I found doing a compression test was easiest when accessing the injector hole. I just made an adapter out of an old injector for my comp tester. Also, don't cheap out on any of the testing devices, I'd suggest getting one from sears at the minimum. I still have the Snapon gauge I bought over 10 years ago. Quality costs.

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Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

2001 BMW 740i E38 M62 (past)
1981 300SD w126 OM617 (past)