How tight must cylinder head be?

Joined
Apr 8, 2022
Messages
51
Fixed the bike, smaller rear sprocket, #70 -> #68 jet, ported cylinder and it works. Its faster, I can tell and the four stroking 'appears' to be gone.

Was able to get the four M8 1.25 threaded rods for the cylinder in without repairing the crankbox threaded hole, the crankbox threaded hole may not be as damaged in the threads as I first imagined and it may have been more to do with the threaded rods themselves. I replaced multiple M8 1.25 threaded rods and nuts from ace hardware / home depot but the metal appears weak and they become stripped as well easily. I bought two new thread rods from Amazon, a stainless steel one and a carbon steel one with oxide finish, I installed the stainless steel one for now and everything appears to be working.

I dont want to tighten the nuts too tightly as I worry about stripping the threads, they're all relativley hand tight with a ratchet set, although one of the nuts appears to be stripped already on the exterior and can not be tighten any further.

question:

1. How tight must the cylinder head be? If the bike runs, does this mean its good? There appears to be tiny gaps with the metal seal between the head and cylinder (see images)
2. The seal between the cylinder and the cylinder head is a thin piece of metal, is this supposed to be airtight? I dont see how it could be 100% airtight as its not a rubber type gasket to form a perfect seal.

images:
 

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weefek

Active Member
Joined
May 14, 2022
Messages
385
head bolts to 144 inch pounds. 12 foot pounds. i do mine to 160 but to each their own.

looks like an aluminum head gasket, if it's that warped you either don't have it torqued enough or something else is wrong. There should be oil/gas/carbon mix pouring out of those areas.

Make sure you torque in an X pattern.
 
Joined
Apr 8, 2022
Messages
51
head bolts to 144 inch pounds. 12 foot pounds. i do mine to 160 but to each their own.

looks like an aluminum head gasket, if it's that warped you either don't have it torqued enough or something else is wrong. There should be oil/gas/carbon mix pouring out of those areas.

Make sure you torque in an X pattern.

Yup, I do an X pattern. I dont have any measurement device to see bolt tightness, would I be good to just eyeball it? If the bike appears to run good, should I be good to go at the very least?

Or would you advise I buy a carbon steel nut (I guess these would be more difficult to strip, no?) and install them instead and try to make tighter. Im looking at these: https://www.amazon.com/uxcell-M8x1-25mm-Metric-Coarse-Hexagon/dp/B07H3VLQXQ
 

weefek

Active Member
Joined
May 14, 2022
Messages
385
If you have experience (a decent amount) you can probably do it without a torque wrench. Don't bother with different nuts it won't make any difference. Buy a decent inch pound wrench.

Generally the 1/2" drive torque wrenches are in foot pounds, generally the 3/8" drive torque wrenches are in inch pounds, but it varies and you could find either in both size.

Get a 3/8" drive torque wrench in foot pounds or inch pounds and you'll be good.

I'm not in the US but I'm sure you could find something cheapish on amazon or at harbor freight. I bought a decent set of both for like 80$ on sale but the biggest cost is the 1/2" drive foot pound torque wrench (not really a wrench it's a damn ratchet) because I also work on cars which require some high torques at certain times.
 

ImpulseRocket89

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Joined
Apr 16, 2014
Messages
361
2. The seal between the cylinder and the cylinder head is a thin piece of metal, is this supposed to be airtight? I dont see how it could be 100% airtight as its not a rubber type gasket to form a perfect seal.
1. Two flat surfaces that mate up almost perfectly actually form one heck of a seal on their own. If they are machined fine enough/smooth enough you can actually achieve a mechanical seal so tight that they can hold some pressure and liquids with almost no leaking at all.
2. The thin bit of metal between the head and cylinder is (supposed to be) made out of a sheet of metal - aluminum or copper usually - that has been annealed. Annealing softens the metal. When you tighten the head nuts down this softer metal deforms and fills in the imperfections in the surfaces on both sides and helps create a perfect seal.
3. As the softer metal of the gasket deforms under pressure it actually undergoes something known as work hardening. This now harder metal is able to withstand the pressures better and is also braced on both sides by even harder metal than itself.

