DIY chainsaw repair

[adowning7]

I bought a Makita dcs7901 of a mate for £100 a while back and it died a couple of weeks ago. I have stripped it down and found a chunk missing out of the piston.

 

I have seen new pot and piston kits on ebay for £50 and I am contemplating having a go at fitting it myself as it is a saw I only use probably once a month.

 

As a novice am I getting ideas above my station?

 

Be as honest as you like!

 

Thanks

 

Adam

[WorcsWuss]

The most complicated bit of this job is compressing the rings and getting the piston back inside the pot.

I've got a home made wooden block to sit the piston on [inch thick piece of ply with a slot cut out to slip round the conrod] and I use the plastic ring compressors that are under 2 quid from Stihl....

You'll need a few other bits, bottom gasket, carb base gaskets, threadlock and the right tools, probably a drift to get the gudgeon pin out etc.... but all these things are pretty cheap...

So long as you have somewhere clean and tidy to work, have a clean and tidy mechanically minded brain and a parts book & workshop manual to make sure you get little things right, like the piston rings the right way up, then it's just Meccano.... And it's very satisfying when they fire up....

That said, I've only ever worked on Stihls.....

[adowning7]

Thanks for the reply; I can only find user manuals with a parts breakdown do you think this will do?

 

I have ordered the new pot and piston so I will let you know how I get on - might not be before christmas!

[WorcsWuss]

Is it this one....? http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=7&ved=0CE8QFjAG&url=http%3A%2F%2Fwww.makita.com%2Fen-us%2Fassets%2Fimages%2Fproducts%2FDCS7901%2Fparts_breakdowns%2FDCS7901-1-10.pdf&ei=N3ajUPHACaXb0QXG0oCgAw&usg=AFQjCNEuZK7hI9X3Wwqn3VQcJrHKbm9t8w&sig2=wHEb-rgT8WFH9l2_PUy45A

 

That will show you what goes where, but you'll still need to take care over orientation of things like your base gasket, piston, rings etc, which are only really covered in the workshop manual....

 

A way round this is to inspect everything very closely as it comes apart and photograph it all in detail.

For instance, the piston will likely have a little punch mark on the edge of the ring channel. This kink is to stop the rings rotating round so the joints line up. The rings will likely be chamfered at the split to create a vee which the dimple sits into. If the rings don't sit correctly, they'll protrude from the piston when you're reassembling and you could snap the top one when attempting to fit the pot...

The new piston should also have an arrow stamped into it, this probably needs to point at the exhaust port.....

 

They're simple enough, but some things are right or wrong and need to be be got right!

[adowning7]

great tip about the arrow on the top of the piston, it can be seen on the picture CYLINDER & PISTON FITS MAKITA DCS6401 DCS7301 & DCS7901 | eBay

 

starting to doubt myself as I would have probably not have given it a thought!

 

That is the only manual I can find so it will have to do. I think I will go down the route of photographing everything and putting aside a good few hours so I might stand a chance of remembering where everything came from!

 

thanks for the advice

[WorcsWuss]

Like murder, the first time is always the hardest.... or so I hear....

 

Take your time, work on a decent large clear surface, under good light [i have an LED magnetic inspection lamp as well which sticks to my vice an illuminates from the side / below which is helpful] and double check everything, twice.

Don't overtighten into threads cut into aluminium castings and make sure everything lines up properly, depending on how the impulse for the carb is fed that can catch you out for instance...

There are some genuine bona fide experts on here who will give far better advice than I can, if in doubt, ask, that's what I do!

 

Oh, and some 'special tools' are invaluable yet surprisingly cheap, don't do what I did recently and try to get by without some of them and make a shameful balls up.....!

[Ray1]

You will also need to find out why the saw went pop in the first place, There is not much point fitting new parts for the same thing to happen 10 mins after its been fixed.

[spudulike]

I always find getting the circlips in more difficult than the rings IMO

[agg221]

I always find getting the circlips in more difficult than the rings IMO

 

I agree. The rings usually have a little step at the ends, which lines up with a peg in the groove. So long as you put the ring the right way up, lining the ends up with the peg, it's pretty simple. I find I can work the ring in gently just by holding the pot between my two hands, fingers pointing down towards the piston and compressing the ring with fingernails. I then check visually that the second ring lines up with the peg, and do the same again. I do tend to put a good wipe of 2-stroke oil over everything before I start, just to make it easier to slip in.

 

So long as you remember to put the gasket in first, have everything clean so no bits of sawdust fall off on it, and make sure the piston is the right way round, it's probably one of the simpler jobs.

 

Have you fished the lump of piston out of the bottom of the crankcase or the muffler? On the subject of the crankcase, while the pot is off, I find you can get a sheet of kitchen roll, hook one edge over the edge of the crank webs and then turn the crank using the flywheel to drag the paper through the case. This picks up the gronk from the the bottom of the case quite well - particularly any metal particles.

 

Alec

[Rich2484]

I always find getting the circlips in more difficult than the rings IMO

 

Too true. Especially when they spring out from the pliers and drop into the crankcase

 

Then you have to try and turn the saw upside down without trying to drop the cylinder off and have to reset it all.

 

The best advise I can give anyone trying this would be to clear a big space on a bench and lay down a cover, either a dust sheet or old bed sheet. Take plenty of pictures so you can refer back to them as to the rebuild. And the most important thing is to take your time. Set aside a morning and if you have to go out them leave it until you come back. Nothing worse than trying to rush a delicate bit because you are about to go out.