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Bolt Torque Setting

Old 08-09-2022, 07:21 AM
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Eastflight
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Question Bolt Torque Setting

What torque should the cylinder head and back plate be used to correctly tighten the bolts on engine sizes between .40 to .61 ?
Probable it doesn't make much difference on these small engines but I'm still curious..!
Old 08-09-2022, 09:26 AM
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fizzwater2
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With 4-40 and 5-40 bolts, I use 12 inch pounds.
Old 08-09-2022, 10:04 AM
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Eastflight
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Originally Posted by fizzwater2 View Post
With 4-40 and 5-40 bolts, I use 12 inch pounds.
Thank you -
So for a OS 46AX engine, the bolts used are M3 3mm (14/125) High Tensile Hex Socket type, is 12 Inch pounds would be a maximum torque?
It seems a bit high for Aluminium.

Last edited by Eastflight; 08-09-2022 at 10:20 AM.
Old 08-09-2022, 10:43 AM
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fizzwater2
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I haven't had a bolt pull out yet. Heads on my race engines get removed / replaced fairly often, and the 12 in lbs was recommended to me by the engine maker.
the M3 bolts are slightly larger diameter than the 4-40's, and I use 12 in lbs on them as well in the Thunder Tiger pro .40's.

Old 08-09-2022, 10:57 AM
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1967brutus
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Originally Posted by Eastflight View Post
Thank you -
So for a OS 46AX engine, the bolts used are M3 3mm (14/125) High Tensile Hex Socket type, is 12 Inch pounds would be a maximum torque?
It seems a bit high for Aluminium.
I cannot come with numbers, because I do not have torque spanners that small, but you know when you unlock your frontdoor from the outside, using the key only? Not the part where you pull back the tongue, but that last little twist where you have to pull back the latch against the spring using the key?

Not much tighter than that.
Another not very accurate description: You know those L-shaped allen keys, the M3 size being about 2,5" or so in lenght? A thumb on the bend, and the tip of the indexfinger at the long end, and no more force than what still is "comfortable".

In the end, tightness is indeed not the main thing, there is a fairly broad range where the head will be sealing properly without damaging the threads. Tightening evenly is more important. It is all a matter of "feel", don't overdo it.

Since you're asking about torque, I assume that you have had some leaks or whatever, leading to the question? Tightening a head is simple: To square the head down, press down on the head in the middle (where the plug is) with one hand. Snug up the bolts in a cross wise pattern by rolling the allen key between thumb and fingertips of the other hand (use the long end in the hex head). Now this is important: When you have snugged up two opposed bolts, check whether tightening the 2nd bolt did not tension up the 1st one. Check by trying to loosen it, should take the same force as when you snugged it up. If that is the case, the head lies on square. If it isn't, there is probably some dirt on the landing face between liner and head.
After snugging up all bolts this way, making sure the head is squared, then tighten, and each bolt should take fairly accurate the same force to turn the same amount of degrees.
For .40~.60 size engines, after properly snugging up, the tightening angle should normally be something like 60 degrees, maybe 90 tops. That is another way of getting bolts equally tightened without overdoing it.
Old 08-09-2022, 01:50 PM
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Eastflight
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Originally Posted by 1967brutus View Post
I cannot come with numbers, because I do not have torque spanners that small, but you know when you unlock your frontdoor from the outside, using the key only? Not the part where you pull back the tongue, but that last little twist where you have to pull back the latch against the spring using the key?

Not much tighter than that.
Another not very accurate description: You know those L-shaped allen keys, the M3 size being about 2,5" or so in lenght? A thumb on the bend, and the tip of the indexfinger at the long end, and no more force than what still is "comfortable".

In the end, tightness is indeed not the main thing, there is a fairly broad range where the head will be sealing properly without damaging the threads. Tightening evenly is more important. It is all a matter of "feel", don't overdo it.

Since you're asking about torque, I assume that you have had some leaks or whatever, leading to the question? Tightening a head is simple: To square the head down, press down on the head in the middle (where the plug is) with one hand. Snug up the bolts in a cross wise pattern by rolling the allen key between thumb and fingertips of the other hand (use the long end in the hex head). Now this is important: When you have snugged up two opposed bolts, check whether tightening the 2nd bolt did not tension up the 1st one. Check by trying to loosen it, should take the same force as when you snugged it up. If that is the case, the head lies on square. If it isn't, there is probably some dirt on the landing face between liner and head.
After snugging up all bolts this way, making sure the head is squared, then tighten, and each bolt should take fairly accurate the same force to turn the same amount of degrees.
For .40~.60 size engines, after properly snugging up, the tightening angle should normally be something like 60 degrees, maybe 90 tops. That is another way of getting bolts equally tightened without overdoing it.
Hi Brutus
Thanks for the info. very helpful.
I use the method that you mentioned with the allen key which works a treat. Generally I have a feel for how much to tighten which seems to have worked over the years.
The reason why I ask for the torque settings is because I have acquired a mini torque driver which has settings between 0.8Nm to 5.0Nm.
As its an instrument for small jobs, I was curious to know how accurate my own settings are.
Nearly all manufactures do not publish such info for small engine torques and so I thought I'd throw the question out there for some hard numbers to work with.....IF available.
But your feedback is much appreciated especially with the angular method for tightening up the head bolts
Old 08-09-2022, 01:55 PM
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Eastflight
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Originally Posted by fizzwater2 View Post
I haven't had a bolt pull out yet. Heads on my race engines get removed / replaced fairly often, and the 12 in lbs was recommended to me by the engine maker.
the M3 bolts are slightly larger diameter than the 4-40's, and I use 12 in lbs on them as well in the Thunder Tiger pro .40's.
That's very useful info thanks. I have not used a torque driver yet on a strip down and glad someone is able to provide some light on the numbers so that I can use the tool.

Old 08-11-2022, 10:48 AM
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I do as others have mentioned - hold the head down tight and just start the screws in until they are touching the head, you don't want to rock the head any when you are starting. Get all of the screws to touch before you start tightening.

I start at 6 in lbs, the first time around, and then make another pass at 12 in lbs. Haven't warped a head or had leaks yet doing it that way. I tighten one, then go straight across and get the one directly opposite it to avoid rocking the head - then I move over two, tighten that one, then directly across, then the remaining two (on heads with 6 screws)



Old 08-11-2022, 12:12 PM
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Eastflight
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Originally Posted by fizzwater2 View Post
I do as others have mentioned - hold the head down tight and just start the screws in until they are touching the head, you don't want to rock the head any when you are starting. Get all of the screws to touch before you start tightening.

I start at 6 in lbs, the first time around, and then make another pass at 12 in lbs. Haven't warped a head or had leaks yet doing it that way. I tighten one, then go straight across and get the one directly opposite it to avoid rocking the head - then I move over two, tighten that one, then directly across, then the remaining two (on heads with 6 screws)
Nice one, thank you

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