How tight must cylinder head be?

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Apr 8, 2022
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54
The Studs & Nuts should be Metric Grade 8.8

Metric Grade 8.8 M8x1.25 bolts max torque is 17 ft lbs but obviously only torque your head bolts to 12 ft lbs

You would be surprised how little bit 12 ft lbs by hand can feel
Buy yourself an Inch lb Torque Wrench to be sure

Ft lb Torque wrenches aren't very accurate under 20 ft lbs

That's probably why most Inch lb torque wrench only go up to 240 inch lbs
Where did you find the 12 ft lbs max for torque on the head bolts? I found a chart here ( https://www.fastenermart.com/files/metric_tighten_torques.pdf) confirming your 17 ft lbs limit for grade 8.8 M8x1.25.

Its tricky though finding M8x1.25 acorns, many dont have a grade at all, the one that did so far was grade 4.8 - ill keep searching.

Update: Pretty much all the acorn nuts are A2/18-8 (308) Stainless Steel. M8x1.25 = 5/16-18 bolt size. This chart (https://www.raajsagarsteels.com/fasteners_nut_bolts_screws_washer_studs/alloy_20_studs.html) says max torque is 132 inch-lbs (11 ft-lbs), so im guessing 18-8 Stainless Steel wont work?
 

Wrench

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Stainless is the same as grade 8.8
Many of the acorns are 18-8 Stainless Steel. M8x1.25 = 5/16-18 bolt size. This chart (https://www.raajsagarsteels.com/fasteners_nut_bolts_screws_washer_studs/alloy_20_studs.html) says max torque is 132 inch-lbs (11 ft-lbs), so im guessing 18-8 Stainless Steel wont work?

Metric stainless steel is called A-2 and it's like grade 8.8

Check the grade at your hardware store, A2 is usually stamped on the bolt or nut

M8x1.25 does not equal 5/16x18 their close, but their a different size and pitch

Where did you find the 12 ft lbs max for torque on the head bolts?
That's not what I said or meant
I said, 12 ft lbs was the torque on your head bolts
 
Joined
Apr 8, 2022
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Stainless is the same as grade 8.8


Metric stainless steel is called A-2 and it's like grade 8.8

Check the grade at your hardware store, A2 is usually stamped on the bolt or nut

M8x1.25 does not equal 5/16x18 their close, but their a different size and pitch


That's not what I said or meant
I said, 12 ft lbs was the torque on your head bolts
Thats good to hear, so all A2 nuts "308" or "18-8" should be sufficient to meet required torque. Im going to order these: https://www.amazon.com/Antrader-Thread-Acorn-Stainless-Steel/dp/B07M9J9VMD

How exactly do you know 12 ft lbs is the torque on the head bolts? Is there a building kit manual specifying this is the amount of torque needed?
 

Wrench

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How exactly do you know 12 ft lbs is the torque on the head bolts? Is there a building kit manual specifying this is the amount of torque needed?
I don't even own a CG 2 stroke Motorbicycle engine but everyone on this Forum says it's 12 ft lbs

Just for Fun, here's a pic of my 2005 4t 138cc Flat Head Whizzer M/B
Torque on these M8 head bolts is 16 ft lbs
 

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ImpulseRocket89

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Apr 16, 2014
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The 12ft-lb figure is a good safe figure that works. When it comes to fasteners there are often charts that display the maximum safe working torque range for a given shank, thread size, and pitch based on material and hardware grade. Grade 8.8 being your generic grade tempered carbon steel nuts and bolts for most purposes. I prefer to use Grade 10.9 for most engine fasteners so I actually upgraded most of my hardware as it is a higher strength grade.

Here is a small chart that helps demonstrate the safe maximum torque of a fastener by grade and given thread size/pitch.
f653b14f887ae3db045846900e340e7a.jpg


Notice the max torque for the M8 x 1.0 Zinc Plated grade 8.8 in this instance is basically 20ft-lb. This is also assuming proper graded hardware which is not likely on these cheap little engines. It is also worth keeping in mind the quality of the aluminum these engines are made out of as well as the quality of the casting itself.

Even the specs listed by a couple of engine builders don't exceed 15ft-lbs when using higher quality billet heads and upgraded hardware.
 

Wrench

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OH I forgot to say I use grade 12.9 bolts and my Whizzer (Westman) cylinder is made from cast iron
No need for me to worry about striping out Chinesium aluminum threads
 

ImpulseRocket89

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Apr 16, 2014
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Where do yall get these high grade bolts?
I am lucky enough to live within a mile of a Fastenal location. Fastenal is specifically a hardware distributor so it makes it really handy to get my hands on damn near anything I can imagine. The really come in handy when I need odd/uncommon sized hardware like a grade 12.9 M14x1.25x277mm bolt I needed for one of my motorcycles (not a cheap bolt btw!).

If you aren't lucky enough to have that at your disposal you can also look into Ace Hardware if you have one nearby. Every location is a bit different but I Have always had good luck getting my hands on a decent selection of metric hardware in both mild and stainless steel.
 

Chainlube

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Jan 5, 2020
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Ended up ordering coupling nuts, they're longer and will have more thread contact and I think it will prevent any stripping and provide good torque: https://www.amazon.com/gp/product/B07RTP85RY
If you're stripping out the nuts or the studs on your engine, then you can't be turning it on right. If you can't easily screw on the nut by hand all the way, then there is something impacted in the threads and it has to be addressed. Acorns and stainless aren't very good for engines, acorns because some times the studs are left a little proud of the surface and the nut bottoms out before it actually does its job. Stainless, because it can't stand repeated heat cycles before it becomes brittle.
 
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