Bolt question- engine mount bracket to flywheel housing

Jud - s/v Sputnik

Super Anarchist
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I just discovered a bolt has vibrated out (it fell out completely; in pic below I’ve partially reinserted it) of the back of my flywheel housing - it holds engine mounting bracket on. It’s the lower bolt visible in the pic. (Years ago, before I acquired this engine, used/rebuilt, the third/middle bolt had sheared off in the engine...)

A very experienced engineer/engine rebuilder/knows-tons-about-a-lot friend said it’s not a huge deal that this vibrated out because this is or should be a “doweled” connection - something holding things in alignment - i.e., the engine mounting bracket to flywheel housing, presumably — independent of the bolts.

A few questions - my calipers are basic plastic ones but seem to show that bolt as 10mm. It’s definitely an obvious metric length -20mm long. Which suggests its an M10 bolt. However, the head fits a 5/8” wrench perfectly (whereas the web is telling me that 17mm is head size for a 10mm bolt - 17mm is too big for this one). And of course a 3/8” bolt typically has a 9/16” head. Perhaps it’s a special Volvo Penta bolt??

The other weird thing is that for the bolt above it, the heads fits a 17mm wrench, as do the three bolts on the starboard side of the engine. Again, lower bolt on port side fits a 5/8” wrench perfectly. (WTF...)

Anyway, threads on it look a bit mashed/worn, which suggests it’s old, not holding well? Doesn’t look like a fine thread either, surprisingly: 10 threads over 20mm length). Also - no lock washer or tabs to hold the bolts in place. Seems like it should have something...but then that would shorten three engagement if I added a lock washer! Presumably now how the engineers intended it.

In short, just trying to figure out what kind of bolt it is and what to replace with if Volvo now longer stocks these, a definitely possibility...and also, go slightly longer and lock washer them?

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Last edited:

Zonker

Super Anarchist
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Many tap and die sets come with these but if you don't have a set, they are pretty cheap.



1659319517967.png

7/16" bolts use a 5/8" wrench

But I bet it's a metric standard bolt. 10mm bolt standard threads are 1.5mm pitch. So a 20mm bolt is 13 threads. I count maybe 14 or 15 threads. Close enough. I'm sure Volvo is NOT making up special thread pitches just for this application. It's an off the shelf bolt. Just use red loctite and re-use.
 
1. Typically, all the bolts performing a particular function will be identical. Pull the others, one at a time, and see what you've got. Metric bolts of different diameters can have the same thread pitch and a size smaller bolt can screw into a larger hole, but will wiggle until tightened.
2. This bolt is in a shear situation and good practice suggests a bolt with an unthreaded shank to carry the load.
3. If the motor did not shift when operated with only one bolt remaining, this mount is carrying NO load. The sheared off middle bolt suggests a hefty load.
4. Factory service manuals often describe common fasteners by size and strength rather than part numbers.
 

slug zitski

Super Anarchist
5,295
911
worldwide
I just discovered a bolt has vibrated out (it fell out completely; in pic below I’ve partially reinserted it) of the back of my flywheel housing - it holds engine mounting bracket on. It’s the lower bolt visible in the pic. (Years ago, before I acquired this engine, used/rebuilt, the third/middle bolt had sheared off in the engine...)

A very experienced engineer/engine rebuilder/knows-tons-about-a-lot friend said it’s not a huge deal that this vibrated out because this is or should be a “doweled” connection - something holding things in alignment - i.e., the engine mounting bracket to flywheel housing, presumably — independent of the bolts.

A few questions - my calipers are basic plastic ones but seem to show that bolt as 10mm. It’s definitely an obvious metric length -20mm long. Which suggests its an M10 bolt. However, the head fits a 5/8” wrench perfectly (whereas the web is telling me that 17mm is head size for a 10mm bolt - 17mm is too big for this one). And of course a 3/8” bolt typically has a 9/16” head. Perhaps it’s a special Volvo Penta bolt??

The other weird thing is that for the bolt above it, the heads fits a 17mm wrench, as do the three bolts on the starboard side of the engine. Again, lower bolt on port side fits a 5/8” wrench perfectly. (WTF...)

Anyway, threads on it look a bit mashed/worn, which suggests it’s old, not holding well? Doesn’t look like a fine thread either, surprisingly: 10 threads over 20mm length). Also - no lock washer or tabs to hold the bolts in place. Seems like it should have something...but then that would shorten three engagement if I added a lock washer! Presumably now how the engineers intended it.

In short, just trying to figure out what kind of bolt it is and what to replace with if Volvo now longer stocks these, a definitely possibility...and also, go slightly longer and lock washer them?

