Makita DCS520 - low compression / bad top end?

[tom050162]

So,

My Makita dcs520 has been a great saw up until the last few uses where i noticed it started smoking quite a bit on startup, it's been down on power and I've found it to be bogging down a lot when getting on the dogs.

a cold compression test returned 70-80PSI which was much lower than what I expected. So I decided to have a look through the exhaust port and it looked a bit scored, I took the top end off and the cylinder head and found some scoring and what looks like pitting on the jug and piston.

What's your guys opinions, I'm think the cylinder, piston and rings all need replacing? Parts a dificult to get hold of but seen a few kits on ebay for around £40-60.

Opinions very welcome as I've only ever dealt with 4-stoke engines before so first stroker rebuild.

 

[9050LX]

How many miles on it? Can you get OEM rings?

 

[tom050162]

How many miles on it? Can you get OEM rings?

Its only had relatively light use from myself, but I only picked it up last year. Seemed pretty poorly looked after when i first got it had a .325 chain on a 3/8 bar and drive sprocket so I don't think the previous owner was too knowledgeable about saws...

New rings are about £15 so don't wanna start throwing money at parts if the cylinder is scrap from all the pitting marks.

 

[9050LX]

I would run it. See what others say.

 

[SteveSr]

Its only had relatively light use from myself, but I only picked it up last year. Seemed pretty poorly looked after when i first got it had a .325 chain on a 3/8 bar and drive sprocket so I don't think the previous owner was too knowledgeable about saws...

New rings are about £15 so don't wanna start throwing money at parts if the cylinder is scrap from all the pitting marks.

Looks like high mileage / crap air filter. What is the ring end gap above the exhaust port?

 

[tom050162]

I put the ring back into the cylinder and measures the ring end gap when square, it seemed very large but I'm not sure if I'm measuring wrong? It too two feeler gauges to start dragging between the ring (0.63mm and 0.60mm) which combines as 1.23mm ring end.

I believe this converts to around 48 thous?

Not sure but I might be measuring it wrong?

 

[SteveSr]

I put the ring back into the cylinder and measures the ring end gap when square, it seemed very large but I'm not sure if I'm measuring wrong? It too two feeler gauges to start dragging between the ring (0.63mm and 0.60mm) which combines as 1.23mm ring end.

I beleive this converts to around 48 thous?

.048" is large and likely the cause of somewhat low compression. What is the bore diameter of the cylinder? Others should reply on acceptable gap/bore dia. clearance

However, your gauge is suspect because the engine would not start/run at 80psi. Gauge MUST have a schrader valve at the tip that fits into the spark plug hole. An automotive tester will NOT be accurate.

Not sure but I might be measuring it wrong?

Probably not. You can use the piston to square up the ring in the bore.

Next measure the piston skirt/bore clearance at the bottom (unworn) part of the cylinder. If the clearance is too large you will need a new piston.

The pits in the plating are not terribly unusual. What matters is the area where the ring travels and above the top of the exhaust port where compression is made.

I would recondition the cylinder with some red scotchbrite on a mandrel and re-inspect. Be sure and clean the cylinder THOROUGHLY with hot water and dish soap to remove any leftover abrasive from the scotchbrite.

https://www.arboristsite.com/thread...um-transfer-without-acid.248325/#post-4595785

Dust and dirt is what caused all of the wear on the piston. ring, and cylinder. You need to figure this out and fix or you'll just ruin any new parts that you throw at it in short order. You may also have an air leak which bypasses the filter although this will usually result in lean seize scoring much worse than you have.

 

[tom050162]

Thanks for the reccomendation steve, i watched a couple tinman videos too and given it a go cleaning it up using the emery cloth and some scotch-brite pads. Looks a lot better to me. Still a few marks but most of the transfer is gone and i cant feel any of the with a nail or metal pick.

Measured the gap between front and rear of piston skirt and cylinder to be 0.10mm (.004), the piston looks relatively good and same thickness around the base.

I think the stock filter might be to blame as I found some small particles on the inside of the screen, any other option to stop this happening? I couldn't find any perforations or cracks in the filter, clean more regularly maybe? I always blow it out after each use anyway...

What do you guys think, fit a new caber ring and run it?

 

[Lightning Performance]

Your ring was toast.
Slap one in there and run it.

 

[tom050162]

Your ring was toast.
Slap one in there and run it.

Thanks a bunch for the help guys!

Looking back on the compression test I did was using a standard automotive gauge with flexi hose which returned 80psi, possibly it wasn't too bad after all. However, I'll repeat with a proper type schrader when I get the new ring fitted.

 

[Lightning Performance]

Thanks a bunch for the help guys!

Looking back on the compression test I did was using a standard automotive gauge with flexi hose which returned 80psi, possibly it wasn't too bad after all. However, I'll repeat with a proper type schrader when I get the new ring fitted.

I don't even bother with compression gauges anymore.

