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Discussion Starter #1
Hey guys,

Do the cylinder head bolts on the 3 cylinder bikes need to be replaced each time the cylinder head is removed? They require a torque angle measurement so I think they are a one time use, but could be wrong.

Manual only addresses this when it says to discard the bolt if you exceed the angle and you haven't yet reached the minimum torque (meaning the bolt is yielding).

It does not specifically say replace each time so just want to check with you guys first as they are 8 bolts and add up to a bit of cost.
 

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I would say you have your answer. Install the bolts and, if any exceed the angle, replace those.
 

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Simple answer, yes replace them all....

Every OEM head bolt i have ever used has stated to replace them after their first stretch. The act of tightening to the proper torque spec means the bolt stretches. Once the bolt stretches the metal gets fatigued. It can no longer go back to it's original dimension. It is permanently compromised.

Bolt stretch is the force that is actually holding your block to the head! Stretch a rubber band in between two fingers... It pulls your fingers back together when you stop exerting opposite force. Metal is the same way but instead of springing back completely, the grains are perminently rearanged and can no longer "spring back".

The question you are really asking is... are the head bolts torque to yeild? Yes. They "Yeild" after being used. Making them compromised.

They do make bolts that are re-usable like ARP. They are able to be used again because they use Alloys that have higher carbon content (harder) and can resist the stretching forces. They spring back after being torqued to the specified rating set by the engineers who created the bolt.

Every piece of metal has a point of no return and a breaking point. Your head bolts are at their point of no return.

Don't believe me? Buy a new head bolt and use a micrometer to measure the length. Then mic the old bolt. The yielded used head bolt will be longer in length. It has stretched beyond what it can take and wont spring back.

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Discussion Starter #4
Simple answer, yes replace them all....

Every OEM head bolt i have ever used has stated to replace them after their first stretch. The act of tightening to the proper torque spec means the bolt stretches. Once the bolt stretches the metal gets fatigued. It can no longer go back to it's original dimension. It is permanently compromised.

Bolt stretch is the force that is actually holding your block to the head! Stretch a rubber band in between two fingers... It pulls your fingers back together when you stop exerting opposite force. Metal is the same way but instead of springing back completely, the grains are perminently rearanged and can no longer "spring back".

The question you are really asking is... are the head bolts torque to yeild? Yes. They "Yeild" after being used. Making them compromised.

They do make bolts that are re-usable like ARP. They are able to be used again because they use Alloys that have higher carbon content (harder) and can resist the stretching forces. They spring back after being torqued to the specified rating set by the engineers who created the bolt.

Every piece of metal has a point of no return and a breaking point. Your head bolts are at their point of no return.

Don't believe me? Buy a new head bolt and use a micrometer to measure the length. Then mic the old bolt. The yielded used head bolt will be longer in length. It has stretched beyond what it can take and wont spring back.

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Thanks very much for the explanation!
 

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@Figifister .... do you think perhaps the factory used that type bolt (like ARP) and that is why they state to replace only if specified torque cannot be achieved within specified angle???
If the bolts required replacement every use the factory instructions would say so.
 

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@Figifister .... do you think perhaps the factory used that type bolt (like ARP) and that is why they state to replace only if specified torque cannot be achieved within specified angle???
If the bolts required replacement every use the factory instructions would say so.
What they are stating is that once the bolt is stretched past the point of no return it is to be discarded..... which is a fancy way of saying they are one time use.

If you were to torque down the head bolt again you would have a very high probability of it snapping. The act of torquing the bolt properly to the specified angle or foot pounds renders it past the point of no return. I don't know of any OEM that uses multi use head bolts.

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The 'stretch' of the bolt is actually holding the head in place like Figifister wrote.

If you are not going past the yield strength it will return almost to it's original length and you could reuse them.
However, this is a thin line and if you crossed that you are not sure so replacing is the best option.
In the past I reused head bolts on Honda CBR600RR's numerous times without issues (tuned engines for race bikes).

The typical thing is that in the F3 you will find 'regular' metric head bolts in 12.9 quality.
So no metric fine thread and not the typical cylinder head bolt shape you'll find in f.e. Japanese bikes.
The tuner I work with did never see this normal type of bolt used as a head bolt.

On my F3 I replaced the cylinder head twice and replaced the bolts; just to be sure.
I looked for an alternative from ARP but sticked with the original MV bolts
 

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That's why I encourage people to measure their own parts. I know ARP supplies a chart to reference the tolerance of stretch length for their bolts.

