Big end wear?

[Arbian]

Stripping a badly neglected 125 in the hope of recovering it but after managing to free a seized piston and getting the barrel off I found the crankcase had a lot of rusty crud inside. There is some damage around the inside joint of the cases which I can only assume was done by the con rod end cap screws scraping against them although they don't seem to get that close. There seems to be a lot of play in the big end and my question is how much is too much?

[Charles uk]

The design clearance on all Seagull big ends is 1.5 to 2.5 thousands on an inch on the diameter, so on a 170/125 the big end crank journal should measure 20.00 mm
& the conrod big end bearing ID should measure no more than 20.075 mm when new.

After several years of running the aluminium big end cap can show significant signs of ovality, so if at any point the ID measures much more than 20.15 mm it's on it's last legs, though might run for quite a bit longer, when they "let go" they often do it "big time"!

The best way to measure this clearance is "Plastigauge" which should be available from most serious motor reconditioner suppliers.

Where on the planet are you?

[Arbian]

Thanks Charles. I am in Perth Scotland. Given that plastigauge is £29 a pack and I am pretty sure there must be more clearance than you state (as there is significant sideways 'wiggle') should I just replace the conrod and cap (if still available) or in such a circumstance is it necessary to have the crank split and new big end bearing fitted (in which case I am wondering if this project is still viable ..... I guess I also need to establish what other woes await me .... the whole motor was in poor shape)

[Charles uk]

I won't be driving past your place, so there's no chance of having a look at it, 500 miles is quite a drive, call me selfish if you like.

Seagull haven't had 170 rods in stock for more than 10 years, as they didn't have a great life expectancy, it's a one piece crank & a two piece rod & we haven't found anything else that will fit.

20 mm big end, 13 mm little end, 76 mm centre to centre.

We did have some new blanks cast for racing with bronze end caps, but after all the machining they ended up costing £120, stupid money, so only racing fundamentalists would spend that much!

There was a crank & rod on ebay last week, I haven't seen it, so can't comment on the condition. If you've got any toolmaker friends, I've still got some cast blanks & might still have some new big end screws.

[Arbian]

Afraid I don't have any tool making friends Charles but thanks for the offer. I have since also found that the flywheel is locked solid on the crank and several days of release oil has made no difference .... think this one may have to go on the back burner!