Top end kits: NOS vs repos?

[Mad Professor]

Start of a new thread.

I've two 038 supers I got for free (bad top ends....).

I plan on comparing one built with nos stihl parts the other with after market parts (where available of course, other "bad" parts might be nos/aftermarket too.....). I will compare such things as fit and finish, tolerances (mikied with machinist guages as delivered) weights (piston/ring/pin /clip assy, anyone consider balance?????), overall quality, and in the end PERFORMANCE.

I plan on keeping one and selling the other to offset cost.

I will refrain from naming the aftermarket companies for the sake of not getting sued...........

This might take a few weeks/months, will keep you all educated/updated!!!!


If you have input, please do, MANY here know more than me............... I've posted about this rebuilding of my free saws (and I thank those that have given me an insight),but I think this is the best way for those involved with rebuilding to have a handle/input.

I am planning on investing in a digital camera, if so pictures will be forthcoming, be patient!!!

Best regards, MP

 

[Lakeside53]

There are several different aftermarket kits - each with their own problems. I know of Italian, Indian, Taiwanese, and Chinese... Are you going to compare them all

I've only used the Italian, and it's good - Tecomec was an OEM supplier to Stihl back in the 038 days..

 

[Mad Professor]

There are several different aftermarket kits - each with their own problems. I know of Italian, Indian, Taiwanese, and Chinese... Are you going to compare them all

I've only used the Italian, and it's good - Tecomec was an OEM supplier to Stihl back in the 038 days..

Andy,

I''ve not enough saws to compare them all (kits)......You and others have been kind enough to recocmemd Tecomec.

As I Said, will check NOS and a repro for specs, for better or worse........

I've seen kits with one ring VS two ring for NOS. From my "racer" days I prefer two for a good long lasting seal.

I will report when I have clue

Best,

Tony

 

[Mad Professor]

O.K. I'm finally ready to start putting some stuff together. Have two 038S I'm converting to 038M.

For preliminary comparisons I've OEM 038S and 038M cyl assys and a Tecomec 038M assy. Sorry don't have a camera for pics at this point. The overall finish of the Tecomec compares well with the OEM Stihls, I'd say chroming of cyl is better on the Stihl but the ports are MUCH smoother on the Tecomec.

I did some preliminary measurements of all three.

None of the cyls had any detectable taper or out of roundness, all the pistons had 0.005-0.006 " of taper being thickest at the skirts. Piston pin dia and pin bore dia. were identical on all 3 assys. Piston assy mass varies quite a bit from 038S ( 94.5 g) to 038M (104.2 + 106.7g).

Note that the Tecomec and OEM 038M cyls differ by 0.001 ", if you mixed the pistons you would have one tight and one loose assy. The Stihl assys have 0.002 " piston/cyl clearance and the Tecomec 0.0025"

The latter should help people out when they ask "is my piston worn out....."

(See Word attachment for comparison table)

P.S. I'll also try to measure up/compare ports/timing for the 038M cyls and post measurements on getting the 038M cyls onto the 038S bottom ends. I do not foresee a problem with the conversion but will have to track down some MagII mufflers as the exhaust ports are much bigger on the Mag vs. Super.

Hope I can find a camera before the parts start getting assembled........

 

[bcorradi]

Great info tony...I'm suprised on how much the oem super vs mag assemblies differ in weight. Are both the super and mag oem kits new? Also one thing you didn't include in your doc, which I assume you will, is the bolt spacing comparison between the mag and super.

 

[Mad Professor]

Great info tony...I'm suprised on how much the oem super vs mag assemblies differ in weight. Are both the super and mag oem kits new? Also one thing you didn't include in your doc, which I assume you will, is the bolt spacing comparison between the mag and super.

Hi Brad,

Yes the mass was that much difference, on a chemistry scale at work.....+/- 0.0001 g didn't report it as that to be exact, some would say how could you do that measurement ........... Out of balance, maybe???? Manual gives same crank/flywheels for all 038s?

I/we, our machinist measured the clearances as best we could, +/- 0.0005". I do not have mics to do better. I stiill will check ring end gaps and figure out bolt pattern exactly.

