Briggs 3hp build questions

G.W

G.W

Well-known member
Messages
764
Reaction score
497
Location
Nyssa Oregon
Alright, I'm looking into building a 3hp up to maybe 6 or 8 hp. Yes I know that's stupid and is gonna be expensive, but I don't care. I've set my mind to it, I'm going to do it. This build is however another year down the road, I'm just getting ideas stirred up. I can find an aluminum flywheel for the 3hp, but no rod. I don't wa t to pay to have one custom made either. I was talking to one of my buddies that used to work at a speed shop in the 70s and 8/s and he thinks I could polish the stock rod and it would survive 6k to 8k rpms. The only issue I can see is the steel oil dipper likes to snap off at about 5k rpm. I was thinking if I could polish the con rod and maybe make a stronger oil dipper I would be able to run the factory rod.

I got to thinking about what material to use, and I don't want to use aluminum, it's too unpredictable sometimes, especially on a smaller part like this. Maybe I could use some thicker steel on the new dipper, or what I was thinking was some Nylon. The only issue I could see with nylon is it flexes a lot, and might become brittle over time from the several heat cycles. Any Ideas you guys have on this would be great. Thanks!
 
G.W

G.W

Well-known member
Messages
764
Reaction score
497
Location
Nyssa Oregon
Denny said:
Not gonna happen! There’s a reason they don’t make parts for them.
Click to expand...
That's exactly why I want to try. What's the worst that happens? I have a flywheel that will bolt to my 5hp and a blown up 3hp that I spent a few hours porting and polishing? I think it's worth a shot if I don't spend too much trying to get a con rod. I'll probably have to weld some block reinforcement around the lifter area and find some stiffer valve springs.
 
G.W

G.W

Well-known member
Messages
764
Reaction score
497
Location
Nyssa Oregon
Here's what the assembly looks like. Not mine, just one I found on the enternet.
Screenshot_20240624-163304.png
That steel triangle on the end is what I am looking to upgrade. The rest of the rod is aluminum and can be polished pretty easily. Polishing it should remove all sharp edges and stress points and make any power transfer smooth through the rod. I was also going to see what pistons of that pore and pin size I could find (if any) and i have a chunk of Aluminum mill stock I want to try and make my own head for. It shouldn't be too difficult to do.
 
Hellion

Hellion

Moderator
Messages
8,481
Reaction score
3,592
DO IT. I'm all for it. There's legions of people with an old 3 HP lying around that came off an edger or a low-end tiller (or a barn find Bonanza minibike) that they are reluctant to use on karts and minibikes because they think they are woefully underpowered.

All the vintage yard karts and minibikes came with 2, 3 and 3.5 HP engines back in the day, it kind of boggles the mind.
 

Attachments

  • Image 197.jpeg
    Image 197.jpeg
    75.4 KB · Views: 0
  • Like
Reactions: G.W
G.W

G.W

Well-known member
Messages
764
Reaction score
497
Location
Nyssa Oregon
Master Hack said:
Buy a 6 or 8 Hp engine. Then spend the left over cash on the local wimmen. That would mo funner!
Click to expand...
Nope, not an option. I already decided to do it, it's getting done. The more people say I shouldn't, the more I want to.
 
G.W

G.W

Well-known member
Messages
764
Reaction score
497
Location
Nyssa Oregon
Hellion said:
DO IT. I'm all for it. There's legions of people with an old 3 HP lying around that came off an edger or a low-end tiller (or a barn find Bonanza minibike) that they are reluctant to use on karts and minibikes because they think they are woefully underpowered.

All the vintage yard karts and minibikes came with 2, 3 and 3.5 HP engines back in the day, it kind of boggles the mind.
Click to expand...
I will. I have a few 3hp, and I think 1 3.5hp that could all be test subjects. The flywheel will fit them all after all. I really want to get at least 6hp and 8000rpm out of it.
 
G.W

G.W

Well-known member
Messages
764
Reaction score
497
Location
Nyssa Oregon
Ok, after looking into things a bit closer I should be able to rev that puppy up on a polished aluminum rod just fine, but the dipper is still an issue. I looked at the Briggs 5hp rods that arc sells, but they have built in oil dippers. Do you guys think I could make one with thicker steel and maybe put some longer rod bolts into it?

Another potential issue could be main journal oiling. Perhaps I could add a small oiling hole in the rod? Where would be the best location be if I did? And would that compromise the rods structure?
 
Hellion

Hellion

Moderator
Messages
8,481
Reaction score
3,592
There's some high strength nylon dippers that you simply bolt to the bottom of the con-rod cap. They're sold on karting sites, but they were for the Raptor only, or Raptor type builds (5 HP 130000 series).
 
G.W

G.W

Well-known member
Messages
764
Reaction score
497
Location
Nyssa Oregon
Hellion said:
There's some high strength nylon dippers that you simply bolt to the bottom of the con-rod cap. They're sold on karting sites, but they were for the Raptor only, or Raptor type builds (5 HP 130000 series).
Click to expand...
Could you maybe post a link? I couldn't find one.
 
G.W

G.W

Well-known member
Messages
764
Reaction score
497
Location
Nyssa Oregon
Does anyone see any issues with running the aluminum rod at 8,000 rpm? I personally think it's maybe pushing it, but entirely possible, and so does my buddy that used to do this stuff to cars for a living.

Also, does anyone know if the Briggs raptor rods were made any different from the stock rods? I read that the stock raptor rods could spin to 8,000 rpm. If the regular rods are made the same I don't see why it's not possible to spin the 3hp rod that fast. I also don't plan to run the engine at 8,000 rpm for a long period of time, I just want it to be able to. It will likely never see more than 6,000 rpm regularly. I want to put it on a bicycle, that's why I want the smaller package.
 
You must log in or register to reply here.
Top