This dates back to 2017 but the true problem only came to light when the 650 engine was being rebuilt after the more recent Bad Thing that happened to the never-to-taken-apart-again VH engine (Part 2)
At the time (2017) I'd invested a lot of time and money building a 650 engine with all the 'best practice' that the internet could come up with. I managed 845 miles before I noticed the oil return getting a bit glittery. Investigations found metal in the sump filter and the bottom of the timing chest, and thereafter cam lobes and followers (all SRM's best) that looked like they'd been running in gravel and every bush and journal looking like it had been running in sand (or worse).
Hows this for 845 miles of careful running-in?
- Big-end shells
The reason for the damage was a mystery for a long time. I even sent the oil away for analysis and that didn't reveal anything. However, I've been working on reclaiming the FH engine but when the time came to put it back together again (after Bad Thing 2) I noticed that its hardly-used and supposed-to-be-polished-all-over billet con-rods were a bit marked up. 'Light bulb' moment!
- Shaved con-rod
- Doubly shaved con-rod
These are Thunder billet rods and are quite a bit wider in section than standard Ariel/BSA rods. The barrels on these engines have cut-aways to allow clearance for the rods, wider rods need the cut-outs to be enlarged. Clearly there was a problem with the enlarging (which was done by people with a lot of expertise in these engines), and the con-rods had been 'shaving' against the barrel skirts. The camshaft runs in a trough across the crankcases directly behind the barrels, it would probably get showered with shavings of hard alloy directly, which would explain why some things looked like they'd been running in gravel.
There are still some questions, like why this problem wasn't noticeable sooner and why the oil analysis didn't find any metal in the oil (although the engine had already had an oil change at about 500 miles) but this does seem to be a very likely factor in the damage.
SG
