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gs850 bored 3mm over, oil cooler necessary?

Chuck78

Chuck78

Forum Sage
Past Site Supporter
I am putting 79 GS850 jugs on my 77 GS750, & boring it out to 72mm to run some old MTC Engineering 10:1 big bore pistons. I think 850's are 69mm stock bore, & 70mm +1.0mm factory os pistons being the largest.
This puts me at 1.5mm shaved off the cylinder sleeve walls for a total of 3mm overbore.
I've been told by some that an oil cooler is needed when going big bore for most applications like this, & a few others said I could get by on this one without. What do you all think???
I'll have to dig out my 79 850 jugs later & measure sleeve's wall thickness. I think 3.9mm is the average for most. I did however measure the 81-83 GS650G jugs that I just picked up for a gs550-to-673cc project, & the wall thickness when I removed the head gasket was a whopping 6.8mm or more, wow! Those engines were higher compression than all of the older gs series engines, 9.4:1 vs 8.7 or 8.8. This could possibly be telling me something about Suzuki's engineering thoughts at the time as far as the wall thickness versus cooling capacity. Maybe I shoulda looked into an 80-86 gs850 wall thickness.
 
Just because the pistons are larger that doesn't mean there is going to be more heat. Now if you twist the throttle more to use the extra power, that's a different story.

Generally speaking, the shim and bucket GS engines are very robust and an oil cooler is not needed. The oil temperature can hit in excess of 300F which is a bit alarming, but empirical evidence suggests the engines tolerate this condition without complaint. If you want the added assurance of a cooler, you would do well to add a set of 750 pump gears to offset the pumping losses of all the extra plumbing. Synthetic oil is also a good hedge against heat since it doesn't thin out as much as mineral oil in elevated temperatures.
 
the wall thickness measured on the top of the block is not the wall thickness of the liner. that includes the oversize flange on the liners to stop them moving up and down in the block when it is all clamped together.
you need to maesure the wall thickness at the bottom, when the block is removed from the engine.
 
Unless you will only be riding at ambient temperatures of less than 60 degrees Fahrenheit, it is always a good idea to add an oil cooler to an air-cooled engine.
Conventional Dino oils break down severely at temps over 240 F.
They have a propensity to coking...

http://www.exxonmobil.com/lubes/exxonmobil/emal/files/TTopic6_OilCoking1.pdf

Of course synthetic motor oils are less prone to coking at even higher temperatures.
With an oil cooler, your stator will also cool better and is less prone to failure.
 
GSScoobie said:
Unless you will only be riding at ambient temperatures of less than 60 degrees Fahrenheit, it is always a good idea to add an oil cooler to an air-cooled engine.
Conventional Dino oils break down severely at temps over 240 F.
They have a propensity to coking...

http://www.exxonmobil.com/lubes/exxonmobil/emal/files/TTopic6_OilCoking1.pdf

Of course synthetic motor oils are less prone to coking at even higher temperatures.
With an oil cooler, your stator will also cool better and is less prone to failure.
Click to expand...


What's "coking"? At first I thought it may hane been a typo, but twice.
Something I have never heard of.
 
i think he is referring to carbon build up, in the UK we say a heavy carbon build up is "coked" up.
more widespread use of the word involves 2 stroke engines
 
Last edited:
Octain said:
What's "coking"? At first I thought it may hane been a typo, but twice.
Something I have never heard of.
Click to expand...
"Coking" might not have been the proper term to use, but I think he is referring to the process whereby the oil breaks down and carbonizes a bit. The carbon bits might be "coke" and are VERY abrasive and destructive.

.
 
The 550e has an OEM oil cooler
the 550L has none but has a deeper sump.

Identical engine otherwise.

There are much larger air cooled engines out there without coolers.
I say no to the cooler unless you are going in heavy stop and go conditions.
 
The eight valve engine had better air flow through the head.
The newer ones didn't.
Oil coolers keep the engine from warming up completely, they are for saps who like leaks.
 
Agemax said:
i think he is referring to carbon build up, in the UK we say a heavy carbon build up is "coked" up.
more widespread use of the word involves 2 stroke engines
Click to expand...

Been goofin with smokers for years. Never heard of this term.
 
Lots of shim and bucket GS bikes around here without oil coolers that have run in excess of 100,000 miles. That right there should tell you that they are not necessary.

The later generation GS's with oil coolers also had a thermostat built into the plumbing, something that's difficult to implement with an aftermarket kit.

My personal opinion is to skip the cooler unless you plan to slog your bike though traffic on a regular basis in high ambient temperatures. Coolers are prone to leakage, reduce oil pressure (unless different gears are installed), and will over cool the oil in some conditions unless a thermostat is used.
 
I don't run an oil cooler on my 10.5-1 GS850. But.....it doesn't get hammered in high ambient air temp conditions.;)
 
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