What's on your bench today?

[spudulike]

Bermuda I reckon!

[peatff]

10 hours ago, htb said:

Near flyingdales or similar?

Near Hallowe'en

[osku]

HVA 540 had really bad oiling for the bar. I took saw a little bit apart and found this. Oil pump intake and output tube was splitted in half. Having exactly same issue with second 540 so it's next on the line.

[spudulike]

Had a MS461 in from a long standing local customer, it felt like the crank had failed or there was a big piston issue. Took a look down the plug hole with the customer there and the crown of the piston looked cracked to me

Had the machine apart and either the strato flange had busted shattering the piston or the piston skirt had shattered taking out the flange but it was a BIG mess. The screws were still in there and tight so the flange hadn't come loose.

All the debris was cleared, the bore looked OK, no indentations in the squish band or ports and the crank ends were straight, the big end looked OK and the rod was incredibly straight.

Fitted a new piston and flange and the saw now runs, am hoping there are no hidden nasty surprises that time will unleash but a risk worth taking.

 

[Ratman]

Whats your assumption spud? A malformed / badly cast piston? Very unusual occurrence, but in in the field we’re in, obviously the unusual is always possible?

[adw]

Possible fatigue cracks from engine over revving, pretty sure i can see a crack appearing above the gugeon pin boss, just a thought.

[Ratman]

Surely over revving of the engine (if the cause of piston failure) would of left evidence in the cylinder bore?

[spudulike]

The saw had been serviced by me about a year ago and had tached it, found it to be over revving, pulled the limiter, backed it down and then re-tached it at around 12500. It than came back as the owners employee had not put the clutch e clip back on correctly and the clutch had grenaded. It was tached again and OK and after repair, even with the bar off, it didn't over rev so know it wasn't down to that.

This work was months ago, one of the lower window supports has a crack in it and the crown has a fine crack running up the shattered skirt and across the crown that looks slightly older but with no signs of impact or poor piston stop use

The piston was in good shape and the machine is also. I reckon the skirt shattered and it took out the strato flange - I had always wondered what damage would occur if this let loose - I know now!

It may be a casting flaw, one of the lower skirt supports has a very slight honeycomb look about it and it is possible the piston just cracked - not a nice one but hope there are no little hidden surprises - worth a punt on a relatively cheap repair on a very decent ground saw.

 

[bmp01]

Pistons are forged (not cast).

You need to look for the classic signs of fatigue failure to identify where the failure started - if you can be bothered. Its where a crack exists while the part are deflecting and rubbing across the crack surface,  causing a distinctive pattern, often crescent shaped. This happens while the engine is running and can be hundreds of thousands of engine cycles. Bits that shatter during the last few hundred revs don't have that appearance. Trouble is most of the evidence is often destroyed by subsequent impact damage.

 

What's a strato flange and what does it bolt to ? Barrel? 

[billpierce]

My beloved 361 started playing up today. Kept pulling the starter out my hand and is behaving like an 064. Starting has really hard and seems to fire early and RIP the starter out your hands. Did manage to start it but didn't feel like it was revving up proper job.

Piston is well. There is a slight playing in the flywheel woodchuck. Perhaps it has slipped slightly. Tried adjusting it and was easy to pull for a few pulls. Then back to tough.

Disappointingly it seems the woodchuck key is forged into the flywheel rather than a replaceable part. So there goes 80quid.

Anyone got any advice or anything to check before I shell out?

Turning the flywheel the bearing did feel a bit lumpy like a 50p rather than super smooth, again dont know if that's super bad?