Yanmar 2GM exhaust elbow bolts

[webcraft]

.
I've taken my exhaust elbow off and all the bolt heads are slightly rounded/corroded.

The bolts appear to be M8 20mm and have a spring washer.

The genuine Yanmar ones are the best part of £10 for three when you pay for delivery, and come without washers.

Does the 7 on the head signify anything important, and if I source these locally, should I select A4 stainless or something else?


- W

 

[Major_Clanger]

The number is a Japanese reference to the construction and tensile strength of the bolt, and equates to a European marking of 8.8.

There's no need to buy genuine. Any stockist of fasteners will carry something suitable at a fraction of the price, just ensure it's the same spec and torqued-up correctly.

 

[Kelpie]

The ones on my 3GM are stainless (from memory).
Be glad you got them out... there was a long thread on here a couple of years ago from a Ballad owner who had to resort to easy-outs. I think copper grease on the replacement bolts is a good idea.

 

[earlybird]

I'd probably use A2 stainless 'cos that's what my local stockist sells, for pennies. IIRC, from my GM owning days, Yanmar tended to use fasteners with non-standard hexagons, eg 12mm AF on M8 fasteners. Maybe to give more spanner clearance in a tight situation?? Might partly explain their high prices.
Second the idea of copper grease.

 

[RichardS]

... if I source these locally, should I select A4 stainless or something else?

Any stainless M8 with the correct thread .... probably 1.25 mm but check as it could be finer .... will be OK. A2 or A4 will also be fine but if A4 is on offer then go for that as the price difference will be small.

Richard

 

[webcraft]

I'd probably use A2 stainless 'cos that's what my local stockist sells, for pennies. IIRC, from my GM owning days, Yanmar tended to use fasteners with non-standard hexagons, eg 12mm AF on M8 fasteners. Maybe to give more spanner clearance in a tight situation?? Might partly explain their high prices.
Second the idea of copper grease.

Aye, they are 12mm heads on M8 bolts. Might have difficulty getting a spanner on one of them if I use standard bolts, but will give it a try.

Thanks as usual to all the helpful posters on here.

- W

 

[webcraft]

Any stainless M8 with the correct thread .... probably 1.25 mm but check as it could be finer .... will be OK. A2 or A4 will also be fine but if A4 is on offer then go for that as the price difference will be small.

Richard

Can get A4 cheaply enough from my local shop (Highland Fasteners, Oban). Was just a bit worried about putting stainless into cast iron, which is what I assume the block is made of.

Slightly curious re. your comments on thread pitch - I thought all M8 etc. were standard?

- W

 

[scottie]

Can get A4 cheaply enough from my local shop (Highland Fasteners, Oban). Was just a bit worried about putting stainless into cast iron, which is what I assume the block is made of.

Slightly curious re. your comments on thread pitch - I thought all M8 etc. were standard?

No you have a choice of coarse or fine threads coarse being the default option

- W

 

[VicS]

Can get A4 cheaply enough from my local shop (Highland Fasteners, Oban). Was just a bit worried about putting stainless into cast iron, which is what I assume the block is made of.

Slightly curious re. your comments on thread pitch - I thought all M8 etc. were standard?

- W

The common metric threads are coarse pitch ( 1.25mm for M8) but you will also find that "fine pitch" threads are available ( 1.0 mm for M8)

compare with Whitworth and BSF or with UNC and UNF

In fact there are some even finer metric threads

See https://www.fullerfasteners.com/tech/basic-metric-thread-chart-m1-m100-2/


BTW the torque specified in the workshop manual for your exhaust bend bolts is 45 Nm

 

[Laysula]

Aye, they are 12mm heads on M8 bolts. Might have difficulty getting a spanner on one of them if I use standard bolts, but will give it a try.

Thanks as usual to all the helpful posters on here.

Would it be possible to use cap heads?


- W

 

[webcraft]

Would it be possible to use cap heads?

Like this:

Good idea, but could these be done up to the necessary torque ? (45NM)

- W

 

[webcraft]

BTW the torque specified in the workshop manual for your exhaust bend bolts is 45 Nm

Is that with or without a spring washer, and how tight is that in layman's terms using an ordinary spanner? Is it as tight as possible, less than that, or stick a scaffolding pole on it ??

- W

 

[earlybird]

45 Nm sounds rather tight to me for M8, can VicS confirm.
Later edit, Just checked; that's what Yanmar specifies. It's 33 lb ft. in "proper" money!