Makes me wonder if BALANCE means a thing OR if people/manufacturers don't care....?

best regards,

Tony

 

[Mad Professor]

P.S. note CLEARANCE from piston/pin, you should not have to freeze that latter to make it fit/press it in. Even/if you "NEED" too, hopefully the rod/bearing/pin will have enough clearance, if not something is wrong.....and/or the corrupted bearing devils will come too see you soon.......

 

[Lakeside53]

ONone of the cyls had any detectable taper or out of roundness, all the pistons had 0.005-0.006 " of taper being thickest at the skirts.
..

When you say "taper", do you mean a linear taper or just the difference in measurement from the top to the bottom.

That would be a strange piston. 6 thou is a lot... It would mean (using your numbers) 8 thou piston to cyinder gap, or if a linear taper, 4 thou at the mid point...

Most pistons I measure are slightly smaller in diameter immediately above the top ring. The best place to measure the "top" is below the bottom ring and above the wrist pin.

Note that the Tecomec and OEM 038M cyls differ by 0.001 ", if you mixed the pistons you would have one tight and one loose assy. The Stihl assys have 0.002 " piston/cyl clearance and the Tecomec 0.0025"

..

What did you use for measurements. Did you mic it with a mic (etc) capable of resolving 0.0001, or 0.0005? 0.0005 makes it hard to measure bores that are graded in 10ths.

Which stihl cylinder do you have? A, B or C? it should be stamped on top.

Piston pin at 0.001 inches would be very loose. The spec on a similar pin is roughly -0.006mm to -0.002mm less then bore.

 

[Lakeside53]

P.S. note CLEARANCE from piston/pin, you should not have to freeze that latter to make it fit/press it in. Even/if you "NEED" too, hopefully the rod/bearing/pin will have enough clearance, if not something is wrong.....and/or the corrupted bearing devils will come too see you soon.......

With the ground pin and reamed bore, at 1 thou.. it will fall in... Stihl once said to heat the piston to 60c, but they relaxed the spec so now it's a finger press at room temp. Most aftermarket are slightly looser.

 

[Lakeside53]

Great info tony...I'm suprised on how much the oem super vs mag assemblies differ in weight. Are both the super and mag oem kits new? Also one thing you didn't include in your doc, which I assume you will, is the bolt spacing comparison between the mag and super.

There's been a lot of AS discussion about 1 cylinder engine "balance". Basically, it isn't "balanced" in the way multi-cylinder engines are. Lighter piston/pin combined weight is better, but too light and other factors creep in.


Mad: you should also weight the pins. It might not be so on the 038, but on the pro stihls 036 and above the OEM pin is ground tapered in the center to reduce weight. Aftermarket kits don't bother and have heavier pins.

 

[Mad Professor]

There's been a lot of AS discussion about 1 cylinder engine "balance". Basically, it isn't "balanced" in the way multi-cylinder engines are. Lighter piston/pin combined weight is better, but too light and other factors creep in.


Mad: you should also weight the pins. It might not be so on the 038, but on the pro stihls 036 and above the OEM pin is ground tapered in the center to reduce weight. Aftermarket kits don't bother and have heavier pins.

Andy, the weights were the whole assys, piston, pin, rings and clips. The taper I measured was at the skirt compared with the top just above the top ring, so yes if the piston is normally (much) undersize here my measurements may have over stated the taper.

 

[Mad Professor]

When you say "taper", do you mean a linear taper or just the difference in measurement from the top to the bottom.

That would be a strange piston. 6 thou is a lot... It would mean (using your numbers) 8 thou piston to cyinder gap, or if a linear taper, 4 thou at the mid point...

Most pistons I measure are slightly smaller in diameter immediately above the top ring. The best place to measure the "top" is below the bottom ring and above the wrist pin.






What did you use for measurements. Did you mic it with a mic (etc) capable of resolving 0.0001, or 0.0005? 0.0005 makes it hard to measure bores that are graded in 10ths.

Which stihl cylinder do you have? A, B or C? it should be stamped on top.

Piston pin at 0.001 inches would be very loose. The spec on a similar pin is roughly -0.006mm to -0.002mm less then bore.

