AD41 sheared bolt in the engine block

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crewejd

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Hi all,
Whilst attempting to slacken the bolt that holds one end of the alternator tensioner, it felt like it was going to shear.
Weeks and weeks of repetitive soaking with various penetrating oils and linctures later, I did the tighten/loosen a bit at a time, but it sheared off on one of those attempts..
The thread and bit of bolt seems to be a long way inside the casting, and using a centred left-handed twist drill didn't get it to budge.
I'm now committed to drilling out followed by re-tapping this but can't find long tap sets that are 7mm/1.25 anywhere.
So... I'm looking for a 7mm/1.25 tap of good quality that is around 120mm long.. (I don't want to have to drill out a broken tap as well!!)
Any suggestions please?
Cheers
 
It's an AD41A - the bolt position is on the pulley end of the engine. The hole is in about the 9-o'clock position.
1618612690314.png

So taking into account the bolt holds into about 20mm, and I'll need enough sticking out of the engine so I can put some sort of tap wrench on it. Hope that makes sense?
Why the damn thing is threaded so deep in the block I think it's likely that it has to take into account that the cam chain cover is between the outside and the block.
Please tell me I don't have to shear off the remaining umpteen bolts holding the cam chain cover on, to re-tap this hole?
 
Why stay with 7mm @1.25 (which seems very unusual). How about going up to 8mm @1 or 1.25 , which seem much more available?

And going the next step up how about sourcing a stud so that you can use a nut on the outer end and possibly overcoming problems in the future?
 
7 x 1.25 isn't an ISO size, which is probably why you've struggled to find a tap; Tracy Tools are good for oddball stuff like that, but double check the pitch and diameter and consider Caraway's suggestion.
 
crewejd said:
Hi all,
Whilst attempting to slacken the bolt that holds one end of the alternator tensioner, it felt like it was going to shear.
Weeks and weeks of repetitive soaking with various penetrating oils and linctures later, I did the tighten/loosen a bit at a time, but it sheared off on one of those attempts..
The thread and bit of bolt seems to be a long way inside the casting, and using a centred left-handed twist drill didn't get it to budge.
I'm now committed to drilling out followed by re-tapping this but can't find long tap sets that are 7mm/1.25 anywhere.
So... I'm looking for a 7mm/1.25 tap of good quality that is around 120mm long.. (I don't want to have to drill out a broken tap as well!!)
Any suggestions please?
Cheers
Click to expand...
Bracket 860271 bolts to the engine through the water pump with 3 bolts , you can just undo the the bracket , the sheared bolt is in the bracket not the engine block , I am assuming you have sheared the top bolt that secures the alternator to the bracket ?
 
Because it’s so difficult to post pics on this site I’d advise you post on Facebook page , Volvo penta marine Diesel engine yacht and boat owners , this is a page I am involved in with its creator surveyor Nick Vass .
Its a lot easier to post pics and explain that this site .
 
If it’s the lower number 5 that secures the adjusting strap I would drill and tap the bracket just above it’s original place , I’d assume the bracket is off so easy fix , M6 will do nicely .

I wouldn’t be drilling the timing case or block for sure as this could open a bigger can of worms by allowing oil leaks from the timing case .
id clean the case up and apply something to block the hole if there is one .
 
volvopaul said:
Because it’s so difficult to post pics on this site I’d advise you post on Facebook page , Volvo penta marine Diesel engine yacht and boat owners , this is a page I am involved in with its creator surveyor Nick Vass .
Its a lot easier to post pics and explain that this site .
Click to expand...
Just a bit of drift.
I agree that this forum software is rubbish but I know the solution to photos.
Photos need to have be in a small file size for this forum software to upload.
So, all you need to do is resize the image and then you can very simply upload your photos using the "Insert Image" icon.
I use Linux these days and have an image resizer built into my file explorer.
But for Windoze, there is an excellent addon to Windoze Explorer called Prish.
The idea is that you can "right click" the photo and resize it quickly prior to uploading it.
Posting pics on this site is then easy and quick but the designers should have allowed larger images to be uploaded - very poor planning IMO.
 
volvopaul said:
Bracket 860271 bolts to the engine through the water pump with 3 bolts , you can just undo the the bracket , the sheared bolt is in the bracket not the engine block , I am assuming you have sheared the top bolt that secures the alternator to the bracket ?
Click to expand...
Hi Paul,
It is the lower bolt for the belt tensioner that sheared. As was said above, bolt type number 5 - this is the bottom bolt for the alternator bracket through the timing cover to the engine block.

After having a better look at this today, (no rain/hail!) the bolt is 8mm diameter. The thread pitch is 1.25.

The whole bolt length is 80mm with a further 7mm of head above the shoulder. The bolt head is 12mm-ish. (corroded a bit plus a bit of paint).
The depth of the bottom of the hole to the shoulder of the timing case is 76mm so I need a plug tap long enough to reach the bottom plus a bit to get hold of with a tap wrench.

I've found a set of taps from Tracy that seem to fit the bill. Just wondering how thick that block casting is, and if I can safely re-tap with the timing case cover on. Hopefully the pilot drill didn't wander..
 
Last edited:
Caraway said:
Why stay with 7mm @1.25 (which seems very unusual). How about going up to 8mm @1 or 1.25 , which seem much more available?

And going the next step up how about sourcing a stud so that you can use a nut on the outer end and possibly overcoming problems in the future?
Click to expand...
Great idea re the stud rather than a bolt. Thanks!
 
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