This ties into what I was getting at talking to @esq'z me, I haven't seen an oem list a stretch length chart in regards to head bolts. They say instead" if you have exceeded the value listed then replace." Or most common, just "replace".

I'd like to know how you would measure for the bolt being too stretched out without a guidline!

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All "torque to yield" cylinder head bolts require replacing whether it's a car or motorcycle motor. ;)
 

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Just spoke with the owner/engine builder at the local Ducati race shop (and MV shop) about this subject.
He is a very reputable and well known professional performance engine builder.
Certain rod bolts are routinely replaced every use (Ducati stock as example) but rarely cylinder head bolts. Head bolts are replaced only if they exceed the torque angle without achieving the torque load, or do not meet measurement specs in other ways.
Correct thread lubricant and proper tools being used, of course.
He has not replaced the head bolts on any MV engine he has worked on at this point.
This guy builds high spec racing engines for professionals from around the country, he is not a local-yokel shade tree engine builder.
So...unless the factory service manual states "Replace the cylinder head bolts at every use" you do NOT have to replace them.
Follow the manual instructions. If the bolt will not torque to spec within the angle referenced THEN you should replace it.
 

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I respectfully disagree. You're just asking for problems. Just because you can achieve the correct torque doesn't mean the bolt isn't compromised. Your playing the lottery with your already finicky and meticulous Italian steed. You really want a "potential" problem with the thing that keeps your cylinder head to the engine block?!?

The engineers who designed the bolt label them TTY inspite of this the owner/engine builder is going to say that you can do it because he has done it in the past?

Did you ask him what steps he takes to insure that they are not compromised? My guess is that his answer would be "well they torque to the proper spec and I haven't had a problem in 30+ years of wrenching."

I would always error the side of caution especially with something so important.

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If the manual say replace them you do....otherwise you are just guessing they need be.
The manual gives a specification and advises that they only need replacement if the specification is exceeded.
The guy I asked builds pro level race bike engines, and has for decades. I will listen to him before I take your word as gospel.
But you can do as you like.
As a 40 year veteran technical manager for Motorsports OEMs, I will follow the factory manual instructions and the respected builders advice.
 

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Discussion Starter #14
Thanks for you help and insight guys. I decided I am going to order new screws to be safe as the logistics of having to get replacement screw from Italy now during covid is worrying.

Here is a screenshot of the manual.

479937
 

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I have yet to replace cylinder head bolts in our F3 race bike, some will have been used 4 - 5 times, have never had one yeild. Conrod bolts are a different matter, I would not be brave enough to reuse them.....
 

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If spending a bit of cash on 8 bolts buys YOU some peace of mind, then why not? --
I would not do it if it meant waiting for a usable bike as long as I waited for the fuel line connectors for mine ;) :LOL:
What is the metric size of these bolts -- are they fine pitch? -- Usable max torque (90%yield strength) for an M8 - 12.9 bolt is 49 - 51 Nm - So the value given in the manual will certainly hit the yield strength, if the bolts are M8. -- The bolts will stretch, but that does not mean that they are compromized. -- If the bolts are M10 - then you are nowhere near the limits ..
 

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@rodprice since you may not have new head bolts to compare them to I was wondering if you could use a micrometer to measure the length of 1 of the head bolts that you deem reusable?

Then when @PTsalas gets his new set maybe he could do the same thing. Would be interesting to compare them.

Also, how long is your engine lasting before you rebuild it? What are the points of failure/ what is replaced every time you take it apart?

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Discussion Starter #18
@rodprice since you may not have new head bolts to compare them to I was wondering if you could use a micrometer to measure the length of 1 of the head bolts that you deem reusable?

Then when @PTsalas gets his new set maybe he could do the same thing. Would be interesting to compare them.

Also, how long is your engine lasting before you rebuild it? What are the points of failure/ what is replaced every time you take it apart?

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Will measure them as soon as parts arrive. Will be in the next month. Will also measure my old bolts for interest.

Would be interesting to compare with Rodprice's repeatedly used bolts.
 

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I'm telling you guys... if the manual does not specifically state to replace with every use then you are wasting money doing so.
As the Technical Service Manager for a Japanese manufacturer, that is what I would tell you if you called in to the technical support line to ask.
Do what you want.....it's your money
 

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Will measure them as soon as parts arrive. Will be in the next month. Will also measure my old bolts for interest.

Would be interesting to compare with Rodprice's repeatedly used bolts.
Great thanks! Yes I agree it will be interesting.

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