Sachs C piston questions for Sachs specialists

So I have this Sachs 505C engine and its' piston is a bit beat up but the bore is salvageable. The difference between it's current piston (which I assume is original) and an A or B piston seems to be it is about 3MM shorter on the bottom intake side. The transfer openings are slightly larger as well. Wrist pin positions are the same from the top.

The question is, can I use an A/B piston of the appropriate diameter and would some trimming to match the original even matter? The current piston measures 50MM on the exhaust side and 47MM on the intake. A and B measure 50MM on both sides.

I had run this jug and piston already on a 504. It ran well but couldn't get much past 33mph using the A piston and new rings. 120PSI compression.

At BDC the piston peaks above the lowest part of the exhaust port by about 2-3MM Is this normal on a C engine and would there be gains if I trimmed the top of the piston a bit?

I gave up and went the Airsal route on the 504 but would like to see what I can get out of this properly running C on a Balboa with just a 15MM carb and pipe.

Re: Sachs C piston questions for Sachs specialists

Until your requested experts arrive , here's this :

The shorter intake skirt should / would allow quicker and a more complete intake charge .

Relieving the piston crown at the ports will advance the port timing proportionately and should help by having the exhaust port fully open . ;)

Re: Sachs C piston questions for Sachs specialists

You can use either a C or D piston the A&B won’t work

Re: Sachs C piston questions for Sachs specialists

> Tim Doll Wrote:

> -------------------------------------------------------

> You can use either a C or D piston the A&B won’t work

Exactly why is that ?

Re: Sachs C piston questions for Sachs specialists

PD, I like the way you're thinking. Any other non-experts feel free to chime in. Remember you can't call yourself a guru but if others do, then it's legit.

Re: Sachs C piston questions for Sachs specialists

20201015_095933.jpg]

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Done. I chose not to modify the piston crown, for now. It should breathe a bit better on intake and the transfers. (edited)

20201015_104337.jpg

Re: Sachs C piston questions for Sachs specialists

man i just went back and re-read the sachs thread not too long ago, I just got done building an engine using that weird euro chrome bore flat exhaust port cylinder and i'm confused as all get out.

from what i understand, the longer d crank is made up for with a shorter piston, which would make sense, in which case the A/B/C would all use the taller piston and the D would be the only short piston.

but i don't think its straight off that simple, because i've noticed that the pistons come in at least 3 different pin heights and i've noticed the same thing you're saying that different models have different skirt lengths even though the pin height is the same. i also think they screwed around with the compression height on the various restricted bikes, making things even more confusing.

i will say, the current setup i'm running, i have a piston that someone chopped about 3 mm off the skirt and it doesn't run for shit. hopefully you didn't just do that by mistake, haha, i think the c/d cylinders might have different port distance and a different intake port location because of the flat port? This is just a SWAG because i haven't measured port timing and basically just threw this bike together out of junk i had laying around that kinda fit just to see how it would run.

unfortunately i 'got out of sachs' years ago and sold all my sachs shit and meant to sell this balboa but then this cool cylinder dropped into my lap and a 505 and i figured what the hell, might as well build it.

if you want any measurements i know i have at least one D piston in a box that i saved for some unknown reason, and i have this weird euro cylinder which was supposed to be a D stroke but i'm running it on an A stroke engine with this chopped up A (?) piston. I think i'm gonna pull it back apart and see if i cant figure out what new piston to order and then do an actual port map with a timing wheel and try to figure out why it runs like doo doo. unfortunately since everything i own (except for this one piston that has 'stock d piston' written on a piece of masking tap e wrapped around it) is from god-knows-where, i really don't know what i'm comparing things to.

my weird euro cylinder 505/a does about 35 topped out with what feels to be pretty decent stock gearing, but it has crummy low end and zero midrange, as soon as you hit any kind of hill it slows to like 20 mph which is totally unacceptable. its got a 15:15 sha and one of those vintage proma gp clamp on chambers.

Re: Sachs C piston questions for Sachs specialists

And the plot thickens... Not that I really know what I'm doing, I am under the assumption that the piston in this C was the original one. I read that Sachs thread several times before posting. It would be nice if someone had a verified C piston to measure and confirm. It was missing from that thread and in my search efforts everywhere else. I'm trying to put together this Balboa at as low a cost as possible, using up junk and free paint and epoxy primer. I will splurge on handlebars, a pipe, cables, and whatever bits needed to make it go.

If this piston performs horribly I still have another used piston to try from my A cylinder. I also have an absolutely miserable 505B lower end that could be built with the Puch cylinder and spacer if I wanted to continue the madness. But that of course would still cost a bit...

Re: Sachs C piston questions for Sachs specialists

♣Slew Foot♣ /

The people in NJ used to get boxed parts in bulk then pick and pack I have seen so many motors that had mixed parts my frame was a p1 and had all g3 stuff a. D crank a piston with a u on the skirt to let it bdc

Re: Sachs C piston questions for Sachs specialists

Yeah if I didn't have this cool cylinder I would probably run a puch or tomos cylinder on this thing or actually just throw the whole works away and build something easier, haha.

