Author Topic: Sachs action  (Read 2092 times)

Big Al

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Sachs action
« on: August 02, 2016, 07:52:42 AM »
Putting together a Schmitt engine for my KR200. This is a rebuilt unit with a new crankshaft and all, inherited in a deal. But an unresponsive dynostarter testing badly. Also finishing off 2 others. One for the next Sport kit to be sold, one for sale as a spare rebuilt engine.

Notes stimulated by shared grief with over tight Sachs cheese headed set screws with Steve Mandell.


Just so you do not feel left out, Steve. I have just had a 4 hour battle to remove a set screw holding the Dynostart stator to a Sachs 200 engine casing. It had a poor head on it, and like the others had been torqued down far to tight. I could not get anything to get a purchase on it. Each attempt took a little more metal off the slot. Again its a long reach, so the standard impact driver bits do not reach, and the nose of the impact driver is to tubby to get in. I removed the brush assembly for fear of damaging them (mint condition, new brushes! but also over tight screws). My long drive bit failed under load, so that is scrap now. The brace and bit failed to move it, as the slot was to poor, and I could not control the engine enough, on my own, to get pressure on it.

So to go shopping to a far distant proper tool shop, or coming up with my own system? I went for the brace and bit's bit, mounted into a long socket. It had a square hole through to the 3/8 inch drive, so the bit sat in that and in two elements of the hex shaft. Not ideal, but pretty good. That into the 1/2 inch converter and so to the impact driver. I selected my double headed copper mallet as the largest mass. This was going to be success or failure, as I fully expected half the cheese head set screw to fail. That would then be Mr Pillar Drill time.

I tensed up the driver, having trapped the engine in a corner. It took three seriously hard larrups to get the bugger moving. It came out badly, as it had pulled out part of the ali thread. The head of the set screw itself was on the point of shearing off half the head, as they can. So very very lucky!

I have just chased the thread out with a tap, in itself not easy as taps are not very long. But fortunately cleaning the thread is less of a job than creating a new, or re-cutting a cross thread at the beginning of the thread. I did this by reversing a 1/4 in extension bar and putting a socket on the end. That joined to a T drive by a hex drive into another socket on the T driver. It was a bit long and cumbersome, but the best I could think of. The thread will go again, but will need some stud lock to be sure it stays put. Need a good lockwasher on it too.

So another engine poorly built with over tensioned fasteners. Can I blame the builder as the dynostater has a dead stator on it? The starter coils have one that has failed rather spectacularly to a short in the insulation. Cause, looking at the bottom of the unit, the yellow insulation has been moved exposing bare wires. Think one has been bent too and it has managed to short. The charge wire has got hot, so will change that too. Eventually I will take a coil off my donor unit and replace the defective coil. Hopefully it will then live again. I have a fully serviced Dynostarter to put on.

Done I then should have two fully rebuilt new crank engines ready for the car. I have yet to do the trick engine with some mods on. But I need the car driving first. I then have a known spare to pop in any car being prepared, before its own engine is taken in hand. As the chassis needs as much tuning as the engine for friction from bearings, suspension, tyres and so on. Dealing with that with a good engine means circuit and bumps are more successfully aimed at one problem at a time.

The moral here is while set screws need to be tight, they do not want to be over tight. A square shanked screwdriver and a spanner just tweaked beyond wrist strength is enough. No impact driver is needed. Interestingly the casing set screws had silicon sealant over them, presumably to stop them turning.. New one on me.

Messerschmitt set, Goggo Darts, Heinkel 175, Fiat Jolly, Autobianchi, Fairthorpe Electron Minor, Borgward, Isuzu Trooper
Citroen BX 17TZD & GTI 16v
Held - MG Magnette ZB & 4/44
For sale - Vellam Isetta, Bamby, AC Type 70, Velorex, Church Pod, Reliant Mk5, KR200,  Saab 96, Bellemy Trials, Citroen BXs

Big Al

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Re: Sachs action
« Reply #1 on: August 02, 2016, 07:53:46 AM »
Having got a number of Sachs engines sat in a row. each with its chosen cylinder, it is time to select a piston.

Now I have never been greatly concerned about re-using old pistons. Clearly they need to be in good condition, and not worn. In the Sachs you can get away with a little skirt damage, as much of this area is not critical, as long as it is smooth. To much missing and the expansion characteristics worked out by the manufacturer starts to be compromised. But a nick from poor action with the conrod, and such, need not be a great concern.

More important is the port areas, for that is your timing and valve. I normally take file to these to ensure they actually match the porting, not shroud the edges. I then chamfer the entire lower edge of the piston, so that a little fuel/oil mixture is left on the cylinder sides, an aid to lubrication, but probably not outright power, as the piston has to scrap it off all by ring.

A check of the gudgeon pin, for fit, and that the clip groove has not been damaged. No point in chancing a dodgy circlip. A loose gudgeon pin really bogs up a cylinder.

