flathead to OHV conversion help

[OwenAC12]

i am looking to convert an old Briggs & Stratton opposed twin flathead engine to OHV. The block is already ruined because the valve guides were destroyed by the previous owner is there any heads from other engines that will bolt on and have the proper spacing for parts could sombody tell me if there is a kit or heads that fit

 

[Edwin Spangler]

AI says:


There are no direct bolt-on OHV heads or conversion kits available for the Briggs & Stratton opposed twin flathead engine.
Converting a flathead (L-head) block to an overhead valve (OHV) configuration is not possible by simply swapping heads because the two designs have fundamentally different architecture built into the engine block itself:

• Camshaft Location and Lobes: In your flathead, the camshaft is located inside the block directly beneath the valves, and the lobes point upward to push the valves open. An OHV engine requires a different camshaft profile, longer pushrods, and a way to route those pushrods from the block up into the cylinder head.
• Oil Galleries and Cooling: The cooling fins and oil lubrication passages on your flathead block are designed strictly for an L-head setup. An OHV head requires oil to be fed up to the rocker arms and drained back down, which your current block cannot accommodate.
• Bolt Pattern and Bore Spacing: The bolt pattern on the flathead block is designed only to seal the combustion chamber over the pistons. No production OHV head (from Briggs or other manufacturers like Kohler or Onan) shares this specific bolt pattern or cylinder spacing.

Your Best Options for the Ruined Block
Since the valve guides are destroyed, you do not need to scrap the engine. You can repair or replace the existing setup using these methods:

• Install Brass/Bronze Valve Guide Bushings: This is the standard machine shop fix for these engines. A machinist can drill out the ruined, cast-in valve guides and press in aftermarket bronze valve guide liners or bushings (such as those made by K-Line). This restores the tight tolerance required for the flathead valves to operate perfectly.
• Source a Replacement Short Block: Because Briggs manufactured millions of these opposed twins (400000 and 420000 series) for lawn tractors from the 1980s through the early 2000s, used parts are highly abundant. You can often find a donor engine with a blown rod but perfectly good valve guides for very little money.
• Upgrade to an Intek V-Twin (OHV): If you specifically want an OHV engine for better fuel efficiency and power, the easiest route is a full engine swap. The Briggs & Stratton Intek V-Twin series shares the same standard 1-inch or 1-1/8-inch crankshaft output diameter and mounting bolt footprint as your old opposed twin, making it a relatively straightforward drop-in replacement for most equipment.

 

[Hellion]

Where’s Partsguy? He showed a OHV conversion head that bolted onto the single cylinder flathead block (probably the 13 c.i.). It was NIFTY.

 

[Sparkwizard]

Where’s Partsguy? He showed a OHV conversion head that bolted onto the single cylinder flathead block (probably the 13 c.i.). It was NIFTY.

Did that head ever work and this poster needs two of them for his twin cylinder engine.

 

[Hellion]

Did that head ever work and this poster needs two of them for his twin cylinder engine.

As I recall it was incomplete and/or somewhat (or a lot) non-viable because there’d be no way to lubricate the top end...?

The block is already ruined because the valve guides were destroyed

Show us pics of what you have. It’s basically not feasible unless you’re a masochist, then anything is possible.
The opposed twins are neat engines by the way.

 

[panchothedog]

I think Edwin's post #2 is dead right on. Impossible, No. Practical, Feasible, Economically worthwhile, not even close. And he did a great job clearly explaining all of the reasons why not.

 

[Sparkwizard]

I'm curious about the damage to the valve guides. I wonder how the PO damaged them. I have had them replaced on a Kohler Magnum 18 horse opposed twin. I think it cost about 200 dollars in about 2016, when we still had a good small engine shop in town.

 

[Hellion]

Is it a vertical crank engine?

Does anyone make an opposed twin industrial engine anymore? They’re not a space saving design and OHV v-twins have taken over. I suppose I could look…

Delving further, I think a rebuild of the old side valve configuration, not changing anything, could be fun. Look at that head design, I’d almost say it’s beautiful:

 

[Thepartsguy]

NO atleast it would never be reliable.

Sheer luck I found out a Kohler head allowed the valves to line up to the guides.

Would need extremely long heli-coils OR an aluminum welder would be much better.


I still have the complete head the head gasket and my home made pushrods. I scrapped the block I drilled.

One day i will re-drill another scrap block and get everything bolted together for a test fire. But not in the near future as I have bigger fish to fry.

 

[Thepartsguy]

It does not matter if the OHV conversion has an open valve train that leaks oil onto the ground pump it from the bottom of the case with a pulse pump. On an OHV the only thing that matters is the cylinder seals to the head.

In the earlier more industrial days of the combustion engine it was perfectly ok for the oil to return to the earth. AS LONG as it oiled what it needed to on the valve train first. EPA crap among safety standards and a bunch of other crap ruined all that.