I think the only way to do it right is sit down and actually do the port timings, I skipped it hoping to get lucky the first time but now I've just gotta take it all back of and redo it so that was dumb...

Re: Sachs C piston questions for Sachs specialists

Th tricky thing is C's are pretty uncommon. Often they weren't even labeled as a C specifically, they just don't have any designation. just 505/1_

So what happened is there's really shitty information about em based on "best I can remember" sorts of speculation.

Anyway I was looking at a bunch of factory parts manuals before and we found there were 2 piston sizes (technically 4 but it was standard and an overbore option for each)

There was one piston that was like 50mm tall listed for C and D, and one 3mm taller listed for A and B. they're all same bore. I also know that putting an A piston in a D jug hits the head, and putting a D piston in an A jug looks too low.

to the best I know, there are no topend differences between 504 and 505, but the 504/1c is even more rare and unknown so maybe that's a unique one.

I've also only got that one year's factory manual and maybe a dozen sachs engines worth of experience and that's hardly exhaustive - likely different years could've seen differences too. Especially later sachs and the clones made by SIS and the clones used on Kamasura also add confusion

and finally, Athena I'm pretty sure sold their piston as a stock replacement. it's the one that comes with the Athena 50cc cylinder : http://www.treatland.tv/sachs-athena-50cc-piston-p/sachs-athena-50cc-piston.htm but I've heard (only anecdotally) that this is a good replacement for sachs, with no definition A vs D or anything.

So yea, this doesn't help much but hopefully frames some more of the existing knowledge.

Re: Sachs C piston questions for Sachs specialists

yea heres something neat, go look on treats at the stock replacement pistons, theres a bunch, but seems to be either 50 or 53mm tall, and 28mm from top of piston to wristpin for the 53s and 27 or 27,5 on the 50mm tall fellas

sortof just convolutes it more tho, but worth noting theres no caveat or designation 504 vs 505 or for specific years, but also that theres more than just 2 identical sizes (edited)

Re: Sachs C piston questions for Sachs specialists

Yeah here's what I don't get, if the cylinders are the same, why are the pistons different lengths by 3mm? I get the compression height thing because of the different cranks but for stock cylinders a 2mm difference in intake port timing would basically make the thing not run.

So my engine was an a, but it was on a g3, I think someone swapped a d cylinder in it at some point and cut 3mm off the skirt, but like I said I'm not much use because the only cylinder I got is wackadoo

Re: Sachs C piston questions for Sachs specialists

Looking at Treats pistons I noticed this.

To the center of wrist pin and total length not including crown.

27-50

27.8-50

27.5-50

27.5-50

There were two more at 28-53 but the 53 was listed as total length including crown.

It looks to me like the piston height from the center of the wrist pin varies from 27mm to 28mm. Stepping up .5mm, .8mm, and 1mm.

What does this up to 1mm variation have on timing and compression?

Re: Sachs C piston questions for Sachs specialists

The picture of the three pistons half way down this page looks like there is minimal difference from the center of the wrist pin to the top. It looks like it's simply various skirt modifications.

https://www.mopedarmy.com/forums/read.php?1,434290,page=1

Re: Sachs C piston questions for Sachs specialists

> Frank Bailey Wrote:

> -------------------------------------------------------

> The picture of the three pistons half way down this page looks like

> there is minimal difference from the center of the wrist pin to the top.

> It looks like it's simply various skirt modifications.

>

> https://www.mopedarmy.com/forums/read.php?1,434290,page=1

Here's that^ post :

> Ja'Far Rogers Wrote:

> -------------------------------------------------------

> Here are Sachs pistons from A, B, D left to right. The C apparently

> I've only come across once it may be a tad rare anyways I don't have the

> piston with me. Notice the height difference of the pistons and the

> location of the piston pins. Also notice the height and width of the

> port windows from A piston to D piston, it's hard to tell on such a bad

> photograph but the D cylinder is slightly wider than the other two but

> substantially shorter.

>

(edited)

Re: Sachs C piston questions for Sachs specialists

Yes, blow that up as large as possible on your screen and measure with tic marks on a piece of paper. Top of wrist pin to the top of the piston are all the same.

Re: Sachs C piston questions for Sachs specialists

Yeah I'm officially motivated enough to go dig my D (should be a d, I'm pretty sure?) Out of storage and measure it.

I think there is something different about the cylinders, I don't have any to measure besides this weird euro one that really means nothing.

That thread is interesting, it kinda contradicts itself a few times, not sure what to make of that.

Re: Sachs C piston questions for Sachs specialists

OK, I used that piston with the trimmed skirt and it shows 100 PSI compression on this C engine. Dellorto Runtong, 64 jet, 28MM header and $35 chamber. It runs pretty well and touches 35 on GPS and can climb modest hills if you get into the power band. Still hoping to get 38 or more. Might try some modest porting. Mostly just widening a bit. My porting skills are modest and maybe this is as good as it gets.

