Wrist pin roller bearing end play

[Notchback]

I am working on a piston design on a 650cc air cooled Yamaha twin. I am using a longer rod in this engine that has a roller bearing on the little end. I need to provide an end play figure to the piston maker. I was thinking .005" should do the job. Does anyone have any thoughts on this? The rod is crank guided and has literally no end play once it is pressed together with the provided crank washers. Thanks for any help that can be provided.

[Kevin Johnson]

http://www.xs650.de/literatur/wsh/pdf/03-motor.pdf

Page 27 lists a 5:1 ratio of big end clearance to small end clearance (at max limits). I am not sure why you would want to deviate from that (apparent) factory ratio?

Roughly, that appears to indicate your small end minimum clearance should be at .0295".

One thing to enquire about on specialist forums is how well/accurately the crank journal indexes to the centerline of the cylinder bores on typical production pieces. This might be the origin of the larger clearance. There are crank shims listed and perhaps this is a means of shifting this alignment. I don't know.

I should also mention that small end play in the spec might refer to rocking the small end of the rod over a small arc rather than my interpretation of a side clearance in a piston. You need to communicate with a 650 expert to clarify that important point.

The forum that this document is hosted on would probably be a good place to ask your question.

[Dan Timberlake]

I'm not sure a roller bearing is the better than a bushing in 4 stroke engines. I'd consider bushing the rods.

As I recall the caged needle bearings (some with spacers beside the rod pin end in the piston guided CZs) in my 2 stroke bikes slip easily between the pin bosses. Say, 0.005 inch total clearance or even more. Does the pin end have slots or holes for oiling?
I might measure the pin boss width of a piston from the donor engine if you are using that needle bearing.

[ZEOHSIX]

There is no way I would ever run a roller bearing in the small end of a connecting rod in a 4 cycle motor if I could avoid it. They arent detonation tolerant, and only belong in a 2 cycle motor because of the marginal lubrication avalible in that engine design. What is this motor? What are you building it for?

[Notchback]

http://www.xs650.de/literatur/wsh/pdf/03-motor.pdf

Page 27 lists a 5:1 ratio of big end clearance to small end clearance (at max limits). I am not sure why you would want to deviate from that (apparent) factory ratio?

Roughly, that appears to indicate your small end minimum clearance should be at .0295".

One thing to enquire about on specialist forums is how well/accurately the crank journal indexes to the centerline of the cylinder bores on typical production pieces. This might be the origin of the larger clearance. There are crank shims listed and perhaps this is a means of shifting this alignment. I don't know.

I should also mention that small end play in the spec might refer to rocking the small end of the rod over a small arc rather than my interpretation of a side clearance in a piston. You need to communicate with a 650 expert to clarify that important point.

The forum that this document is hosted on would probably be a good place to ask your question.

Kevin, thank you for posting the info. I own the manual in question. However, in this case I am not deviating from anything because the engine did not come from the factory with a roller bearing in the little end. I am a member of multiple specialized forums on the Yamahas and they are great for many issues regarding bike and engine assembly but not technically related issues with regard to engine modifications.

[Notchback]

I'm not sure a roller bearing is the better than a bushing in 4 stroke engines. I'd consider bushing the rods.

As I recall the caged needle bearings (some with spacers beside the rod pin end in the piston guided CZs) in my 2 stroke bikes slip easily between the pin bosses. Say, 0.005 inch total clearance or even more. Does the pin end have slots or holes for oiling?
I might measure the pin boss width of a piston from the donor engine if you are using that needle bearing.

Dan, unfortunately I do not have a donor piston but this is a good point and a great starting place. The rod has an oil hole on top for roller lubrication.

[Notchback]

There is no way I would ever run a roller bearing in the small end of a connecting rod in a 4 cycle motor if I could avoid it. They arent detonation tolerant, and only belong in a 2 cycle motor because of the marginal lubrication avalible in that engine design. What is this motor? What are you building it for?

It would be helpful if we could remain on topic here. I am not looking to justify running a roller bearing vs. a bushing. A discussion on pin boss expansion of a 4032 forging would be more helpful. A visit to the topic of what causes detonation may be helpful to you in determining whether a pre or post ignition event can be more destructive to a naturally aspirated engine. View my pressure trace examples. This is not a race motor, it is a 10 to 1 engine on 93 octane with a standard bore. Thanks guys.

[Kevin Johnson]

Kevin, thank you for posting the info. I own the manual in question. However, in this case I am not deviating from anything because the engine did not come from the factory with a roller bearing in the little end. I am a member of multiple specialized forums on the Yamahas and they are great for many issues regarding bike and engine assembly but not technically related issues with regard to engine modifications.

viewtopic.php?f=15&t=26934 suggests .005 to .006" in line with your initial thoughts for a piston guided rod (I know yours is not). Simply use this value as a minima to bracket your analysis.

