12hp Cast Iron Briggs Question

[Achto 27,623]

I'm currently working on a Briggs 302431 0144-01 (30cu in, 3 7/16" bore) The bore on this engine is terrible, the ridge ring on the top & bottom is big enough to trip over. They used to make a .030 over piston for this engine but they are now NLA. My thought is to install a piston from a 3200000 series engine (32cu in, 3 9/16" bore)  These pistons are readily available. The only challenge that I for see is the weight difference, to compensate for this I plan on shaving some material from the lower skirt on the new piston. In the end making the new piston match the weight of the old one. This would keep the engine balanced.

 

Just wondering if anyone has done this & if so do you have any other tips??

[oldredrider 2,548]

If a standard size piston is available, you could sleeve the cylinder and bore it to match the new piston.

[WHX?? 48,834]

Sounds like a plan but going from 7 theenst to 9 seems like a bigger  jump than the sleeve can handle? So maybe old red is on to something? 

Guessing the plan to go to 14 HP isn't panning out? Or is the 14 the 9/16 piston? No chance with the commoness of the block  can't find a donor? Or is not that common?

Edited January 7, 2021 by WHX24

[Achto 27,623]

33 minutes ago, WHX24 said:

Guessing the plan to go to 14 HP isn't panning out? Or is the 14 the 9/16 piston? No chance with the commoness of the block  can't find a donor? Or is not that common?

 

The 3 9/16" piston would be the 14hp up grade. The cylinder walls are 5/16" think now, taking them down to 1/4" thick does not really concern me too much. The 12, 14, & 16hp all use the same connecting rod part number. The part number for the cranks from 12hp to the 14,16hp is different. Stroke is the same on all but the 14 uses a different counter balancer & the 16 uses two counter balancers.

[Achto 27,623]

@Stepney

Have you had any experience with these?

[WHX?? 48,834]

12 hours ago, Achto said:

cylinder walls are 5/16"

Would have never guessed that thick. 

[WHX?? 48,834]

Since your into your bore refresh groove Dan I will be dropping off that Onan carcass today....see what you think if it can be saved with just a re-ring. Also check out the the wear on one of the rod big ends and crank journal. I put match marks on the pistons and cylinders if the pistons can be reused. Hope fully they can. if they can't its looks like the cost might get way out of hand. 

 

I think your plan on going with the 14 HP piston is sound then. The liner will never miss that metal removed. As far as the piston weight difference is just get them close. Guys Do not just ditch the counter balances as with Kohlers? Guessing they are timed and just as much as a pain as with Kohlers? 

Edited January 7, 2021 by WHX24

[Achto 27,623]

31 minutes ago, WHX24 said:

Guys Do not just ditch the counter balances as with Kohlers?

 

I have not found anything that suggests getting rid of the counter balance on these Briggs engines. Timing them looks pretty easy.

[roadapples 6,983]

3 hours ago, WHX24 said:

Since your into your bore refresh groove Dan I will be dropping off that Onan carcass today....see what you think if it can be saved with just a re-ring. Also check out the the wear on one of the rod big ends and crank journal. I put match marks on the pistons and cylinders if the pistons can be reused. Hope fully they can. if they can't its looks like the cost might get way out of hand. 

 

 

Must be nice to have your very own mechanic on speed dial 

[WHX?? 48,834]

25 minutes ago, roadapples said:

Must be nice to have your very own mechanic on speed dial 

'Specially one that knows what he's doin! 

[Achto 27,623]

1 hour ago, WHX24 said:

Specially one that knows what he's doin! 

 

WAIT!!! I didn't know that was requirement. I'm gonna have to re-read my job description. 

Edited January 7, 2021 by Achto

[Stepney 2,325]

Those engines already shake like a leaf.. cant help but wonder how 'much worse' it would be if you just shaved a little off the oversized piston.

We did far far worse ethical machine work sins with the old B&S engines hopping them up with old used parts.. never had issues to report. Especially the old boxers, and the cast iron 23 series. 

5hp flathead was a favorite subject too..

 

[Greentored 3,216]

On 1/6/2021 at 8:50 PM, Achto said:

 

 The cylinder walls are 5/16" think now, taking them down to 1/4" thick does not really concern me too much. 

Absolutely wouldn't think twice about this.  We shoot for no less than .180-.200 wall thickness on the thrust sides on a V8- if its going to be a 'hot' build.

Grab an old Harley shovelhead jug once, youd be shocked at how thin they are near the bottom.  Yes they have been known to break but not super common.  I had a set of .080 over jugs on my stroker, bet the walls weren't .060 thick on the last lower couple inches!

For piece of mind I eventually replaced the jugs, but not before 10 good years of beating the tar out of it with zero issues.

[Achto 27,623]

37 minutes ago, Greentored said:

Grab an old Harley shovelhead jug once, youd be shocked at how thin they are near the bottom.  Yes they have been known to break but not super common.  I had a set of .080 over jugs on my stroker, bet the walls weren't .060 thick on the last lower couple inches!

 

I used to do a fair amount of 883 to 1200 conversions. There is 1/4" difference in the the piston diameter from one to the other. The walls looked pretty thin after boring but there was still plenty left if you needed to go up to .030 over some day. My biggest engine size jump was on my old 74cu in Shovel, went from 3 7/16" bore to 3 5/8" bore. Of course this required new jugs, boring the cases out bigger, cutting the fire ring bigger in the heads, rebalance the flywheels, etc, $$$$$.  With the addition of stoker flywheels I ended up with a  93cu in engine. Was a very fun bike that was capable of putting quite a few Evo's in the dust.

Edited January 8, 2021 by Achto

[WHX?? 48,834]

12 minutes ago, Achto said:

Shovel, went from 3 7/16" bore to 3 5/8" bore.

Yah that was a consideration when I did my shovel motor ... price just got way out of hand tho for just a run to get cigs  bar hopper bike so stayed stock 74 Always just liked the way those stroker jugs looked tho.  Opted to spend on eye candy... 

This kit went for about 2500 dealer list in my day but nice to see you can go to 84 ci for less than 2500! 

 

Sry ... getting 

[Achto 27,623]

2 minutes ago, WHX24 said:

Sry ... getting 

 

No worries Jim.

[Greentored 3,216]

25 minutes ago, WHX24 said:

 

 

Sry ... getting 

 

Now wait a minute. It was my understanding that this is the norm, not the exception