GS 850 Piston Options

[49er]

I became frustrated that after nearly 30 years of their existance, owners of 850's only had a choice of two oversized pistons, at the stock 8.8-1 CR. These are 0.5 and 1mm O/S.

I approached John Noonan at JE Pistons to make me a set of pistons at 2mm O/S (71mm), with a 10.5-1 CR. My idea was to leave the squish area as stock, the valve pockets at standard depth and maintain the original piston weight.

I made a mould of the standard chamber and piston dome to confirm the calculations of area/volume, and give John a base to work from for the final product. I asked him to get the weight of the new pistons as close as he could to the stock weight.

The pistons arrived fresh off the CNC mill and they were "things of beauty".
I remember thinking what a shame it was to cover them with cylinders, a head and carbon.

John got the weights to within 9 grams of stock. I carefully removed the extra weight off the inside of the crowns, avoiding going under 4 mm in thickness, especially around the valve pockets. Some ali was also removed directly under the gudgeon boses where they mold into the crown.

Heavy duty wrist pins were supplied and they also weighed 9 grams heavier than the stock pins. My calcs figured that these pins could survive a range of CR's up to around 16-1. I ended up buying new stock pins and ground them back in length to match the circlip positions. The new pins were now 1.5 grams lighter than stock. This meant that I only needed to remove 7.5 grams from the pistons to replicate the stock weight.

These JE Pistons are 5mm shorter in length than the stock ones.
The reason for retaining the stock weights was that I didn't want a slightly out of balance engine. Spliting the crankcase at such a moderate mileage was not in the plan.

As a result of increasing the capacity and CR, the torgue has increased significantly in the 4000-9000rpm range.
There is now a huge increase in engine braking on decel into corners.

When pricing this conversion, I compared the cost of these pistons to a set of 2 Hog aftermarket pistons. They compared slightly cheaper in NZ dollars. The other advantage is you can custom to any parameters you wish. I never intend to alter any other tuning parameters, hence the decision to retaining the stock valve clearances and squish area.

I run this setup with the stock VM26SS carbs, airbox and a Cycle Works 4-1 slash pipe.

After 1500kms, I highly recommend JE Pistons as a serious contender for a performance option for the 850 engine. This mod has turned an already peppy performer into something quite special.

I am prepared to share my technology at no cost to other GSR members should they wish to fit a set of these pistons at their next rebuild.

 

[bwringer]

VERY interesting!

One of the most common questions around here is "how can I get more power out of this 850?" and until now, the only answer was "find a 1000G or 1100G engine".

Any problems with detonation/pinging? Can you use pump gas? What octane (might want to provide a translation into US octane numbers if you can)?

Any trouble starting, or is the stock starter motor adequate?

What sort of jetting changes were needed?

Where did you find/how did you make a 2mm OS head gasket? Did you use heavy duty head studs? If so, where did you find them?



And finally, the BIG question...

How much?

 

[chiphead]

Yea, exactly what bw said.

 

[49er]

VERY interesting!

One of the most common questions around here is "how can I get more power out of this 850?" and until now, the only answer was "find a 1000G or 1100G engine".

Any problems with detonation/pinging? Can you use pump gas? What octane (might want to provide a translation into US octane numbers if you can)?

Any trouble starting, or is the stock starter motor adequate?

What sort of jetting changes were needed?

Where did you find/how did you make a 2mm OS head gasket? Did you use heavy duty head studs? If so, where did you find them?



And finally, the BIG question...

How much?

Brian
I spent some considerable time last night doing a reply to this, only to have it lost, through the site being shutdown for maintenance. I know maintenance must be done, but can't we have a provision to save any incoming posts.

Briefly, ignition timing is stock including centrifugal advance. No detonation problems using 98 octane. Still using NGK B8ES plugs.
No jetting changes, only adjustment to pilot circuit.
No starter problems. Stock Vesrah head gasket 72mm O/D bores. Stock studs torqued to 30ft lbs.
Cost US$680 for pistons, rings, HD pins and circlips.
PM me for more details. Early re-torque of head is vital.
Cheers
Ian

 

[49er]

Brian
I spent some considerable time last night doing a reply to this, only to have it lost, through the site being shutdown for maintenance. I know maintenance must be done, but can't we have a provision to save any incoming posts.

Briefly, ignition timing is stock including centrifugal advance. No detonation problems using 98 octane. Still using NGK B8SE plugs.
No jetting changes, only adjustment to pilot circuit.
No starter problems. Stock Vesrah head gasket 72mm O/D bores. Stock studs torqued to 30ft lbs.
Cost US$680 for pistons, rings, HD pins and circlips.
PM me for more details. Early re-torque of head is vital.
Cheers
Ian

I apologise for the tone and abruptness of my previous reply.
As a result, I need to clarify a few points.

Pops used Premium pump gas(octane rating not recorded but would have been aroung 100+ in that era, and leaded) on his Yoshi GS1000's when running 11.5 CR. He used an electronic ignition utilizing the stock static advance and stock automatic advance timings. He did add 1 deg extra advance at 7000rmp. This is the reason I started with stock settings on my 10.5-1 setup. I thought that I may have to slow the mechanical advance rate by stiffening the springs, but found that was not the case. By running 98 octane pump gas, I have had no detonation issues.


When I said that there were no jetting changes, I meant that there was no change from what I was running with the 4-1 pipe (before fitting the new pistons). I'm not sure what the Mikuni sizes are for my jetting (Keith pointed out the differrences between the Mikuni and Dyna sizing methods in another thread), but can tell you their orifice diameters. My pilot jets are 0.4mm and the main jets are 1.06mm. I didn't record the needle numbers but they are set in the fourth groove. I have a set of metric jet drills that I have used for many years on other tuning projects. I just re-drill until I get the results I'm after. I have a reliable formula, so I don't need to do much drilling.

The mention of retaining the stock squish area in my first post, was referring primarily to the depth of the area. The domes on the new pistons start in exactly the same position as the stock pistons, relative to the hemi-spherical chamber. The only difference is an additional 1mm extra width of squish all the way around the chamber (caused by the increase in diameter of new pistons).

The note about re-torqueing the head early needs clarification. Vesrah gaskets for the 850 have large rolled over holes around the oil galley studs (outer studs each end). These are fitted with individual oil seals and are designed to sit snugly inside the large holes on the main gasket. The accuracy of their fit varies from stud to stud. There is potential for oil and or compression to bypass/leak in this and adjacent areas of the head gasket. For this reason I decided to do 2 re-torques, the 1st at 750kms and the 2nd at 2000kms. When I did the 1st re-torque, these were the only studs that required significient re-torqueing. They needed an extra 1/2 turn to regain the correct torque. IMO there is a likelyhood of gasket failure in this area if your re-torque is left too late. This would apply to a lesser degree for OEM rebuilds as well when using Vesrah head gaskets.

Many members will balk at the price of this option. It is costly, but custom means you can set the parameters to suit your whims. I don't expect many will want to go down this path, but I am willing to help others that do.