Help! Trying to prevent con Rod failure

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williambriggs

New Member
#1
#1
Hi, I'm new on this forum. Im currently working on a project trying to increase the horsepower on a old mini bike engine (Briggs 3hp) without using any aftermarket parts. I did a lot of work to get more power and rpm. Now I want to increase to tolerances of the internals.

The stock connecting rod is die casted aluminum alloy 380. On other forums I've researched that the stock rod usually breaks at RPMs above 4,500. Is there a way to increase the strength of the rod to keep it from breaking at higher RPMs. I thought about heat treating the rod, but I don't know the proper procedure for die cast aluminum. Also it is possible to weaken the metal If it were to be heat treated twice, because it may have been heat treated during the manufacturing processes. If you know of a good way to strengthen cast aluminum, or have any good ideas that may help me please let me know thanks.
 
CarPlayLB

CarPlayLB

Well-Known Member
#3
#3
I am no metallurgist by any stretch of anyone's imagination...but I think trying to heat treat again may make it brittle...just my 2 cents and change coming!

Maybe take the rod and smooth out the casting flash...less stress risers may help!
 
W

williambriggs

New Member
#4
#4
Port and polish, Methanol jet, valve springs, shaved the head, modded the stock carb to take in air through the choke. There's other small stuff that I've done and other thing that I'm planning on doing.
 
Phil1958

Phil1958

#5
#5
williambriggs said:
Port and polish, Methanol jet, valve springs, shaved the head, modded the stock carb to take in air through the choke. There's other small stuff that I've done and other thing that I'm planning on doing.
Click to expand...
welcome aboard william, this is a fun place.

sounds like you've been really busy!

you should read the section about modifying engines.

i have one of those engines and would never consider removing the governor or do
anything to increase the rpm with those ancient cast iron flywheels. they are bombs
waiting to go off.

:thumbsup:

phil
 
super nate

super nate

Member
#7
#7
you cannot reheat treat the stock rod. it wont work, the stock rods are made to putt around at slow speeds, if you want dependability put a billet flywheel a billet rod and a forged piston in it, it really doesn't cost that much compaired to a hole the size of a softball in your case.
 
tundrabeagle3

tundrabeagle3

Member
#8
#8
Not sure about 3 hp Briggs but my 5 hp Briggs that I had in the past had only the rod dipper rip off . I have seen some people use nylon rod dipper but have never seen any for sale.
 
neo71665

neo71665

Member
#9
#9
super nate said:
you cannot reheat treat the stock rod. it wont work, the stock rods are made to putt around at slow speeds, if you want dependability put a billet flywheel a billet rod and a forged piston in it, it really doesn't cost that much compaired to a hole the size of a softball in your case.
Click to expand...
A hole in the case would be the least of the concerns when an engine sitting between my legs blows apart.
 
super nate

super nate

Member
#10
#10
another old school trick is to strap the rod...you take a ujoint strap and flatten it out a little with a hammer and form it to take the shape of the cap. place it on the cap and tighten it down..the strap should be drawn down when you tighten it. this is what people did before there were billet rods..
 
KB2ROCKET

KB2ROCKET

Active Member
#11
#11
Just in case you did not know this here is some more advice or info. For you follow at your own risk
When you shaved the head for more compression the roof of the combustion chamber moved closer to the piston now the flow to and from the cylinder and to the valves is narrower restrictions flow but what you can do to fix this is to grind away the metal starting at the edge of the cylinder and create a clear path to the valves . Warning warning warning do not disturb the valve seat !!
Do not grind away the side of the cylinder in excess never grind in the area the top piston ring will travel. In my opinion flow is more important than compression
Good luck
 
SAS289

SAS289

Well-Known Member
#13
#13
From the creative mods I have seen in this forum I'm surprised no one has come up with a 1/4 steel seat plate to protect those genitals. lol
 
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