One of the most common questions is “what about oil filtration?” The 235 had never had “full pressure” oil filtration, and bypass (cannister) filters were never standard equipment, although by the late fifties, most trucks and passenger cars were equipped with the bypass filter option. Even the first 265 V8 came with only a bypass filter. Not until 1958 did Chevy equip the 261 heavy truck motor with a modern full flow spin-on oil filter. GMC’s, likewise, never had full flow remote filters, even though they were of “full pressure” insert bearing design since 1939.


We can only guess why it took so long for Chevy to adopt “full-flow”. Oil technology was still fairly primitive in the ‘50s, although Chrysler offered “full-flow” remote filtration in the late 1940’s. For generations accustomed to easy “oil and filter” changes on more modern cars, it seems antiquated to lack this feature on the 235. And the bypass filter is not much of a solution either! This method of lubrication diverts a small amount of oil (and oil pressure) to a cannister filter and returns it back to the pan, where it immediately mixes in with the dirty, unfiltered oil! The cannister elements are messy to change, sometimes expensive and hard to find.


Inliners have been modifying their GMC’s and 235’s for full-flow oil filtration for decades. The modification is straightforward and can be done with common hand and power tools (although you may feel more comfortable trusting your machinist). BUT! BEWARE! - by drilling into the block, chips and filings will be introduced into the lubrication passages that can destroy bearings after only a few minutes of operation! So it is critical that these passages be flushed out, thoroughly cleaned and blown dry with compressed air. This is best accomplished on a “BARE BLOCK” before installing cam bearings and crankshaft, with all oil galley plugs removed for access to the main oil galley.

            Dick Larrowe, longtime Inliner and founder of Stovebolt Engine Co., has prepared the following description of how to modify a 235/261 block for “full-flow” oil filtration. Our thanks to Dick for this great writeup, also to our Inliners tech advisors Tom Langdon and K. Pat Smith .  

For the GMC version, thanks to club founder "12 Port Charlie" Baker who did the original back in May of 1981. Charlie's description is derived from "California Bill" Fisher's 1955 GMC Speed Manual (Otherwise known as The Little Yellow Book or the "GMC Bible"). The reference to the Chrysler filter is from this document, and for all practical purposes, can be disregarded today. Any number of remote filter heads can be found at off-road suppliers, speed shops and wrecking yards.

                                                                                        - Jack Halton