 

[RichardS]

Is that with or without a spring washer, and how tight is that in layman's terms using an ordinary spanner? Is it as tight as possible, less than that, or stick a scaffolding pole on it ??

- W

You should really never do any bolt up as tight as possible unless you have to. That is the way the the typical garage mechanic seems to approach these matters but we can all be much better than that, even without a torque wrench.

45Nm is definitely tight ..... with a typical 12/13mm ring spanner you need a couple of fingers pull at the opposite end of the spanner and you will be tight enough. It's not a mission-critical seal and doesn't need to be that tight so if it doesn't leak when you get the engine hot then it's fine.

The problem arises if you use a socket wrench, which is what garage mechanics do. In that case I would pull on the wrench with the two fingers about half-way along it's length. If you use your strength on the business end of a wrench you will be liable to strip the thread in the casting unless it is is perfect condition. :ambivalence:

Richard

 

[vyv_cox]

.
I've taken my exhaust elbow off and all the bolt heads are slightly rounded/corroded.

The bolts appear to be M8 20mm and have a spring washer.

The genuine Yanmar ones are the best part of £10 for three when you pay for delivery, and come without washers.

Does the 7 on the head signify anything important, and if I source these locally, should I select A4 stainless or something else?


- W

As pointed out in post #2, these are not stainless steel bolts, they are HT steel equivalent to 8.8. Widely available, even Screwfix sell them in bags of 50 or more. Copperslip is useful with exhaust bolts but don't overtighten. No need at all for stainless steel bolts, which in any case have very low yield points and are likely to stretch.

 

[Laysula]

Like this:
Yes.

Good idea, but could these be done up to the necessary torque ? (45NM)

- W

 

[VicS]

Is that with or without a spring washer, and how tight is that in layman's terms using an ordinary spanner? Is it as tight as possible, less than that, or stick a scaffolding pole on it ??

- W

It would be with a spring washer if that is what is normally fitted but it would be with unlubricated threads.!
It is higher than I expected but I checked with two manuals and Earlybird has found the same figure !

As a general guide Yanmar say 23 - 27Nm ( 17 - 20 lbf.ft ) for M8 when no other torque is specified in the manual

45Nm is more than I can manage comfortably with the short pattern ring spanners I normally use although within my capability with a 1/2" drive socket set and its normal tommy bar. However what I can now manage must be viewed against my now much reduced physical strength compared with a few decades ago ( Getting he skin off a rice pudding is now quite a task!)

If the thread is lubricated the tightening torque should be reduced

[table="width: 300, class: grid, align: center"]
[tr]
[td]lubricant[/td]
[td]% reduction[/td]
[/tr]
[tr]
[td]none[/td]
[td]0[/td]
[/tr]
[tr]
[td]graphite grease[/td]
[td]50 - 55[/td]
[/tr]
[tr]
[td]white grease[/td]
[td]35 -45[/td]
[/tr]
[tr]
[td]SAE 30 oil[/td]
[td]35 -45[/td]
[/tr]
[tr]
[td]SAE 40 oil[/td]
[td]30 -40[/td]
[/tr]
[/table] Source: Enginerring Toolbox

Richard S suggests a very much lower torque ......... what you have to decide is who is correct Yanmar or Richard . Perhaps he is allowing for being well greased although he does not say.

 

[Spyro]

Probably not at all critical. Enough to compress the gasket and tight enough not to come undone.

 

[macd]

Yanmar tended to use fasteners with non-standard hexagons, eg 12mm AF on M8 fasteners.

Aye, they are 12mm heads on M8 bolts.

12mm hex on 8mm bolts is 'standard' with many Japanese products. (And 8mm hex on 6mm is becoming so in many applications.)

 

[webcraft]

As pointed out in post #2, these are not stainless steel bolts, they are HT steel equivalent to 8.8. Widely available, even Screwfix sell them in bags of 50 or more. Copperslip is useful with exhaust bolts but don't overtighten. No need at all for stainless steel bolts, which in any case have very low yield points and are likely to stretch.

It is cheaper for me to get 3 SS bolts locally than to buy a bag of 50 HT bolts from Screwfix or similar. Are there downsides to using SS bolts? You say 'likely ot stretch' - how much of a real problem is this when lots of boats have lots of bits held together by SS bolts/threads

- W