Poly 318 Oil Bypass Valve &

Spin-on Filter Adapters

A-block, Hemi, and Hemi Poly Oil Bypass Valve

All A-blocks have an oil hole drilled through the block underneath the #5 (rear) main cap on the right side. In many early A-blocks from 1956 – 1961 with reports of some 1962 – 1963 blocks, the factory installed a bypass valve in the block (Figures 1 – 5), similar to many 1950’s Hemi and Hemi-block poly engines. The purpose was to recirculate unfiltered oil in the event the oil filter clogged and pressure grew too high in the canister. Mopar realized three issues with the design by the early 60s: (1) if the oil filter clogged, there were major issues with either lack of oil maintenance or fragmenting parts that would not be helped with a bypass, (2) the new-style spin-on filters had a higher bypass pressure rating than the block’s bypass, so the block bypass would trip before the spin-on filter causing unfiltered oil to circulate and (3) the block bypass valves could corrode and come apart, sending debris through the oiling system while also bypassing the filter permanently. To address the issues, Mopar began plugging the passage with a cast plug, and eventually they simply did not drill the passage completely through. Engines running both earlier cartridge and later spin-on filters should always have the internal bypass valve removed and the passage plugged.

There are two methods to plug the passage of which I know:

Threaded Plug (Figure 6): I doubt I invented this idea, but here it is drafted into a visual and explained. The oil passage is 0.38” diameter, slightly larger than the .036” used for 7/16” threads. Tap the passage 7/16”-14 and plug it with a 7/16”-14 black-oxide Allen-head plug using red Loctite and torqueing it down tight. Make sure to thread below the filter intake passage to where when the plug is in place, oil can still flow from the pump to the filter.

Soft Plug: If one can source a soft plug, the bore is typically 0.38″ diameter. Drive the plug down below the filter intake passage using a long bolt as a driver to where when the plug is in place, oil can still flow from the pump to the filter (Figure 5).

Aftermarket bypass plug (Figure 6): The only available bypass plugs carried by Speedway Motors (910-11048), Donovan (2600-0190), and Hot Head Research (20120) are made for early hemis and hemi-block polys and are too large (0.552” x 1.650”). They will not fit the A-block.


early poly A-block oiling system
Figure 1: 1956 – 1961 System with Bypass
later poly A-block oiling system
Figure 2: 1962 – 1966 System without Bypass
Poly A-block bypass valve
Figure 3: Bypass Valve Exploded View
hemi block bypass valve
Figure 4: Oil Bypass Valve (schematic for a hemi block but similar to A-block)
Poly A-block Factory Plugged Oil Gallery
Figure 5: Poly A-block Factory Plugged Oil Gallery (1963 block)
Justin Lotspeich's threaded oil bypass plug
Figure 5: Soft or Threaded Bypass (drawing by J. Lotspeich)
Aftermarket hemi block bypass plug
Figure 6: Hemi Aftermarket Plug (will not fit A-blocks)

Spin-on Filter Adapters

LA spin-on filter hardware, including aftermarket adapters, can be fitted onto A-blocks with cartridge filters. If running a spin-on filter, it is important to plug the block bypass described above (if applicable) since it has a lower activation psi than the spin-on bypass and can lead to unfiltered oil circulating.

For Factory Flow:

Use a factory style LA oil-filter adapter with Felpro 70522 fiber gasket to seal to the block. Tacking the gasket in place with a few dabs of grease helps. Tighten the bolt to 25 ft.lbs. Rubber gaskets are out there, but they have a tendency of being difficult to seal compared to the fiber gasket.

factory LA oil filter adapter
Factory Style Adapter, Stud, and Gasket

For Aftermarket High-flow:

Use the factory style adapter plate but drill additional holes for greater volume.

Poly A-block High Volume Oil Filter Adapter
Poly A-block High Volume Oil Filter Adapter (modification & photo J. Lotspeich)

Use a Crown Automotive stud CWA-53007563AB and a Canton 22-575 adapter. Along with higher flow, this adapter utilizes an o-ring against the block that negates the need for the fiber gasket.

Canton 22-575 Mopar oil filter adapter
Canton High-flow Adapter
Crown Auto CWA-53007563AB Mopar oil filter adapter stud
Crown Automotive Stud