View attachment 531570

View attachment 531571
There seems to be no washer under the bolts ..bad news

install with a washer and thread locker
 

sailak

Super Anarchist
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AK
No washer from the factory, and they should both be identical... If it has loosened up and moved around some there is probably damage to the threads in the other one too, and damage to the internal threads in the bellhousing. Sounds like someone might have chased the threads or made it "fit" with an SAE bolt or something like that. I would get new bolts (I think that is Volvo Penta # 970948 and the parts page lists the quantity as 5 total) and try and thread them by hand. If they don't go try and chase the threads with a tap and inspect for damage. Might need to start thinking about threaded inserts or tapping larger holes if there is enough material there.
 

longy

Overlord of Anarchy
6,518
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San Diego
Adding a washer will not change the bolt strength, as long as there is still enuff bolt length engaged by the threads. Measure depth of threaded hole, get longet bolt if needed. That bolt will be a standard metric size, look on bolt head for strength markings. The damaged threads visible in pic are wear from bracket, not damage to threaded hole. As the bolt loosened, the braket was able to vibrate against the threads, rounding them over as seen in the pic.
 

mcpusc

New member
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14
PNW, USA
whereas the web is telling me that 17mm is head size for a 10mm bolt
your bolt head is probably 16mm — there are *at least* three different standards for head size per bolt size on metric bolts; an M10 ISO bolt will have a 16mm head, an M10 DIN bolt 17mm, and an M10 JIS bolt 14mm (those use a flange head). Chart from https://grassrootsmotorsports.com/articles/understanding-metric-hardware/:
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since it can be difficult to get anything but the "standard" bolt for the area of the world you're in, it's not uncommon to find mis-matched bolt heads if a bolt's been replaced with local parts.
 

Son of Hans

Member
153
76
San Diego
I was actually going to post that exact same chart, but was too lazy to hunt it down! However, it doesn't show thread pitches - JIS will be 1.25mm while the others will be either 1.00 or 1.50. As an aside, notice that JIS requires a 12mm wrench while the others want 13mm. Wrench sets rarely include both and which one you get depends on whether the target market is Japanese or Euro car owners. Also notice that there is no size calling for a 9mm wrench, yet my first set included one. In nearly 50 years of wrenching, I have found exactly one use for a 9mm - it is the grub screw that holds the bushing for the clutch cross-shaft on an air-cooled VW transmission, and perpetuated to this day in Hewland MK series gearboxes.
 

Blitz

Super Anarchist
1,516
112
The fastener section of your local hardware store should have a size and thread guage, find the hole it fits in and there you go.
 
Clearly, this engine has been painted, so conclusions about what came from the factory based on paint are questionable.

An undersized hex probably indicates a non-factory bolt, either different spec or smaller thread.

The metric system uses the same thread pitches for several diameters, producing the ability to sometimes actually screw a next-size-smaller bolt into a threaded hole and unless you notice the excessive slop, there is (considerably) reduced thread engagement, A.K.A. weaker attachment.

Thread gauges work nicely on external threads (bolts) but are difficult to use on internal threads (holes). Find a bolt that fits the threads without slop and use the thread gauge on the bolt.

The other bolts attaching motor mounts to this engine should all be the same thread.

The sheared bolt might indicate that the suspect bolt was there before the shear event but wasn't carrying its share of the load.

The bracket could have shifted slightly after shearing the middle bolt, so that a correct-size bottom bolt would not fit but a slightly smaller one would.

All this suggests that inspecting the upper bolt is in order.

I'd remove the bracket, extract the sheared end of the middle bolt and replace all three bolts, using the proper size and strength.
 

DrewR

Utility Infielder
1,211
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Buzzards Bay, MA
Thread gauges work nicely on external threads (bolts) but are difficult to use on internal threads (holes). Find a bolt that fits the threads without slop and use the thread gauge on the bolt.

Which is why i've used one of these, works for both internal and external holes.
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Just about any hardware store will probably have that. You could re use but it's likely you can get a new easy. I would not use loctite for this as getting out might be a pain later. I would use a small amount of antiseze and torque based on lubricated bolt tightening spec, you can find all over online. Probably a grade 8.8 or 10.9 M10, grade is usually on the head. Run around all of them with the torque wrench and pop and you should be fine.
 

El Borracho

Sam’s friend
6,442
2,458
Pacific Rim
M8 and 5/16” hardware are often confused. Even US manufacturers and suppliers will mix them up. Takes a sharp eye to reveal with a typical thread gauge. Loose handyman nuts will fit both. Engine blocks not so much. Head size gives a vague clue about 50% of the time.

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