The same concept is used very often in hydraulics with copper seals on banjo fittings. Metal gaskets are much stronger/more durable than paper or rubber seals.

You will find this concept hard at work in most cars on the road. The seal on the oil pan drain bolt is usually aluminum. The previously mentioned banjo fitting seals on your brakes are copper or aluminum. Most modern cars use a multi-layered steel head gasket as well.
 

Wrench

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Joined
Nov 14, 2018
Messages
5,220
I dont want to tighten the nuts too tightly as I worry about stripping the threads, they're all relativley hand tight with a ratchet set, although one of the nuts appears to be stripped already on the exterior and can not be tighten any further


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
 

Chainlube

Well-Known Member
Joined
Jan 5, 2020
Messages
6,251
Fixed the bike, smaller rear sprocket, #70 -> #68 jet, ported cylinder and it works. Its faster, I can tell and the four stroking 'appears' to be gone.

Was able to get the four M8 1.25 threaded rods for the cylinder in without repairing the crankbox threaded hole, the crankbox threaded hole may not be as damaged in the threads as I first imagined and it may have been more to do with the threaded rods themselves. I replaced multiple M8 1.25 threaded rods and nuts from ace hardware / home depot but the metal appears weak and they become stripped as well easily. I bought two new thread rods from Amazon, a stainless steel one and a carbon steel one with oxide finish, I installed the stainless steel one for now and everything appears to be working.

I dont want to tighten the nuts too tightly as I worry about stripping the threads, they're all relativley hand tight with a ratchet set, although one of the nuts appears to be stripped already on the exterior and can not be tighten any further.

question:

1. How tight must the cylinder head be? If the bike runs, does this mean its good? There appears to be tiny gaps with the metal seal between the head and cylinder (see images)
2. The seal between the cylinder and the cylinder head is a thin piece of metal, is this supposed to be airtight? I dont see how it could be 100% airtight as its not a rubber type gasket to form a perfect seal.

images:
Get a new head gasket, and check that the head is flat by placing it on a piece of glass.
 

weefek

Active Member
Joined
May 14, 2022
Messages
385
Personally I prefer Copper head gasket and a properly "decked" (flattened) mating surface, which means sanding them both with a piece of glass as a backer for true flatness.
 
Joined
Apr 8, 2022
Messages
51
Researching nut grades and types (most are grade 8.8, even the "carbon steel" ones) - im thinking the reason its stripping is because nuts are shorter and have less surface area for the thread contact so theres more stress on the threads - the acorn nuts that came with the kit are longer and more thread on thread contact and less chance of stripping (Even though mine did strip) - so im looking at buying either new acorn nuts or coupling nuts.

Never used a torque wrench, is it necessary? - You say torque to 12 ft lbs, so set it to 144 inch on the wrench? Looking at getting this https://www.amazon.com/Neiko-03714A-Adjustable-Vanadium-Inch-Pound/dp/B000I7ZDN4
 

Chainlube

Well-Known Member
Joined
Jan 5, 2020
Messages
6,251
Researching nut grades and types (most are grade 8.8, even the "carbon steel" ones) - im thinking the reason its stripping is because nuts are shorter and have less surface area for the thread contact so theres more stress on the threads - the acorn nuts that came with the kit are longer and more thread on thread contact and less chance of stripping (Even though mine did strip) - so im looking at buying either new acorn nuts or coupling nuts.

Never used a torque wrench, is it necessary? - You say torque to 12 ft lbs, so set it to 144 inch on the wrench? Looking at getting this https://www.amazon.com/Neiko-03714A-Adjustable-Vanadium-Inch-Pound/dp/B000I7ZDN4
Torqueing the head is important because if the pressure isn't the same on all the fasteners the head will warp causing a loss of compression. That looks like a good wrench, don't need a lot of length, it's not a wheel you're torqueing down.
 
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