Re: Sachs C piston questions for Sachs specialists

If that cylinder and head are a single unit , you consider taking maybe .030" off the bottom of the mount flange . Or possibly off the cases , but I have no idea how that might work or even if the case would allow for it .

Anyway , removing that much should up the compression by around 10 pounds .

Using a thinner base gasket would just add to that .

Re: Sachs C piston questions for Sachs specialists

PD, Yes single head/jug unit. I was just thinking of doing this as well if I try some porting. I want to use a degree wheel just so I can see the port timing. I'm hesitant to raise the exhaust so as to preserve what low end I have. I tried to port a B cylinder earlier and the results were disappointing. It currently starts easily, idles, and runs well enough I don't want to muck it up. Its fun. (edited)

Re: Sachs C piston questions for Sachs specialists

> Frank Bailey Wrote:

> -------------------------------------------------------

> PD, Yes single head/jug unit. I was just thinking of doing this as well

> if I try some porting. I want to use a degree wheel just so I can see

> the port timing. I'm hesitant to raise the exhaust so as to preserve

> what low end I have. I tried to port a B cylinder earlier and the

> results were disappointing. It currently starts easily, idles, and runs

> well enough I don't want to muck it up. Its fun.

I did the base mount 'thinning' on my Simplex , which also has a one-piece cylinder and head . I gained near 20 pounds compression .

Keep us in the loop , whatever you do . ;) (edited)

Re: Sachs C piston questions for Sachs specialists

The cylinders aren't the same, just in case that isn't clear. The A and B are taller pistons and taller cylinders to match. If you put an A piston in a C or D cylinder it will smash into to top of the jug. Like they gotta be matched pair, piston and cylinder. Nothing to do with stroke.

The kit pistons like the Athena might split the difference to make up for the 42 vs 44mm cranks, hence the 27, 27.5 or 28mm wristpin height.

Like, the D has the 44mm stroke, C is presumably same topend with 42mm stroke. which means at tdc it sits 1mm less squish, and takes the compression down a bunch. But the ports are in the middle, so if you picture the piston motion as 2 sine waves, one with a 42mm and other 44mm amplitude, they still hit the ports around the middle at basically the same time, they just stop 1mm short at top and bottom. This means the exhaust might not fully open and compression is lower. You could shift the whole piston up 1mm which would recover compression but block more exhaust and effectively lower all the ports I expect this was accounted for and designed into the kit versions and why the 27 to 28 variations in pin to crown height

Re: Sachs C piston questions for Sachs specialists

> Born to be WillD Wrote:

> -------------------------------------------------------

> The cylinders aren't the same, just in case that isn't clear. The A and

> B are taller pistons and taller cylinders to match. If you put an A

> piston in a C or D cylinder it will smash into to top of the jug. Like

> they gotta be matched pair, piston and cylinder. Nothing to do with

> stroke.

I'm going to be removing my cylinder soon and at least widen the intake and exhaust ports a bit. I'll compare cylinder lengths between the A, B, and C cylinders again. For now, all three pistons ARE the same with the exception of the intake side skirt length, openings by the transfers, and the ring position. The wrist pin to the top of the piston distance are all the same.

My modified A or B piston (can't remember which I used) does not bang the top of the C cylinder. At BDC the piston does not completely open the exhaust port and of course, falls short of reaching the full compression of a D crank engine.

I'm assuming each of my cylinders had their original pistons, but who knows.

Appreciate the input.

Re: Sachs C piston questions for Sachs specialists

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OK, so I measured the jugs. All three measure 95MM from the top to bottom on the inside, to the lowest part of the steel liners.

The top of the intake ports are about 55MM-56MM, the C is about 55MM top and 66MM bottom. These were measured with a metric straight edge so give or take .5MM or so for eyeballing it.

I did a very gentle exhaust porting with a rat-tail file and 220 grit sandpaper. Widened maybe 1MM total and up just a bit, but really more of a polish and eliminated the base gasket. Compression now measures 115PSI.

Upjetted from 63 to 65. Timing is set as per stock, 11-42 gearing.

I annoyed the neighbors and tested it but it still mostly goes 30-33 on GPS. I'll have to continue experimenting with carburetion and airboxes. Airbox seems to make a big difference on these Sachs. Maybe this is about as good as it gets with this gearing. It will climb most small hills but you of course need to get a run at it and hit at least 20 to get in the powerband.

Maybe I'll get rid of this SHA abomination and go with a round Bing. (edited)

Re: Sachs C piston questions for Sachs specialists

Shawn Barley /

🙏🙏👍👍👍👍👍👍 tha is to all the gurus for the great info

Re: Sachs C piston questions for Sachs specialists

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Went with this setup after seals and bearings. 15MM "Bing" with 2.17 atomizer, single mark needle, 64 jet, drilled 24 slide. It handles 10% hills and is just fast enough to be fun. 11/42 sprockets.

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