Why NEW CRANKCASE BOLTS

[Budd4U]

Question, why do you have to replace the crankcase bolts on a V-Twin when you open the engine? Why can't you just wire brush them and use tread locker on the old ones? From what I see is that if the treads are good and clean you should be able to just use a good tread locker and torque them are usual.
Budd

 

[ILENGINE]

Some of those bolts are setup with a certain elasticity that will not return to their prestretched length when removed for reuse. I was thinking that one of the Briggs sump gaskets comes with new bolts. May be the V twin.

 

[sgkent]

I can't answer that definitely but as an automotive machinist for many years, some of those practices are considered "best practices," and are not 100% infallible depending on the quality of the new bolts. Some bolts come pretreated with threadlock, and the assumption is that is better than threadlock on an old oily rusty bolt. Also the zinc plating often wears away when bolts are cleaned. That said, I have seen new upgraded automotive rod bolts stretch an unacceptable amount. Also some metals crystalize over time, and the manufacturer may wish to avoid dealing with that possibility. If they are available, and the manufacturer recommends them, I would use them.

 

[Budd4U]

I can't answer that definitely but as an automotive machinist for many years, some of those practices are considered "best practices," and are not 100% infallible depending on the quality of the new bolts. Some bolts come pretreated with threadlock, and the assumption is that is better than threadlock on an old oily rusty bolt. Also the zinc plating often wears away when bolts are cleaned. That said, I have seen new upgraded automotive rod bolts stretch an unacceptable amount. Also some metals crystalize over time, and the manufacturer may wish to avoid dealing with that possibility. If they are available, and the manufacturer recommends them, I would use them.

I understand what you are saying but working in a few machine shops over the years it's hard for me to believe that a 5/16 bolt torqued to 200 inch pounds / 17 ft pounds actually stretch's the bolts. Your thoughts, I actually torqued a 5/16 old crankcase bolt to 40 ft pounds in an old case and measured it both before and after and it was within .0015" difference and those bolts are a little over 2.250" long.

 

[bertsmobile1]

Probably being a bit annally rententive
However remember those 200 inch pounds of torque are only 100% if it is a dry NEW bolt in a CLEAN hole unless of course other wise specified

 

[sgkent]

I was thinking of some very high end racing rod bolts when I wrote it, that really stretched unacceptably. That said, all bolts stretch some. The key to proper torque is to be tight enough that the bolt does not load and unload every cycle or it will induce fatigue. Meaning that the torque has to be greater than the bolt sees in the loaded mode. If the bolt sees say 30 lbs force at its max, then it needs to be torqued to more than 30 lbs. That is if it is a tension bolt and not a shear bolt, which is a completely different type bolt. If you torque it to 35 lbs (assuming the engineer got it right) then the bolt never feels less that 35 lbs. But if you torqued it to say 25 lbs, then it sees a fluctuating 5 lb load every cycle. That will eventually fatigue it or the threads it is screwed into.

 

[bertsmobile1]

I rather think required clamping force is the determining factor
Otherwise all bolts would be finger tight + 1/2 turn

Then there is the shear strength of the material the bolt is threaded into which is a cylinder the diameter of the thread x the length of the thread that is in the part.
A lot more than just overcoming the back off torque

 

[StarTech]

It is because Briggs was having the screws to loosen during operations. If you read the instructions, you are to clean the crankcase threads with a thread chaser and not a regular tap to not be cutting new threads. Briggs also increased the torque from 200 in/lbs to 325 in/lbs on the series that they included the new screws with the sump gasket. The new screws also has a thread sealant applied.

Just note Briggs might these specs at anytime which is why you need to read the enclosed paper instructions.

On the last 49M I did, the sump gasket looked like expanded steel in two place where the sump had loosen. This where they want you check the flatness of the sump and crankcase with a straight edge.

 

[Tinkerer200]

The V twin B&S engine has a reputation of leaking sump gaskets and the replacement one is expensive coming with new high strenth bolts for increased torque specs. I made my own gasket and bought new Grade 8 bolts in the bulk from local farm supply store at a fraction of B&S price. Works for me - so far.

Walt Conner

 

[Hammermechanicman]

I have never bothered to use new bolts with a briggs sump gasket <gasp!>. Steel bolts in an aluminum case. Chase the threads and clean the holes with brake clean and a dab of blue locktite and torque to spec. Done quite a few and never had a problem. A couple hundred inch pounds on a 5/16" bolt isn't all that much.