piston pin bearing

turnofftheradio

turnofftheradio

Member
I don't want to beat the topic to death, and I have read as much as humanly possible on the subject, but i want to replace my piston pin bearing.

I've replaced every other bearing, besides the loose mess in the clutch gear. My last round of parts I bought from a great guy who knew the source of my engine, which was a mystery to me. BGF, the batch I have had a 40% failure. Mine chose to blow up the mag. side bearing. So, with the vendor/ reputation in mind I'm going to replace the piston pin bearing now as well. I've heard good things about the sthil bearing. Is this still the best? I do not want to use a general service 10x14x13 or 10x14x15. It's got to be for wrist pins. I'm not going to do the lower one.

anyone have a favorite piston pin bearing?
 
most are 10x14x13 and they leave alot of play for the bearing to slide side to side on the wrist pin. I havent had any issues with it, personally, but I just don't like the sloppiness. I'm using a bearing a little bit wider (havent measured it but it's quite possibly a 10x14x15 bearing) and it fits snug between the wrist pin mounts. If I were to recommend a size, it would be that one. As for the brands, well, a good quality bearing will do, really. These engines don't rev very high for 2-stroke engines, so a general service bearing from SKF or a japanese company will suffice. Stihl bearings seem a bit pricey, and there was a silver one for Minarelli race engines on www.treatland.tv I'm pretty sure.

In the end, however, vibration reduction is the name of the game to keep your bearings in tact. out of the box, the 66cc engine is crazy unbalanced and is capable of shaking the whole bike apart (some even at the welds). Reducing the vibrations will keep your bearings, seals, overall engine, and even bike, in one piece. Lightening the piston is one way to achieve this, and balancing the crank is another. I prefer the piston weight approach, as it gives you a much wider "sweet spot". Less weight to throw around is better than counter balancing a weight at a specific speed. My piston weighs 97g if I remember correctly, a solid 13g less than stock. At that weight, my motor only ever vibrates a lot if there is a misfire (4-stroking or otherwise) and it's comfortable for long rides at any RPM.
 
well, it's been settled for me. I decided on the juice bushing. then I went to order it and they are out of stock till august? I ordered the sthil bearing. We'll see how it does. If I kill this engine I have much better options nowadays with the balanced motors that are available.
 
GearNut said:
Subscribed with interest. I wanna see just how good this Sthil bearing really is.
Click to expand...
I imagine as a bgf engine we will find the connecting rod the next weak point. Really, I was more excited to give that juice bushing a try with this, but the sthil bearing should be the best replacements for the original size bearing. I considered a wider bearing seriously, but so many people have tons of miles on stock ones, I'd be safe keeping variables to a minimum.
 
got it. fits well.



threw another bearing in this shot to show that the sthil bearing much more closely resembles normal pin bearings. this one happens to be from a suzuki ts185. the stock bearing is of a general service design it appears (note the ribbed area in the center)

order left-right
stock bgf, sthil, ts185



.wee.
 
Thank you very much for showing the comparison of the bearings.
How well does the Sthil bearing fit the con rod with wrist pin installed. Would any honing be involved on the con rod?

I keep pondering sleeves installed on either side of the bearing to take up the space between the bearing ends and the wrist pin bosses of the piston. But... what material to use?
It must be light weight and very durable. I really don't think aluminum would last. Steel would be too heavy unless lightning holes were drilled. Titanium would be expensive but durable enough. Magnesium... I don't really know enough about it's durability to comment.
Something must be done though to prevent the bearing from walking on the wrist pin or early failure will be eminent.
 
GearNut said:
Thank you very much for showing the comparison of the bearings.
How well does the Sthil bearing fit the con rod with wrist pin installed. Would any honing be involved on the con rod?

I keep pondering sleeves installed on either side of the bearing to take up the space between the bearing ends and the wrist pin bosses of the piston. But... what material to use?
It must be light weight and very durable. I really don't think aluminum would last. Steel would be too heavy unless lightning holes were drilled. Titanium would be expensive but durable enough. Magnesium... I don't really know enough about it's durability to comment.
Something must be done though to prevent the bearing from walking on the wrist pin or early failure will be eminent.
Click to expand...

I came really close to shimming the top, if I did I think bronze would have been my metal of choice. Why did I not? As assembled, my con rod appears to be slightly off center, I think the shin may cause more problems than it solves in this engine. If I could shin the bottom to match, or my crank was more accurate perhaps I would consider an upper shim.
The sthil bearing fit my con rod very well with no additional prep to the pin or rod
 
[QUOTE="GearNut]

I keep pondering sleeves installed on either side of the bearing to take up the space between the bearing ends and the wrist pin bosses of the piston. But... what material to use?
It must be light weight and very durable. I really don't think aluminum would last. Steel would be too heavy unless lightning holes were drilled. Titanium would be expensive but durable enough. Magnesium... I don't really know enough about it's durability to comment.
Something must be done though to prevent the bearing from walking on the wrist pin or early failure will be eminent.[/QUOTE]
The PK 80 style of motors I sell have longer piston bosses so the needle bearing can not move excessively from side to side.

My older Starfire pistons have a lot of play.

Much easier and a better solution is to get another $10 piston which has longer bosses.
 
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the stihl bearing sems to have longer roller rods amd be of higher quality. can you post the part # and web site you ordered from ? i am interested in this as well
 
As it turned out, I changed a few things on this. My jug was pretty poorly finished so I swapped it for an extra I had picked up from a garage sale, had a new piston too. I have some pics somewhere, it would have taken hours of work to make the same jug I had I had sitting there. Didn't really want to have to break in the engine again though.
 
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I got to take my first spin on the new bearings. They seem great and there may even be a small amount of vibration reduction. Since I used the new top end I'm still in break in, but it seems way better than last time, great response all around. I used a slightly thicker base gasket. I'm not the guy looking to add compression. I lean towards economy/longevity rather than top speed, but I do need 30mph and get it. This will be faster than last time I think. No problem in traffic already. My last top end could not do this at break in. Sthil = Good bearing so far.
 
I am hoping that is proves itself a winner. I have an engine on my bench that I want to rework including all bearings and seals.
 
GearNut said:
Thank you very much for showing the comparison of the bearings.
How well does the Sthil bearing fit the con rod with wrist pin installed. Would any honing be involved on the con rod?

I keep pondering sleeves installed on either side of the bearing to take up the space between the bearing ends and the wrist pin bosses of the piston. But... what material to use?
It must be light weight and very durable. I really don't think aluminum would last. Steel would be too heavy unless lightning holes were drilled. Titanium would be expensive but durable enough. Magnesium... I don't really know enough about it's durability to comment.
Something must be done though to prevent the bearing from walking on the wrist pin or early failure will be eminent.
Click to expand...

The best thing I have ever used for this is teflon or other HDPE type plastic capable of withstanding high temps. I have used teflon scrap from industrial work for a pair of washers on the pin to keep the pin bearing centered between the bosses and on the rod journal.
I don't have any scientific theory to prove it, but I'm absolutely POSITIVE the bearing takes a much worse beating when half of it is hanging out in the breeze. Half the surface area if nothing else right?
Besides, I've seen the difference myself. The particular engine I refer to would eat a top end bearing about every three months like clockwork, even using high quality german bearings. When we came up with the washers to hold the bearing centered, it stopped munching parts. Running several years now with no more failures.
I once saw the same thing in here somewhere, so I know I'm not imagining things.
 
I have no idea where to get some teflon doweling but I can get Delrin at the local surplus store. I wonder if it is temperature resistant enough?
 
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