Bultaco model 212 piston

[saddleback]

I have a Bultaco model 212  250 alpina. The piston  in this bike has a full length skirt on the intake side. Would anyone know how much a stock piston intake skirt is shortened?         Thanks.  Dean

[model80]

Not really grasping what you are asking.

[section swept]

A standard piston will have the same length skirt all round. Shortening the intake side skirt will uncover the intake port sooner but you might need to use a reed valve. Shortening the exhaust skirt side serves no purpose as the crankcase pressure will escape down the exhaust port. Shortening the intake side of the piston skirt needs care, you would only remove a section the same width as the intake port in the cylinder barrel. The height is the crucial measurement and this depends on what you want your engine to do. 

[feetupfun]

Bultacos of that era used the same basic piston but the induction side skirt was shortened on the models that were designed for top end performance (Pursang, Astro) to increase inlet port duration. Not having a reed valve meant that shortening the skirt was a compromise and took away from the performance at low RPM.

Alpinas and Sherpa Ts that I have worked on mostly have had the full length inlet skirt but some have had pistons with shortened inlet skirts (and other porting changes) that were previous owners work. People do fiddle around with engines.

The important thing is that the inlet skirt provides the correct inlet timing. If you want to be certain of the inlet timing, you can confirm the inlet timing with a protractor or "degrees wheel" against standard timing.

I rode a Bultaco with similar port timing to the M212 (M49) for a couple of years before I pulled it apart and discovered that it had a shortened inlet skirt (about 4mm). After the rebore and now with an original type MAHLE piston that had the full length inlet skirt, there was an improvement in low RPM power, but it would have also had improved ring sealing as well as the more conservative inlet timing.

[lorenzo]

feetupfun - 

Mmmm.... that's interesting, and a timely subject for me, as I am currently  trying to sort a piston/barrel for my mod. 85 Alpina.

11 hours ago, feetupfun said:

People do fiddle around with engines.

The liner of the original cylinder has effectively been ruined by some PO idiot with a grinder.  The best cylinder I have (from a mod. 124) is on STD. bore and Mahle piston, but this has the shortest rear skirt of any 250 piston  that I have ....

(I have a few new Sherpa pistons on the shelf (Mahle and Dinamin) and I've also kept some of the old pistons I've replaced in the past, for the purposes of reference and comparison etc. in the future.)

All the 350 (83.2mm. & oversizes) pistons have front and rear skirts of equal length.

All the 250 (72.0mm. & oversizes) pistons have a rear skirt shorter than the front ;  this also applies to the later 240 (71.0mm. & oversizes) pistons.

I think this explains the reason for Saddleback's question raised in his o.p.

I know that mod. 212 is a 250, but unfortunately I do not know what piston would have been fitted originally, although it seems likely it would have had a shortened rear skirt.

Edited April 24, 2018 by lorenzo
Clarity

[feetupfun]

13 hours ago, lorenzo said:

feetupfun - 

Mmmm.... that's interesting, and a timely subject for me, as I am currently  trying to sort a piston/barrel for my mod. 85 Alpina.

The liner of the original cylinder has effectively been ruined by some PO idiot with a grinder.  The best cylinder I have (from a mod. 124) is on STD. bore and Mahle piston, but this has the shortest rear skirt of any 250 piston  that I have ....

(I have a few new Sherpa pistons on the shelf (Mahle and Dinamin) and I've also kept some of the old pistons I've replaced in the past, for the purposes of reference and comparison etc. in the future.)

All the 350 (83.2mm. & oversizes) pistons have front and rear skirts of equal length.

All the 250 (72.0mm. & oversizes) pistons have a rear skirt shorter than the front ;  this also applies to the later 240 (71.0mm. & oversizes) pistons.

I think this explains the reason for Saddleback's question raised in his o.p.

I know that mod. 212 is a 250, but unfortunately I do not know what piston would have been fitted originally, although it seems likely it would have had a shortened rear skirt.

