I was looking forward to starting my engine build this past weekend. I then thought it best to check everything leading up to assembly one last time. I’ve invested so much energy and money into this project, I can’t bear the thought of a KERBLAMMO because I made a silly mistake. I’m doing this by the book (Harry Pellow that is), and he spends several pages advising to check the running clearances.
For example: Rod bore diameter – crank journal diameter – (2 x bearing thickness) = Connecting rod running clearance
I spent a better part of the weekend measuring the crank, connecting rod bore, and bearings. First with digital calipers which did not result in very consistent results. Cheap tool, that only went to hundredths of a mm. So I started over and did it a second time with calibrated Vernier micrometer. Which forced me to learn a new skill via YouTube. Documented in mind-numbing detail in a spreadsheet.
In both instances I ended up with negative numbers when I calculated the running clearances. Which as I understand it, means the components would be touching when assembled. That can’t be right.
I’ll be the first to admit that the most likely culprit is my inexperience, combined with tools that may not be ideal for the job. My connecting rod bearing measurements for example come out higher than the First Under range.
So off back to my trusted machinist with parts in hand to request measurements for said calculations. He confirmed what I suspected, in that I did not have the correct tools to perform the measurements. Vernier micrometer was fine for the crank journals, but neither that nor calipers would work for the curved inner diameter (ID) of a bearing, connecting rod, or case bore.
While respectful of my attention to detail, and patient with my questions, he did not quite share my perceived level of importance in calculating the running clearance. My machinist’s reasoning includes:
- He doesn’t calculate running tolerance via prescribed formula, and doesn’t suspect most professionals do either. They are measuring and reworking the components to a spec, and selecting appropriate bearings.
- Variance in the production of bearings is basically a non-issue. They are far more precise than the measuring tools most people have available to check them. They would not be in business very long if they couldn’t make precise bearings.
- Not that impressed with Mr. Pellow. He wrote a book. He made a name for himself. It doesn’t mean that The Maestro’s way is necessarily the best way, and certainly not the only way.
He did at the end of the day offer to perform the requested measurements for my records and peace of mind. With a bit of reservation, I ultimately decided to let it go. My machinist has been the source of most of my parts (i.e. crank which was within standard tolerance as measured by him) and all of my machine work (connecting rods and align bore for the case).
I guess the leason learned is to pick a great machinist. If you can’t trust his measurement, work, and ultimately judgement, you are going to have problems no matter what.
I think I’ve officially run out of excuses to start assembly.
