Piston etc for Sachs 1251/6B GP (long)

[anders]

Hello,
As I am new to the Pog (and actually don't qualify as I only have a Monark) I need to run a quick introduction.

I am an Norwegian aged 45, and have been a motorcyclist all my life. Right now, I race Speedway bikes and my 2 sons (aged 16 and 22) are into MX, all 3 of us just for the fun of it. As I said, I, or rather my 16 year old son, has a 1973 Monark ISDT used for Vintage MX in Scandinavia. He is racing a few vintage races each year. This years's race in Linköping, Sweden, was a big disappointment, as the Monark did not cooperate wery well. It ran OK during practice, but failed all the 3 races.

From the Monark records in Sweden, we can see that this engine is what Monark called the GP engine in Scandinavia, the Monark-tuned 6B. The ports does not match the GS-Pro port sizes, but it's ports are even wider than the Pro. I don't know if the cylinder is an original GP, or if it's a home-ported affair. Anyway, it runs quite well (when it runs, that is). We started out with the original Bing, but have later gone to a 26mm Mikuni (off my wife's RZ350). As the intake is almost 32mm inner diameter at the spigot, I guess we could run it with a 30 or perhaps a 32mm Mikuni.

Now we have opened the engine, and found that the piston is broken. The top ring is gone, but not without a trace, as we can very clearly see all the places where parts of the ring has been pressed into the piston top and the head. I plan to do a full rebuild, including a new bottom end.

So we need a new piston, the cylinder is 55mm. The current piston is a Meteor, a brand that is still available here in Norway, but my impression is this is no race-quality piston. Original Sachs pistons are also availeble here, but only the type for the iron cylinder.
I have an offer for a Wiseco 163p4, and it's measurements are quite similar to the Meteor, apart from the fact that the distance from the pin to the top is approx 1mm higher, and the overall height is approx 3mm shorter. All in all, this means higher compression and longer intake duration, both things were in fact reccomended as tuning tricks from Monark USA in their description on how to tune the cylinder as a GS-Pro.

Does anyone here know a good source for race-quality pistons for these engines, pistons that match the wide transfer ports?
I would prefer to buy pistons from someone that knows these engines, and are able to offer some advice.

While we are at it, I might also replace the old Motoplat with an internal rotor PVL. Any experience using this ignition, compared to the Motoplat?

I have seen that people use both 30 and 32mm Mikunis on these engines. Any thought on this? The bike is used for MX, but the rider is no pro.

Also, are there anyone offering (modern style) pipes for these engines? My impression is that the original Monark pipe is not too good performance-wise.

I'm sorry for this long posting, but I would really like to have this engine running well again.

All advice are highly appreciated

Anders Hørtvedt
1973 Monark ISDT

[terry]

Hello and welcome. I have a PENTON ans Sachs web site. I sell new and used and reproduction PENTON/KTM and Sachs motor parts. I do have brand new original Sachs pistons in stock with rings and pin and clips.
Thanks Again
Terry Everett/Ohio
United States

//www.pentonmotorcycles.com

[DKWRACER]

Anders första, welome till de Penton ägarna grupperar Website, är det riktigt en internationell plats, som välkomnar några, och alla venues av KTM och Sachs drev maskiner. Du might försök "det sökande" särdrag med kategorien som den är Monark. Det finns fåtalet, som har äktat att erfara, med ifrågasätter dig har frågat. God lycka med din "quest" och uppehälle som vi som informeras av ditt, fortskrider för att dela! Tom Brosius

[anders]

quote:Originally posted by DKWRACER

Anders första, welome till de Penton ägarna grupperar Website, är det riktigt en internationell plats, som välkomnar några, och alla venues av KTM och Sachs drev maskiner.

Tom, takk for den skandinaviske hilsenen, hvor lærte du svensk?

