Topic: Piston-Pins - 15mm vs 17mm

[Old Scrambler]

We know that Honda used 17mm pins on the later CB900C and other larger displacement motors. When building a CB750 for high-compression and/or large displacement, is the 17mm pin preferred?  Is it stronger?  I'm curious and the answers may be helpful for builders that start small and intend to go big later on. On my own high-compression 762cc build, I used rods with 17mm small ends and bushed them to accept the 15mm pins that match my J&E pistons. If I build a 900+ motor I'm thinking of ordering pistons with 17mm pins

[gschuld]

I'm curious about this as well.  I know that there have been many 100hp plus big bore motors, 1160 strokers and beyond, running high compression, big cams, etc. All on 15mm pins.  I would think that it would be a very impressive 762 motor that could approach thosepower levels.

Perhaps extreme high rpms could account for the extra stress, your motor was intended to run to 11,000 or so right?

George

[dragracer]

Have you ever looked into boring the wrist pin hole on the piston to 17mm versus bushing the rod end?? We do this with the KZ 17mm pistons when using the 83/84 GPZ cranks in the earlier engines. The GPZ's have 18mm rods. Not sure if there's enough material between the wrist pin hole and the oil ring land to open it up from 15mm to 17mm. Seems like good insurance to use the bigger wrist pin on an engine turning 11,000 plus rpm's in an endurance setting as long as the piston isn't compromised.

[Old Scrambler]

George..........my motor already revs past 11,000 under load as tested on a dyno   Now we are tweaking a few things so it may go another few hundred. Peak HP was in the mid-10s.

Drag Racer...........Yes, we thought about boring the piston-pin holes, but determined it was safer to stay away from the ring-lands. On custom-made pistons the thought was to only have 2 rings. My thoughts are that the smaller pins may serve a better and stronger purpose. Less weight, less stress, and I'm thinking it may be better to oil-cool the piston with less retained heat in the smaller steel pin. 

[kmb69]

We know that Honda used 17mm pins on the later CB900C and other larger displacement motors......

Did you mean a CB1000C? The 1982 CB900C, it's last year of production, uses pin 13111-300-000 according to the parts manual. That is a 15mm pin from the SOHC CB750.
Only the CB1000C and the CB1100F use 17mm pins, 13111-413-000 and 13111-KM1-000, respectively, from that era of DOHC's.

[Old Scrambler]

One of these days I'm going to get to the bottom this........or better yet 'on-top'..............I purchased 2-sets of used rods that were advertised as: CB900C although one seller also added CB1000.  Both sets have 17mm pins.  We used the set without squirters due to concern for oil-pump capacity. The other set came with pistons and pins and are packed away for later use. Both sets came complete with rod bolts and shells. 

[bwaller]

Don't shoot the messenger, this is only for perspective. I'm not sure how many years used a 15mm pin, but the 2006 GSX-R1000 used a 15mm pin. Bore & stroke 73x59mm, 12.5:1CR, and 178hp @11,000rpm.

Honestly I don't know but how many pin failures with a SOHC CB750 based N/A engine have been documented. Sure they get destroyed when things go bad. Big Jay has great tool steel pins that ought to handle mucho power.

[bwaller]

The other side of it is with old style pistons needing long 15mm thick walled pins, the weight of a nice tapered 17mm pin wouldn't be any heavier and certainly stronger.

[FunJimmy]

Don't shoot the messenger, this is only for perspective. I'm not sure how many years used a 15mm pin, but the 2006 GSX-R1000 used a 15mm pin. Bore & stroke 73x59mm, 12.5:1CR, and 178hp @11,000rpm.

Honestly I don't know but how many pin failures with a SOHC CB750 based N/A engine have been documented. Sure they get destroyed when things go bad. Big Jay has great tool steel pins that ought to handle mucho power.

Agreed. Are you chasing ghosts or is there a bonified problem with 15mm pins?

[kmb69]

One of these days I'm going to get to the bottom this........or better yet 'on-top'..............I purchased 2-sets of used rods that were advertised as: CB900C although one seller also added CB1000.  Both sets have 17mm pins.  We used the set without squirters due to concern for oil-pump capacity. The other set came with pistons and pins and are packed away for later use. Both sets came complete with rod bolts and shells. 

Dennis, If you have a set of 17mm pin rods without oil "squirters", I'm not sure what you have. I have several sets of 900 rods and they all have oil "squirters" and 15mm pins.

Plus, if you are taking some eBay Seller's description of what they sold you for Gospel, uh oh! Been there, seen that!     

[Big Jay]

A high strength 15mm pin like we made ( I think they are discontinued) will handle 200 hp in a GSXR1000. I believe Russ is using 15s in the Sorcerer and the triple.

[Old Scrambler]

Thanks, Big Jay

Keith, one of these days I'll get the answer to my ROD confusion. 

[Bill/BentON Racing]

413 pin is a CB400T if I remember correctly, Bill

[turboguzzi]

interesting discussion, i think diameter peaked some 10 years ago, arriving also to 18mm at one point.
 later motors than that suzook, like bmw s1000RR are 17, but if a gsxr1000 seems to run ok on 15mm, wouldnt worry too much.

oil jet towards bottom of the pistons surely helps these pins survive better.

[MRieck]

A short, thin walled 17mm pin would be lighter and stronger than just about any 15mm pin. Modern pins have gone the way of thin wall, larger OD but short length. They are light too. Reminds me of rear axles and swingarm "bolts".