Again, back to manufacturing tolerances. If you plan on minimizing this value, which the above represents, along with the existing minimal side play value at the big end then you absolutely must know the locative accuracy of the crankshaft assembly with respect to the bore centerlines, hence the piston centerline. If you do not determine this (or spec a large leeway or "blueprint" the motor) then it is quite likely that the installed rod will have a permanent side loaded torque on it which would be quickly disastrous for many types of roller bearings. With such a long production history, it would be unwise to expect a set value.

Perhaps you could get some buddies on the list with standard piston-rod assemblies to use feeler gauges to see what the stock side clearance values are. That would have factory engineering tolerances built in, so-to-speak. If you tighten that up then you need to do careful measuring on your particular block/engine-case and crankshaft.

[Notchback]

Kevin, thank you for posting the info. I own the manual in question. However, in this case I am not deviating from anything because the engine did not come from the factory with a roller bearing in the little end. I am a member of multiple specialized forums on the Yamahas and they are great for many issues regarding bike and engine assembly but not technically related issues with regard to engine modifications.

viewtopic.php?f=15&t=26934 suggests .005 to .006" in line with your initial thoughts for a piston guided rod (I know yours is not). Simply use this value as a minima to bracket your analysis.

Again, back to manufacturing tolerances. If you plan on minimizing this value, which the above represents, along with the existing minimal side play value at the big end then you absolutely must know the locative accuracy of the crankshaft assembly with respect to the bore centerlines, hence the piston centerline. If you do not determine this (or spec a large leeway or "blueprint" the motor) then it is quite likely that the installed rod will have a permanent side loaded torque on it which would be quickly disastrous for many types of roller bearings. With such a long production history, it would be unwise to expect a set value.

Perhaps you could get some buddies on the list with standard piston-rod assemblies to use feeler gauges to see what the stock side clearance values are. That would have factory engineering tolerances built in, so-to-speak. If you tighten that up then you need to do careful measuring on your particular block/engine-case and crankshaft.

Thanks this is helpful.

[MadBill]

Further to Kevin's last post, we encountered a similar situation with an SBF build using NOS NASCAR pistons. We discovered the rod little ends were actually wider than the space between the pin bosses, so we had to clearance the latter. Not wanting to whittle out any more material than necessary, we left only ~ 0.010" per side, but very carefully checked that with the assembled rod pressed up hard against the journal, the rod little end fell naturally into the pin boss gap and didn't bottom out as the rod was shifted left and right along the journal. We've since run a full season without any issues.
Presumably the Yamaha crankcases are split vertically, so to see the underside of the assembled pistons, perhaps the rotating assembly and barrel could be fitted into one half for the purpose.

[Notchback]

Further to Kevin's last post, we encountered a similar situation with an SBF build using NOS NASCAR pistons. We discovered the rod little ends were actually wider than the space between the pin bosses, so we had to clearance the latter. Not wanting to whittle out any more material than necessary, we left only ~ 0.010" per side, but very carefully checked that with the assembled rod pressed up hard against the journal, the rod little end fell naturally into the pin boss gap and didn't bottom out as the rod was shifted left and right along the journal. We've since run a full season without any issues.
Presumably the Yamaha crankcases are split vertically, so to see the underside of the assembled pistons, perhaps the rotating assembly and barrel could be fitted into one half for the purpose.

Thank you, very helpful. I believe the pin bosses expand toward cyl. Wall so this is promising.

[Chris1]

Possibly oversize and shim to accommodate correct centered location - similar to big end.

Are you looking to cure small end galling issues with this engine type?

[Notchback]

Just wanted to re-visit this thread briefly because my pistons have arrived and I am at a point where I need to decide to go crank guided rod or piston guided. I had the piston maker set my little end roller bearing end play to .005" (which I can always enlarge later if I so decide) which subsequently also sets my rod side to side clearance at .005" because the side to side clearance for the rod when measuring at the crankshaft is specified to be between .012" and .024" which the engine builder sets when pressing the crank together. So, even if I set the big end side clearance at .012" my rod will still be guided by the piston. My dilemma is do I open up my little end roller bearing end play to a figure greater than .012", say .020" just to keep the rod as a crank guided piece (which is stock). An issue that concerned me was not getting enough oil to the little end roller bearing. There is an oil feed hole on top of the rod but it is obviously splash lubricated not pressure fed. If the end play is .005" I worry that not enough oil will get in from the side. Once again I'm not trying to turn this into a thread about justifying running a little end roller bearing, just trying to get some input on clearances, rod set-up, etc. Thanks!!

[Kevin Johnson]

Capillary action is your friend.

[piston guy]

.005 end play will have the rod rubbing the pin bosses for sure. I would recommend .030 as a minimum. At that they still might touch slightly. Rod deflection may be more than you are anticipating.