Anders

Anders Hørtvedt
1973 Monark ISDT

[DKWRACER]

Hi Anders, thought I might brush up on my poor Swedish language skills which were necessary in comms with my X...[8D]
I sure hope you find the proper piston for your project. To protect most of my racing pistons, I have them coated by Swaintech here in the USA. Their coatings consist of a TBC dome coating which helps the piston run cooler and reducing the swelling of the piston causing the dreaded "squeeked" piston, they also offer a skirt coating that extends from the bottom of the ring to the bottom of the skirts, might be something to look into etc...Good luck...Tom Brosius [^]

[Bryson Williams]

Greetings from Oregon, I own a bunch of those there Monarks, I'm interested in your cylinder,specifically your porting.  You mention your porting is actually wider then the GS PRO spec's.  

Would you mind sharing the port spec's measurments?. Regarding the pistons there is such a piston called a GS piston which basically is a repinned piston.  In reviewing one of the GS pistons they are noticably pinned different.  

I guess my question is that, IF the GS pinned pistons are right there on the GS and GS PRO? Ported bikes. I don't see how you could port wider and not knowingy expose the rings to either the intake or exhaust port. OR was that ring exposure by design, is there an advantage?.

Very interesting.

Dirk Williams  

[anders]

quote:Originally posted by Bryson Williams

I'm interested in your cylinder,specifically your porting.  You mention your porting is actually wider then the GS PRO spec's.  

Would you mind sharing the port spec's measurments?.

I guess my question is that, IF the GS pinned pistons are right there on the GS and GS PRO? Ported bikes. I don't see how you could port wider and not knowingy expose the rings to either the intake or exhaust port. OR was that ring exposure by design, is there an advantage?.

This is a quick list of the port sizes, as hastly measured right now:
Inlet 39.5mm wide, 25mm high, 65mm from top of cylinder
Exhaust 35.7mm wide, 24.8mm high, 28mm from top of cylinder
Transfer 26.2mm wide, 15mm high, 41mm from top of cylinder

Compared to the GS PRO port chart at http://www.vintagemonark.com/images/modifications.jpg the exhaust and inlet are wider, while the transfer is narrower, probably more equal to a standard 6B? I guess this also means that I can do with a standard piston?
I cannot see that the transfer is raised 2 mm at the rear, as the chart suggests.

From what I can see now, and compared to the vintagemonark chart, I would guess this is a porting that is done by some swedish motocrosser to try to hang on to the big guys with the factory-prepared GS and GP engines. Or perhaps the GP engine was like this all the time? I don't have too much experience with 2-stroke porting, but to me it seems like the transfer ports don't match the inlet and exhaust?

I cannot reall tell if this is a better or worse porting than anything else, because we have nothing to compare to in a scinetific manner. I have seen another Monark that was slightly quicker than this one on a very long straight (running flat out on 1000m oval), but that might also be due to gearing, or the 26mm Mikuni.

The piston found in the cylinder, had 2 rings, one at 6 o'clock (facing rearwards), and one at approx half past 7, in between the inlet and left-hand transfer ports. As far as I have been able to find out, the Wiseco pistons are pinned similarily. Are these the standard piston's pinning?

I guess the ring breakage of the original piston is due to one of 3 causes;
1: inferior quality
2: Too sharp edges of the ports
3: Too wide ports.

Any thoughts on this? Perhaps I should round off the exhaust and transfer port top and bottom edges?

Anders


Anders Hørtvedt
1973 Monark ISDT

[Bryson Williams]

Good morning and Happy Thanksgiving to everybody.  IN looking at a stock sachs piston I see that they are pinned at 6 and about 7 1/2 like you mention.  The GP designator is also interesting to me, I've never heard of that before and wonder what it really means.  

In looking over the Swedish register for Monarks I have looked at the bikes that are GP tagged.  I only noticed one visual difference,on one of the bikes, that being a Pro Fab swing arm.

I have been trying to locate some of those GS pinned pistons if I locate any I'll email you.  It seems to me that your problem is in fact that the ring ends are exposed to the ports which is hanging up.  

Another thought is to re pin the stock piston ring posts, which is a real pain, however its a potential solution to your problem.

Dirk Williams