2-71 Head Gasket Issues Experienced and Background Information
During the process of rebuilding and testing the engine I experience head gasket issues resulting in coolant leaks to oil and compression seal leakage into the cooling system. This article covers the background of the head gasket design and the issues relating to the engine build that caused the problem.
Background: Head gasket upgrades were made to Series 71 engines, in the early years of production. I do not know the exact year of the change, but a service manual for 6-71s indicates the transition serial number for the 6-71 was 6A16283. Considering the number of 6-71’s built in WWII (http://usautoindustryworldwartwo.com/General%20Motors/detroit-diesel.htm ) and that 6A472101 was built in 1995, and 6A 90964 was built in 1960, it would appear the change was made in the early or mid 1940’s. A 1944 Gray Marine Manual I have still only shows the original low block design. All series 71’s were upgraded with the exception of the 2-71, which retained the original 1938 “low block” design until it went out of production in 1987. The new high block design used individual compression gaskets and “o-ring” seals for oil and coolant versus the original 2 piece body gasket. The result of the new design was that much more of the head bolt loading went to the compression gaskets, or at least in a more controlled manner. I assume the changes were made to facilitate the use of larger injectors, power ratings and or turbocharging. For example 90 cubic mm injectors were used in high output 6-71s in WWII. All 2-71’s had 60 or 70 cubic mm through out the production period. It was the only engine in the series (other than the 1-71) not to be eventually turbocharged or fitted with the higher compression ratio 18.7:1 pistons in non turbo configurations.
My failure analysis shows my original J-B weld repair of the erosion around the water holes in the block did not hold up well. It is also likely the higher cylinder pressures from the 18.7 compression ratio pistons and the added boost (2 inches Hg) due to the coated blower, in combination with larger output injectors caused higher cylinder pressure resulting in the compression gasket leakage (bubbles in the radiator). I also later discovered the aftermarket head gasket I used was roughly .015 inch thinner and only had 4 vs 5 steel layers. The effect of this would have been to raise the nominal compression ration to roughly 19.6:1 further increasing cylinder pressures! This was only identified after removing the second gasket and comparing it to the 2nd and 3rd. More on this follows.
My initial fix pursued was machining steel inserts for the block to fix the erosion and changing to 17:1 turbo pistons and rings to decrease cylinder pressure. Pictures showing the modifications to the eroded water holes appear later in the picture story. The liners were shimmed to near .006 inch protrusion to increase the load on the compression seal. While the engine runs good with the 17:1 pistons, I decided to go back to the 18:7:1 compression ratio and perhaps limit the injector size and or retard injection timing, limiting the maximum rpm and maybe increasing head bolt load to deal with cylinder pressures. My reason for changing back is the startability of the engine has been greatly diminished and the start up white smoke is much worse.
Previously the engine had almost instant starts with virtually no white smoke even at around 32 deg F. With 17:1 pistons it white smokes for a significant period of time after starting even at above 70 deg F. This morning on a 42 degree cold start I need to use ether and the start and clean-up was not impressive. With the white smoke (unburned fuel) it is a nuisance when starting inside my garage/shop. I’ll publish more on this in the future.
In studying injector combinations I was able to obtain information on the injector P&B (Plunger and Bushing) assemblies I had experimented with 5228658 (N75) and 5228661 (N80). N-80 has 4% higher effective stroke or theoretical output than the N-75, which I previously understood, however I found the N-80 has approximately 3.6 degrees of additional timing advance build in! At 1.460 injector timing height, the theoretical beginning of injection would have been at approximately 20 degrees before TDC, which is quite advanced. One new possibility is that my brief experimentation with the N-80 may have contributed to higher cylinder pressures and the head gasket compression seal leakage!
Since rebuilding with new 18.7:1 pistons and using the thicker OEM head gasket, the startability is restored and there is clean-up of the white smoke within seconds or starting. There is a small amount of white smoke for a few seconds not previously there with the first build. I contribute this to the first build probably being closer to 19:6 :1 due to the thinner head gasket. I also sense a bit more black smoke when highly loaded. The lower smoke with the thinner gasket might be due to the lower piston to head clearance, which give more squish velocity and a higher percentage of air in the bowl at TDC (higher k-factor). The higher cylinder pressure density would also have reduced the spray penetration, perhaps resulting in less spray impingement on the piston and more optimum combustion.
I saw some suspicious wear or coloration near the top of the liners, initially I thought this may have been due to the coolant in the oil and or in combination with the higher cylinder pressures and or insufficient lubrication. Detroit diesel used different oil ring packages, typically high tension oil rings for injectors below 60 mm3, reduced tension oil rings for non turbo engines above 60 mm3 and a single lower oil scraper in the lower groove for turbo engines with larger injectors. To be most robust relative to lubrication, I used the turbo oil rings in my 2nd and 3rd builds due to the high output injectors I’m running with the increased boost. Upon tear down of the 17:1 build, the wear marks and coloration appeared similar. My conclusion is it may have been normal as the fire-ring had not yet fully seated. For both the 2nd and 3rd builds I did a slight re-hone and the areas cleaned up quickly.