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.