark plug thread. Use a spark plug as a sump plug and connect an HT lead to it just to confuse anyone who notices it.  :D

 
Based on a 15.9 measured thru hole I'd hazard a guess at M18x2.5 (ie a standard M18 bolt pitch, a 15.5 tap drill) or quite possibly even 3/8 BSP!!  I'm not kidding, seen worse.  ;)  If the engine came from Spain, then I'd lean to M18, if England then BSP.  :D
Alternative would be 11/16 BSF (~15.5) or 11/16 perhaps BSW or UNC (both ~15.1).
Given it's likely been tapped out in situ in less than ideal circumstances the ID of the thread will likely be quite a bit larger than the theoretical tapping drill size.

There might be other even stranger alternatives if the person who did it was particularly disturbed.
 
Mystery solved it appears that the re threading must have been done on a flat base as it is nice and square and an M16 plug fits in there perfectly, maybe I got that 15.9mm wrong (no glasses on)
 
Just as an aside.... On one trip to Breganze, I asked Sg. R. Oro esq. to change my oil, as it was a bit overdue. He didn't go near the sump plug, but put a lump of wood under the LH centrestand foot and leant the bike hard to the RHS. He then undid the (triangular-ish) gearchange cover and let the dead dinosaur juice out from there. Nipped the cover up, (tapping to and fro to ensure smoth gearchange I was pleased to see), fresh oil in atop and there were were - done.

Hag.
 
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