I do have a M198 (a 238cc Sherpa T contemporary to the M212 Alpina) and it came with the original piston, so I will dig that out and take a photo

[feetupfun]

12 minutes ago, feetupfun said:

I do have a M198 (a 238cc Sherpa T contemporary to the M212 Alpina) and it came with the original piston, so I will dig that out and take a photo

Well my memory was wrong about the M198 engine being apart. I found the motor re-assembled and mounted in the frame!!!

I did however measure up some 250cc pistons that had come out of M85, M85 and M49 engines and they were all different rear skirt lengths, and the rear skirts had all been shortened. I measured from the crowns to the edges of the inlet skirts and got 56mm, 58mm and 59mm.

One of the M85 cylinders had had the top edge of exhaust port raised in one window but not the other window, so I know what you mean about being ruined by someone with a grinder (and is why I bought a second M85)

[aaronh17]

Hello, I’m new to this sight but have had several Bultacos over the years. I’ve never been on a Sherpa T but have had a matching M-49 for close to 20 years (basket case)

 In my late 40s now, kids almost on their on and I’m beginning to lean back to my passion of fixing some of my Buls to putt around on.  I have always been under the idea that a reed valved 2 stroke is better than non reed, but I very rarely see Alpinas and Sherpa T bikes fitted with reed valves.  

Wouldn’t an M-107 square finned cylinder equipped with a reed (which I have) be the ultimate set up on an M-49? I know the expansion chamber would need modded to work with this, but just trying to pick some minds, I have the correct round cylinder that came on the m-49. I get these wild ideas and over time I convince myself that they are good ones.  Any thoughts?  Thanks

[feetupfun]

On 4/28/2018 at 11:16 AM, aaronh17 said:

Hello, I’m new to this sight but have had several Bultacos over the years. I’ve never been on a Sherpa T but have had a matching M-49 for close to 20 years (basket case)

 In my late 40s now, kids almost on their on and I’m beginning to lean back to my passion of fixing some of my Buls to putt around on.  I have always been under the idea that a reed valved 2 stroke is better than non reed, but I very rarely see Alpinas and Sherpa T bikes fitted with reed valves.  

Wouldn’t an M-107 square finned cylinder equipped with a reed (which I have) be the ultimate set up on an M-49? I know the expansion chamber would need modded to work with this, but just trying to pick some minds, I have the correct round cylinder that came on the m-49. I get these wild ideas and over time I convince myself that they are good ones.  Any thoughts?  Thanks

It depends on what you are seeking to be able to say if is the ultimate set-up or not. With the square barrel, it would no longer look like an M49. If that doesn't matter to you then it sounds like a fun experiment.

If you haven't seen many Sherpa Ts with reed valves, maybe you are looking in the wrong place. In some parts of the world it is a popular mod on late model Sherpa Ts, but not on early models like the M49. I suspect that the reason for this is that if someone wants a competitive Sherpa T, they start with a late model rather than an M49 due to the superior chassis dynamics. If people want to enjoy the awesome look of an M49, they would probably not want to make it look unusual/hybrid/ambiguous which would happen with a square cylinder or a reed valve.

It's fairly rare to see reed-valved Alpinas probably because the main benefit for the Sherpa T motor is improvement in smoothness and even running of the power, which is not an important issue with Alpinas.

[saddleback]

On April 24, 2018 at 8:22 PM, feetupfun said:

Well my memory was wrong about the M198 engine being apart. I found the motor re-assembled and mounted in the frame!!!

I did however measure up some 250cc pistons that had come out of M85, M85 and M49 engines and they were all different rear skirt lengths, and the rear skirts had all been shortened. I measured from the crowns to the edges of the inlet skirts and got 56mm, 58mm and 59mm.