The mics were 0.0005" so all the measurements are +/- 0.0005". The large clearance measured for the pin may have been due to the radius of the pin hole being a little smaller than the radius of the measuring tool. I'll have to have a closer look......


The 038S cylinder had no stamping on the top but the piston was marked B.

The 038M cylinder was marked B and also in smaller letters 7 with 75K beneath it. The piston marked BE.

Questions concerning OEM Sthil pistons:1) can you specify A, B, or C when ordering new to match your cyl? 2) Is A or C the largest/smallest?, 3) I assume a loose mismatch will result in piston slap, will a tight mismatch be so tight it will sieze?

 

[slabmaster]

You will find there are huge weight differences in pins.That is where most extra weight will be.Also measure between wristpin bosses for side play.I find there is alot of side play between bosses on aftermarket pistons.

 

[bcorradi]

I know A is the largest. Andy will probably be able to shed some light on the percentage of difference between the A, B, and C levels.

 

[Mad Professor]

You will find there are huge weight differences in pins.That is where most extra weight will be.Also measure between wristpin bosses for side play.I find there is alot of side play between bosses on aftermarket pistons.

Thanks, I'll measure that too for each!!!!

 

[Lakeside53]

I know A is the largest. Andy will probably be able to shed some light on the percentage of difference between the A, B, and C levels.

A is the smallest cylinder (tightest)... C the largest (looser). I haven't seen new production C for years.

Pistons that come with cylinders are often matched (less now as manf tolerances/processes are much better). Replacement pistons (typically AB) are sized suitable for any cylinder.

The differences... I don't have the exact details, but it's a few 10ths... (0.0001's) on the small bores, and a few more on the larger.

 

[bcorradi]

See what I know...I always thought A was the most desired level so maybe i just assumed the largest.

 

[Mad Professor]

A is the smallest cylinder (tightest)... C the largest (looser). I haven't seen new production C for years.

Pistons that come with cylinders are often matched (less now as manf tolerances/processes are much better). Replacement pistons (typically AB) are sized suitable for any cylinder.

The differences... I don't have the exact details, but it's a few 10ths... (0.0001's) on the small bores, and a few more on the larger.

Thanks, thats good to know. I was afraid they differed by 0.001 and mix/match would cause a failure. I just brought some NOS pistons for a 036 (another it's dead/take it away) and a spare for a second saw I have.

I cant' thank you guys enough for all I've learned here.....

 

[PES+]

Just curious

Can you post a picture of your mics? I am most interested in which inside type you used.

Also how many axis points and at how many points in the piston travel area did you measure?

The cylinders are quite accurate before they are run but when you do run the one you keep for a while and really work it you will be pretty amazed at how far from your original measurements the cylinder will mic (still within great usable tolerance but far from new and unused)

A very good project you have taken on and the information will be quite useful for members.

Who was the guy that was going to try to clean up the aluminum from a wiped cylinder with a boring bar?

I just wonder how he made out and what he found.

 

[Mad Professor]

Can you post a picture of your mics? I am most interested in which inside type you used.

Also how many axis points and at how many points in the piston travel area did you measure?

The cylinders are quite accurate before they are run but when you do run the one you keep for a while and really work it you will be pretty amazed at how far from your original measurements the cylinder will mic (still within great usable tolerance but far from new and unused)

A very good project you have taken on and the information will be quite useful for members.

Who was the guy that was going to try to clean up the aluminum from a wiped cylinder with a boring bar?

I just wonder how he made out and what he found.

For the cylinders we used a snap gauge that was measured against the mics. The cylinders seemed uniform throughout with the mics we were using +/-0.0005.

We checked the pistons at the bottom of the skirt and near the crown in several places, in retrospect I should have taken some measurements mid-piston. As stated there was "taper" from skirt to crown but no out of round detected.

Our machinist at work checked the pin bores, I'll have to check on what type of mic he used, it was not a snap gauge. All the equipment in our machine shop is first rate (for the stated tolerances) although I'd have to check the brands.

Sorry no digital camera (yet) so can't provide any pics.


P.S.Our machinist got a chuckle when I directed him to a "hotsaw" website with a 500cc "bikesaw". He is a motor head too with lots of hotrod chevys, I think one has a big block "merlin" motor....