Poly 318 Oil Bypass Valve &

Spin-on Filter Adapters

(applicable to 277, 301, 303, 313, 326 A-blocks and some content applicable to Gen I Hemi and Hemi-block Poly Engines)

Poly A-block, Hemi, and Hemi-block 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 blocks, the factory installed a bypass valve in the block (Figures 1 – 6), similar to Gen I Hemi (e.g. 331, 354, 392) and Hemi-block poly (e.g. 270, 315) engines. The one-way valve includes a steel housing with an internal spring and ball valve. The purpose of the valve was to open and recirculate unfiltered oil in the event the canister oil filter clogged and oil pressure grew too high. Mopar realized three issues with the design by the early 60s:

  1. If the oil filter element clogged, there were major issues with either lack of oil maintenance or fragmenting parts that would likely destroy engine components regardless of bypassing the filter.
  2. The new spin-on filter design had a higher bypass pressure rating than the block’s bypass valve, so the block bypass would trip before the spin-on filter causing unfiltered oil to circulate through the engine even though the spin-on filter element was not clogged.
  3. The block bypass valves could corrode and come apart, sending debris through the oiling system while also bypassing the filter permanently. Once the spring or ball valve came apart, the oil filter would be permanently bypassed.

To address the issues, Mopar began plugging the passage with a cast plug in blocks already drilled and eventually stopped drilling the passage completely through to where no plug was needed. Unfortunately for many 1956 – 1961 A-block owners, many have their oil filters bypassed due to the corroded valve. For example, the last 1957 – 1961 poly 318 engines I tore down that had the bypass valve installed were missing the spring and ball where the filter was completely bypassed. 1956 – 1962 engines running both earlier cartridge and later spin-on filters should always have the internal bypass valve inspected and if present removed and the passage plugged.

Removal and Plugging the A-block Valve

Ideally, the bypass valve should be removed and plugged with the engine disassembled during a rebuild or overhaul so that all the oil galleries can be washed out with a brush and blown out well with compressed air to remove all metal shavings. However, the removal and plugging can happen with the engine in the vehicle, but there is a risk that shavings will remain downstream the filter and could be flushed into the system and damage components. To confirm if the engine has and to remove the bypass valve, you must drop the oil pan, remove the oil pump, and remove the rear main cap. After confirming the engine has an oil bypass valve or the remaining housing still in place, remove the oil pressure switch or pipe plug from the top of the engine, insert a length of 1/4″ steel rod or all-thread as a drift, and hammer out the bypass valve housing. I have encountered some valve housings that refuse to be driven out, and they require drilling out from the bottom. If doing so with the crank installed, an extra-long drill bit will keep the drill motor away from the crankshaft journal. Once the valve housing is drilled out, remove all debris and oil residue from the gallery walls. So long as no debris is higher than the new plug, any remaining debris will flush into the oil filter and be captured.

I know of two methods to plug the passage:

Soft Plug: The bore is typically 0.38″ diameter but should be confirmed before sourcing a soft plug. Use a deep-cup plug since dish and shallow-cup plugs will try to flip on their axis in the hole. Apply a liberal coat of red Loctite to the outside of the plug. Using a drift or long bolt, drive the plug into the block until there is 1-5/8″ – 1-3/4″ distance between the plug and the main cap pad. Check that the plug is driven in above the filter intake passage so oil can flow unabated from the pump to the filter (Figure 7). Do not drive the plug in too deep because it can block the filter outlet passage located above it.

Threaded Plug (Figure 7): 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 torquing it to 35 ft.lbs. against the lip where the threads stop. Make sure to tap far enough above the filter intake passage to where when the plug is in place, oil can still flow unabated from the pump to the filter. The issue with this method is that the tap and/or extension must be long enough to reach far enough into the bore, and tapping hard cast iron can be difficult.

Aftermarket bypass plug (Figure 8): The only available bypass plugs carried by Speedway Motors (910-11048), Donovan (2600-0190), and Hot Heads Research (20120) are made for Gen I Hemis and Hemi-block polys and are too large (0.552” x 1.650”) for A-blocks. No similar plug exists for 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
Poly A-block bypass valve
Figure 4: A-block Bypass Valve (photo of a 1958 A318 with the check ball missing but valve body intact)
hemi block bypass valve
Figure 5: Oil Bypass Valve (schematic for a Hemi block but similar to A-block)
Poly A-block Factory Plugged Oil Gallery
Figure 6: Poly A-block Factory Plugged Oil Gallery (1963 block)
Poly A-block Oil Bypass Removal Plug
Figure 7: Soft or Threaded Bypass (drawing by J. Lotspeich)
Aftermarket hemi block bypass plug
Figure 8: Hemi Aftermarket Plug (will not fit A-blocks)

Spin-on Filter Adapters

LA 273, 318, 340, 360 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 throughout the engine.

For Factory Flow Rates:

Use a factory style LA oil-filter adapter and stud with a Felpro 70522 fiber gasket to seal to the block. Do not use rubber gaskets since they are prone to squeezing out and leaking. Tacking the gasket in place with a few dabs of grease helps during installation. Tighten the bolt to 25 ft.lbs.

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

For High-flow Rates:

Use the factory style adapter plate but drill additional holes for greater volume. See my technical article on other performance oil system modifications.

Poly A-block High Volume Oil Filter Adapter
Poly A-block High Volume Oil Filter Adapter

Another option is to use 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 22-575
Crown Auto CWA-53007563AB Mopar oil filter adapter stud
Crown Automotive Stud CWA-53007563AB