One of the M85 cylinders had had the top edge of exhaust port raised in one window but not the other window, so I know what you mean about being ruined by someone with a grinder (and is why I bought a second M85)

Hi, The 58 or 59 mm sounds like a possibility.. In A.Graham Bell's book two stroke performance tuning he talks about a old Bultaco matador(model 26 ?) as having a inlet open period of 150 degrees. As the two engines are so similar ,before altering to piston the timing in degrees needs to be checked out.   Thanks much for the help.   Dean

[saddleback]

On April 27, 2018 at 8:16 PM, aaronh17 said:

Hello, I’m new to this sight but have had several Bultacos over the years. I’ve never been on a Sherpa T but have had a matching M-49 for close to 20 years (basket case)

 In my late 40s now, kids almost on their on and I’m beginning to lean back to my passion of fixing some of my Buls to putt around on.  I have always been under the idea that a reed valved 2 stroke is better than non reed, but I very rarely see Alpinas and Sherpa T bikes fitted with reed valves.  

Wouldn’t an M-107 square finned cylinder equipped with a reed (which I have) be the ultimate set up on an M-49? I know the expansion chamber would need modded to work with this, but just trying to pick some minds, I have the correct round cylinder that came on the m-49. I get these wild ideas and over time I convince myself that they are good ones.  Any thoughts?  Thanks

 

On April 29, 2018 at 4:42 AM, feetupfun said:

It depends on what you are seeking to be able to say if is the ultimate set-up or not. With the square barrel, it would no longer look like an M49. If that doesn't matter to you then it sounds like a fun experiment.

If you haven't seen many Sherpa Ts with reed valves, maybe you are looking in the wrong place. In some parts of the world it is a popular mod on late model Sherpa Ts, but not on early models like the M49. I suspect that the reason for this is that if someone wants a competitive Sherpa T, they start with a late model rather than an M49 due to the superior chassis dynamics. If people want to enjoy the awesome look of an M49, they would probably not want to make it look unusual/hybrid/ambiguous which would happen with a square cylinder or a reed valve.

It's fairly rare to see reed-valved Alpinas probably because the main benefit for the Sherpa T motor is improvement in smoothness and even running of the power, which is not an important issue with Alpinas.

My fathers model 49 Sherpa T had a reed installed in 70 or so. A DH reed install by a Mc Collough  engineer . Dale Herbranson?   A small 4 petal reed with two boost ports,actual ports cut in the cylinder not slots as is the normal now.  Anyway the engine became very responsive not always a good thing in a trials section.  A new liner was fitted and tuning was returned to stock.  I still have the bike it's a 325cc round barrel now.  More on that if anyone is interested.    A.Gram Bell's two stroke tuning book has a article on adding a reed on a matador with 360 degree open timing and a power port setup.   With dyno results.     A good read.     Thanks.    Dean

[pschrauber]

If you want to get an answer to your question and may be a NOS piston to your Bultaco Alpina model 212 you might ask Orlando Calonder, he was the main Importer for Bultaco Europe main land has all the knowledge papers documentations and spare parts.

He lives in Switzerland speaks German, French, Spanish, Catalan and Italian, but not English he will also most likely not communicate in English, there must have been some issues in the past between UK and him so here you have to be very sensitive.

As an example: I've got the original main road reg. card for my Bultaco model to get the bike road registered in all the countries he imported the bikes back then which was super useful to get my bike through as to mine the papers where lost. He also has most parts and all technical information for Bultaco's in general from trials, Enduro, MX to street bikes.

He has an own internet page, www.orlandocalondersa.com but easiest way to communicate or to do orders is by phone, letter and fax.

[nigel dabster]

so what were the two holes in the 199b 340 pistons for then?

[saddleback]

On May 7, 2018 at 12:56 AM, nigel dabster said:

so what were the two holes in the 199b 340 pistons for then?

Back to A.Graham Bells book. He says the small holes and worm ports in the cylinder liner are to cool the piston crown .They promote airflow under the piston crown.   Dean

[nigel dabster]

strange application if the case in a trials motor?