Sludge trap plug welding, good idea ?

Fast Eddie · Oct 12, 2025

I remember that when folk raced with Triumph cranks, the place they broke most commonly was through the sludge trap.

When you look at how aggressively some are centre punched, it looks like a pretty obvious stress riser to me.

I got to thinking…if one were to chamfer the plug, what about welding it in? My thinking is it would add a lot of strength, and it would negate the need to centre punch.

It would be very easy to machine it to remove it, although with modern detergent oils, I doubt the sludge trap would ever fill up.

Any thoughts on this y’all ??

grandpaul · Oct 12, 2025

Perhaps a tiny weld spot. I would NEVER weld the whole circumference.

Fast Eddie · Oct 12, 2025

grandpaul said:
Perhaps a tiny weld spot. I would NEVER weld the whole circumference.

Because ?

marshg246 · Oct 12, 2025

Fast Eddie said:
I remember that when folk raced with Triumph cranks, the place they broke most commonly was through the sludge trap.

When you look at how aggressively some are centre punched, it looks like a pretty obvious stress riser to me.

I got to thinking…if one were to chamfer the plug, what about welding it in? My thinking is it would add a lot of strength, and it would negate the need to centre punch.

It would be very easy to machine it to remove it, although with modern detergent oils, I doubt the sludge trap would ever fill up.

Any thoughts on this y’all ??

Not a good welder or a metallurgist of any kind, but it seems to me that if the weld penetration is not deep, no appreciable strength would be added and if the penetration is deep enough to add strength, then when machining it out many/most threads would be gone.

I know what you mean though - I've seen a ridiculous number of punches and ridiculously deep.

On the other hand, probably could remove the trap insert, weld it permanently, and install an oil filter and never worry about it again. After all, there are millions (billions?) of engines running every day without sludge traps in the crank.

Fast Eddie · Oct 12, 2025

marshg246 said:
On the other hand, probably could remove the trap insert, weld it permanently, and install an oil filter and never worry about it again. After all, there are millions (billions?) of engines running every day without sludge traps in the crank.

Exactly Greg. My thinking is that with modern detergent oils and a paper filter in the system the sludge trap would remain sludge-less almost indefinitely.

grandpaul · Oct 12, 2025

Fast Eddie said:
Because ?

1. No need, a spot weld would be sufficient.
2. A full weld would be enough to invite a re-balance, not always necessary.
3. I know there's a third reason, but I just can't think of it at the moment!

I do agree that a high quality filter, properly installed, is VERY beneficial to ANY motorcycle.

baz · Oct 12, 2025

grandpaul said:
1. No need, a spot weld would be sufficient.
2. A full weld would be enough to invite a re-balance, not always necessary.
3. I know there's a third reason, but I just can't think of it at the moment!

I do agree that a high quality filter, properly installed, is VERY beneficial to ANY motorcycle.

I think the reason Eddie is talking about fully welding the plug is to strengthen up that whole area
Not to stop the plug from undoing

baz · Oct 12, 2025

Fast Eddie said:
I remember that when folk raced with Triumph cranks, the place they broke most commonly was through the sludge trap.

When you look at how aggressively some are centre punched, it looks like a pretty obvious stress riser to me.

I got to thinking…if one were to chamfer the plug, what about welding it in? My thinking is it would add a lot of strength, and it would negate the need to centre punch.

It would be very easy to machine it to remove it, although with modern detergent oils, I doubt the sludge trap would ever fill up.

Any thoughts on this y’all ??

I'm not sure on the metallurgy but a small chamfer on the plug
And a light countersink on the crank then fully welded and ground flush would be fine IMO

grandpaul · Oct 12, 2025

baz said:
I think the reason Eddie is talking about fully welding the plug is to strengthen up that whole area
Not to stop the plug from undoing

I would think if it has NO visible cracks, there is naught to do but to perhaps magnaflux or penetrant dye test, then carefully peen-punch the new PLUG edge into the existing relief divot in the crank (after Locktite on threads).

Fast Eddie · Oct 12, 2025

grandpaul said:
I would think if it has NO visible cracks, there is naught to do but to perhaps magnaflux or penetrant dye test, then carefully peen-punch the new PLUG edge into the existing relief divot in the crank (after Locktite on threads).

We are looking at things differently here GP. I’m not looking at repairing a crank etc, I’m looking at removing, or at least strengthening, an inherent potential failure mode.

Fast Eddie · Oct 12, 2025

baz said:
I think the reason Eddie is talking about fully welding the plug is to strengthen up that whole area
Not to stop the plug from undoing

Correct.

grandpaul · Oct 12, 2025

Fast Eddie said:
We are looking at things differently here GP. I’m not looking at repairing a crank etc, I’m looking at removing, or at least strengthening, an inherent potential failure mode.

Yes, I know what you are talking about, but if the crank has no actual damage, any future issues can be mitigated "in house".

UNLESS you are going to be riding hard regularly, or racing, then it becomes worthwhile.

t ingermanson · Oct 12, 2025

Not sure the heat is worth it. You have to get the crank to a molten state pretty dang close to that big end. When you get it that hot, you create a Heat Affected Zone, which is when the molecules get all wonky and disorganized, instead of a smooth and relatively linear grain pattern. That dramatically reduces the strength, and you'd have to re-treat the crank to get back the metal to its original strength.

If you're a betting man, is a lack of a small stress riser better than an HAZ? Don't know, but I wouldn't bet my crank on it.

Last time I opened up my crank after only using modern oils (cracked case on a 5 year old rebuild), there was a surprising amount of sludge. I imagine there's a certain amount of solids from cylinder/ring break-in that just will be.

Fast Eddie · Oct 12, 2025

grandpaul said:
UNLESS you are going to be riding hard regularly, or racing, then it becomes worthwhile.

Exactly.

What I’m getting at is what looks like a ‘collection of weaknesses’ to my eye.

The size of the hole, the fact that it’s threaded (not smooth, chamfered, polished, etc) and the centre punching on top of these things.

Seems to me it all adds up to quite a potential weakness.

And although they’ll comfortably handle sensible road use almost indefinitely, using high revs will likely find one of these weaknesses sooner or later.

One option is to have a billet crank custom made. I’m just thinking that a careful look at this sludge trap area could solve the problem much cheaper…

Fast Eddie · Oct 12, 2025

t ingermanson said:
Not sure the heat is worth it. You have to get the crank to a molten state pretty dang close to that big end. When you get it that hot, you create a Heat Affected Zone, which is when the molecules get all wonky and disorganized, instead of a smooth and relatively linear grain pattern. That dramatically reduces the strength, and you'd have to re-treat the crank to get back the metal to its original strength.

If you're a betting man, is a lack of a small stress riser better than an HAZ? Don't know, but I wouldn't bet my crank on it.

Last time I opened up my crank after only using modern oils (cracked case on a 5 year old rebuild), there was a surprising amount of sludge. I imagine there's a certain amount of solids from cylinder/ring break-in that just will be.

That’s something I was also wondering about. My uneducated gut feel is that it would be fine. But the safe option would be laser welding.

Matchless · Oct 13, 2025

The reason modern engines don't need sludge traps cleaning out is because they don't have one. Even with a proper filter the sludge trap in a Brit twin would centrifuge enough crap to require cleaning during it's life time.

Sludge trap plug welding, good idea ?

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