Pro tip:
If it hurts when your finger is blown off the hole it has plenty of compression. If it turns over when picked up by the starter rope it does not have good compression. If it never moved it needs help

 

[SteveSr]

Thanks for the reccomendation steve, i watched a couple tinman videos too and given it a go cleaning it up using the emery cloth and some scotch-brite pads. Looks a lot better to me. Still a few marks but most of the transfer is gone and i cant feel any of the with a nail or metal pick.

I didn't really see any transfer in the photos. What you have is wear/damage caused by dirt ingestion.

Measured the gap between front and rear of piston skirt and cylinder to be 0.10mm (.004), the piston looks relatively good and same thickness around the base.

.004" is probably about 2x what it should be new. Yes, it will run like this but the piston slap will rob power and accelerate skirt wear. Time to start looking for a spare top-end.

I think the stock filter might be to blame as I found some small particles on the inside of the screen, any other option to stop this happening? I couldn't find any perforations or cracks in the filter, clean more regularly maybe? I always blow it out after each use anyway...

That is a mesh filter and as you have found out will let all kinds of fines through and cause wear/damage. Stihl quit using these decades ago. Is this AF aftermarket or OEM? In any case I would replace it with a brand new OEM AF.

 

[tom050162]

I didn't really see any transfer in the photos. What you have is wear/damage caused by dirt ingestion.


.004" is probably about 2x what it should be new. Yes, it will run like this but the piston slap will rob power and accelerate skirt wear. Time to start looking for a spare top-end.


That is a mesh filter and as you have found out will let all kinds of fines through and cause wear/damage. Stihl quit using these decades ago. Is this AF aftermarket or OEM? In any case I would replace it with a brand new OEM AF.

Struggling to find anything other than a Chinese aftermarket top end thats sold on ebay. After reading reviews it seems they're rather hit and miss so I'm somewhat reluctant to fit one in case it ceases up and knackers the bottom end. I'll see how it performs when the new ring turns up, it's a caber so should hopefully help a bunch!

As for the piston to cylinder gap i think I measured it wrong as I was using feeler gauges along one side of the skirt rather than actually measuring different between piston diameter and bore diameter. I'll see if I can get a micrometer and bore dial and re-measure correctly.

In terms of the air filter I'm fairly certain it's an oem makita/dolmar filter, however, again I've struggled sourcing a replacement but I'll keep looking.

 

[9050LX]

Bet it runs fine with new rings. Learn how to tune it.

 

[tom050162]

Bet it runs fine with new rings. Learn how to tune it.

Watched quite a few videos from Buckin and tinman on tuning it by ear so I've got s fairly good idea in terms of making sure it's running fat enough not to overheat it. Hopefully the new ring brings some power back. It never used to struggle pulling an 18" bar with a 3/8 full house chisel chain but last few uses its been bogging in the cut. We shall see. Maybe I just need go buy a bigger capacity saw

 

[SteveSr]

Struggling to find anything other than a Chinese aftermarket top end thats sold on ebay. After reading reviews it seems they're rather hit and miss so I'm somewhat reluctant to fit one in case it ceases up and knackers the bottom end. I'll see how it performs when the new ring turns up, it's a caber so should hopefully help a bunch!

You can also set a search in Ebay and wait for a good used P&C to appear.

As for the piston to cylinder gap i think I measured it wrong as I was using feeler gauges along one side of the skirt rather than actually measuring different between piston diameter and bore diameter. I'll see if I can get a micrometer and bore dial and re-measure correctly.

Feeler gauges should be thin enough to bend to the curvature of the cylinder. It's not perfect but should get you close.

In terms of the air filter I'm fairly certain it's an oem makita/dolmar filter, however, again I've struggled sourcing a replacement but I'll keep looking.

Get out your bright lights and magnifying glasses and see if you can find a Makita logo or part number on it anywhere. If the filter can be split down the middle (like some Stihls) the marking may be on the inside. If you cannot find any relevant marks the filter is likely AM junk.

 

[tom050162]

When measuring piston to cylinder clearance should the measurement be halved if using feeler gauges to account for the fact the measurement is one sided and the piston will be up against the cylinder wall on the opposite side to what is being measured? I assume this is correct as using a dial indicator and micrometer the measurement is taking into account the diameter and difference between the two measurements across the diameter of the piston, rather than the total gap?

Just a thought.

 

[tom050162]

So I've been a bit stupid... turns out the caber ring I ordered for the Makita was in fact far too big for the piston. After some measuring and googling it turns out the saw isn't actually a dcs520 as i originally though and seems to be a dcs430 (43cc 40mm piston rather than 52cc 44mm piston).

My next question is, will a dcs520 (dolmar 111) top end fit on the 430? I'm sure I've read about them being interchangeable somewhere but I'm unable to locate it. Any advice would be much appreciated, seems like a decent upgrade from the 43cc to 52cc displacement.