The most important area is the ring grooves, and pins. The grooves need to show no signs of wear. I have had ones where some kind of vibration has been set up, or a snapped ring has remained in place, but waggled its free end into a nasty gash. If your economical, these pistons can be fitted with only two rings, and used in a spare engine, of the get you home type. Though I have had occasion where a two ring, one groove damaged, piston provided for a very quick engine. Less friction, for one thing. So its worth a try, if you reach the privileged position of excess engines. I do not know if Villiers guys run one ring in a spare engine on such a damaged piston, but I am willing to bet a few got home on one ring after a bit of bad luck.
The other check is on the ring stake pin. It should be there! Its to stop the ring turning in the groove like a four stroke, as eventual the ring end will find the port, and get caught in it, getting sheared off and often taking out the bore and piston in the process. Though rings have been known to disappear out of the port leaving no descernable damage at all! Quite clearly that is a bad thing to allow to happen, nonetheless. They stake pins have been known to fall out. It should not be worn, either. You can replace them, but it gives me the willies, as how do I know it will stay there.

A good clean up on the piston. Try and remove scuff marks, particularly if they are from attempted seizures. The ali gets dragged and can create a high spot. If you have a piston like that, then it will need longer to run back in, as those high spots will want to create friction spots. You need to wear them off in a controlled manner. Cleaning the top of the piston is a moot point. A layer of carbon actually acts as a heat shield, but unless you know your pistons, its nice to have the front of the piston proven by the marking, and the size of the piston proven.

Rings can be reused. But for a good unit I fit new, as you would. The old rings get retained for spare pistons. The ratio of salvaged rings that are usable, to old pistons, is not that great. So any that can be got off are worth having, in my opinion. I am not great at recording their size, but the crucial thing is to pop them loose into the bore they might be used for. The gap left not ring will tell you if its going to be any good. Rings can be made to go slightly undersized, but the danger is they will brake, creating damage far in excess of a saving of over compression, instead of new rings.

I have a box with about 16 Sachs pistons in it. Half are standard bore of course, so of marginal use, save I have a lot of little used barrels, having had the pick of 35 years of stuff going passed. Most are clean, but others are not yet serviced. So I selected my favoured candidates yesterday. In two cases they are unserviced.

So the first job is to get the old rings off. In most cases folks leave the rings on. Either gummed by deposits into place during use, or left to corrode in the atmosphere, into place. So the first round is to see if they will loosen. Normally I get them soaked in light oil. Good fingernails are handy. A sharp small screwdriver and some old feeler gauges. If you can lift an end then you can chase the loose part round the ring by working gently at it. The screwdriver will nick away at the end. The thicker feeler gauge will eventual slide under the ring. Often times, though, the ring will not have it. This is a great job while listening to some good sounds in the evening.

So then its onto the cooker. The Aga offered the top oven, but the gas cooker has a simmer ring. Note boiling in water will restrict the heat to near 100 degrees Celsius. You want it hotter than that. So buzz it on that. Go off and find a bit of hard wood, back of a wire brush is quite good, and an oven mitten. The piston will get good and hot. If your lucky the expansion of the ali is great enough over the iron rings to break the bond of crude holding it in. The ring will magically loosen on its own. Otherwise its on with the mitten, and in hand rotate the piston while firmly batting the rings in turn to break the bond of crude. If not a success return to heat and fetch the sharp screw driver.
Good and hot now place the screwdriver against a end of a ring and and hit the other end with your bat of wood. Normally the end of the ring will ease out. You end up with loose ends, but often still a trapped ring on the other side of the piston. I now go for more oil. The problem is that while hand hot is a help, its not really enough heat to get that crude loose. So you need to return to working the end of the ring. It will soon be clear if your going to win or not. Eventually your persuasion will be to forceful and the ring will fail. But at least you tried.

So this morning I have one piston with its three rings off. The other has one ring nipped in by a seizure bruise, the other two defy movement so far. I remember this first oversize piston. Its done about 2,000 miles and kept nipping up. I think I was to greedy in the tolerances, wanting it tight, for max performance. An own goal. It was my first steel conrod engine. In the event the cylinder was honed clean, and that took it out to where it should have been in the first place. A new piston saw that unit cover many miles, after running in again, I still have it. The abused piston is still a good piston, but needs a good polish and running in again. The rings are good, if I can get them out. A set of rings is what, now, £35? Worth a couple of evenings fidgeting I would say.
Messerschmitt set, Goggo Darts, Heinkel 175, Fiat Jolly, Autobianchi, Fairthorpe Electron Minor, Borgward, Isuzu Trooper
Citroen BX 17TZD & GTI 16v
Held - MG Magnette ZB & 4/44
For sale - Vellam Isetta, Bamby, AC Type 70, Velorex, Church Pod, Reliant Mk5, KR200,  Saab 96, Bellemy Trials, Citroen BXs