Tohatsu M9.8 2-stroke - head bolts torque settings

Strolls

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Nov 22, 2019
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Hello,

I have a year 2000 Tohatsu M9.8 2-stroke outboard, and I was hoping someone here could tell me what torque settings to use when reinstalling the head.

Mine looks the exact same as in the photos posted by Smash in this previous thread, except his looks cleaner than mine.

I have the service manual mentioned by pvanv in that thread - just google "003-21035-1 SERVICE MANUAL" and you'll find the PDF - but it doesn't seem to give the torque settings for the head bolts.

I'm not alone in having this problem - googling the matter finds a number of people asking the same question, but no answers!

Thanks for reading,

Stroller.
 

pvanv

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Apr 20, 2008
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6,082
Standard torque values are on p2-5. The head bolts are not unique. Do pay attention to tightening sequence, and torque in steps... 50% first pass, 75% second pass, 90% third pass, and final torque last pass.
 

Strolls

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Thanks for your help, Paul.

I assumed the head bolts would be listed explicitly, and not just the standard values.

Can I just check I've got it right, please?

The head bolts are 6mm diameter (10mm head), so I use the M6 range, or 5-6 N-m? (44-53 in-lb / 3-5 ft-lb)

The reason I double check is that my torque wrench doesn't go down that far. Looks like a Sealey STW1012 will be the best option.
 

pvanv

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Apr 20, 2008
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That's the minimum value. As long as you follow a pattern and torque evenly, you can go up to 20 foot pounds. But go in steps.
 

Sea Rider

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Sep 20, 2008
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10,635
Check if the torque specs is casted in front of the cylinder head, 10 mm head bolts are usually torqued to 25 NM in 3 steps, say 10-20-25 NM. If the head has 10 bolts the torque sequence is :


9-10 UP
5-6
1-2 ---start middle head and torque in order
4-3
8-7 DOWN

To torque those bolts use a 10-50 NM Norbar Torque Wrench...

Happy Boating
 
Last edited:

Strolls

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Nov 22, 2019
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Check if the torque specs is casted in front of the cylinder head, 10 mm head bolts are usually torqued to 25 NM in 3 steps, say 10-20-25 NM.
Nope, there's one on the exhaust cover, but I assume that refers to the big bolt in the middle of it.

The specifications on 2-5 of the service manual, as mentioned by pvanv, are the same as on this page, except at that link it specifies that they "DO apply to head gasket torques".

Maybe I've got it arse about face, but I always thought the published torque values were a maximum or simply to be adhered to, rather than a minimum. I thought the point of specified torque values was to ensure it's fitted properly, but also to avoid overtightening, which risks stripping threats or warping the head. The values you and pvanv are several times those specified by Tohatsu.

I write this not meaning to cast doubt on what you say, just because I'm inexperienced and nervous about damaging things.
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
Messages
10,635
Sorry a senior moment, 13 mm head bolts are usually torqued to 25 NM, exhaust cover 10 mm ones to 8 -10 NM.

If the 9.8 HP exhaust cover has a upper 1/2 inch/13 mm bolt different one from the10 mm ones, that's a flushing port where you connect a flushing adapter sold separately

The min-max torque specs mentioned on the 2 strokes Service Manual for each size bolt are referential only. The exhaust cover has it's own torque value cast on the cover, check if there's one on the middle cylinder head, if so should be 25 NM ?

If exhaust cover and cylinder head bolts were found rusted install new ones and clean all bolt's threads before installation, don't grease them. Re torque all head and exhaust bolts back to specs after first 5-10 run hours, gaskets will have ceded a bit which is quite normal.

Happy Boating
 
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