170 Postmortem

[andrew]

Finally opened up a 170 I've had a few years. When I got it, the flywheel was loose but would only turn a few degrees in either direction, so I expected it had gone the usual way of conrod/big end failure. At first this seemed to be the case. But this motor has obviously seen a long service life, so it is one of the lucky few that for some reason were stronger than the rest. Can tell it was painted at least twice, and motor was in otherwise good condition, so it seems that it was frequently maintained as well.

Which made me take a second look. Then I realised that the last time it was reassembled, there was no wire put through big end screws to keep them from rattling loose. I think that might have done it! Sad way for an otherwise good example to go.

[Charles uk]

A very informative picture, thank you Andrew!

Were you the first to open up, as there looks to be a mark on the rod where the locking wire has marked the heat blueing, but it's a job to see.

If you get the rod off the crank we would love to see pictures of the damage done.

Perhaps readers will now understand why we say "strip before you start if you have the merest hint of a doubt, with regard to the 160, 170, 125 & 5R"!

[Charles uk]

Here's a picture I was sent today in response to your post, this one went bang!

[andrew]

You were right about the locking wire marks, they're clear in several spots. Guess it didn't last long at all without it!

Here can see the slight bend at the base of the rod:

threads stripped out of conrod:

and big end cap bent out a bit

[Charles uk]

Andrew
Here are pictures of some 170 rods I have in my rod drawer.


Rod A is a rod I removed from a running 170 that I thought I ought to check, no noise no play, but bent & showing major signs of heat distress with some slight signs of scaring on the bearing surfaces both sides, note there is a bronze shell in the top side of the bearing that yours seems to have devoured.
Rod A also has a fully machined aluminium end cap this might be what stopped it from self-destructing, I don't know where it came from, it's the only example I've seen.

Rod B is an unmachined bare casting, note the end cap is cast in situ to ease machining.

Rod C is the last known new/unused Seagull made 170 rod, No it's not for sale it's a pattern!

Rod D is the last of the redesigned rods I had made to increase the rods life expectancy & it's ability to to do it's job in racing 170's, with phosphor bronze on both sides of the big end journal, No it's not for sale it's a pattern!