XS 400 Seca restoration

[Gaston]

Hi all!

I've started to restore an XS400 Seca (1982) to it's former glory after being out of traffic since 1995. When I disassembled the engine the pistons looked like the attached photos. Since I haven't done any engine work before I don't know if the pistons need replacing or not? The bottom gasket was broken which I think explains the amount of soot on the top.

Could someone please advice?

Kind regards!

/Gaston

 

[BBS360]

Welcome to the forum!

Bottom gasket shouldn't have anything to do with it. They're a bit sooty but don't look bad at all. How much carbon is deposited on the pistons has to do with the fuel/air mixture and quality of combustion.

Unless there's visible damage how they look is totally irrelevant.

How many miles on the odometer?
Were you able to do a compression test before disassembly?
Get some feeler gauges and check all piston/cylinder/ring clearances listed in the manual.
That will tell you piston condition.
Getting the cylinders checked with bore gauges will give you the complete story.

There's a Seca manual posted in the Garage, thread "Yamaha XS400 Manuals".

 

[Gaston]

Thanks BBS360!

Unfortunately I didn't do a compression test before disassembly.
The bike hasn't moved since 1995 but have approx. 2300 km on the dial which should correspond to about 1430 miles...

I will do as suggested and check the clearances.

Thanks once again!

 

[BBS360]

Pretty low miles. I'd expect the measurements to be right where they should be unless there was any corrosion damage from sitting so long.

Double check everything since you have the engine apart but I'd bet you should be able to give everything a light cleaning, reassemble, and be good to go.
An acetone scrub, Pin-Sol, or something similar should work well to clean up the dirty pistons. Just don't use anything that could damage the aluminum.

Check the rubber seals too. Might have hardened up with age.

 

[Gaston]

Sorry! I was a bit too fast with the numbers. Should be 23 000 km which corresponds to 14290 miles

 

[BBS360]

The pistons in both my bikes looked a lot worse when I got them.
My SJ is only at 11,000 km and my 360 is also at around 23,000km.

Hope everything works out well.

 

[16VGTIDave]

Those pistons appear to have lots of oil deposits on them. My guess would be that the rings were never seated correctly and that the engine had a significant amount of blow-by as well as low compression.