Oil Cooler For 750 Twins?

Back in 1982, I remember I called one of the companies that made aftermarket oil coolers to ask if they made a cooler for my 1979 750 twin.

He said no, they didn't make enough twins to make it worth their $$ to sell a kit for them.

Anybody know if they ever offered one for the 750 twins?

I know the coolers themselves are pretty universal, but it's the fittings to the engine that they have to make. The guy said they have to make a special bleed fitting for the hole where the oil pressure sending unit mounts and a special drain plug with a return hose fitting.

Here's a link to a post over at another forum where a the exact topic was discussed. I hope nobody minds the cross-site reference...

I just looked at the linked thread to the twin owners forum, and according to the text it is a full flow type oil cooler. I'm not going to comment on it until I get a chance to look at my service manual out in the shop. That's not the way I remember the oil routing but my memory is sometimes faulty

If it does work as a full flow system, then that would be a good way to do it.

Looks like he built his own. I was looking for an aftermarket kit that is the "parallel" kind where it runs a little oil off the high pressure outlet of the pump where the oil pressure sending unit goes and returns it to the bottom of the oil pan.

That's called a "Bypass oil cooler".

Do you really want to bleed off oil pressure and volume?

Kawasaki used a full flow oil cooler on the GPz750's and ZR7's.

steell wrote:

That's called a "Bypass oil cooler".

Do you really want to bleed off oil pressure and volume?

Kawasaki used a full flow oil cooler on the GPz750's and ZR7's.

I suppose I really don't want to reduce the internal oil pressure, but flow volume should not change. The guy who designed the coolers said you add more oil to the system to account for what's traveling in the external route.

I'm not sure how much pressure would have to bleed off to make the oil pass through the cooler. maybe only a few PSI, I agree that it's never a good thing to bleed off pressure (especially in an older engine which drops to about 20 PSI at hot idle).

If your oil pump moves 2 gallons per minute, and the oil cooler passes 1 gallon per minute back to the oil pan, that leaves you with 1 gallon per minute of flow through the motor, a reduction of 1 gallon per minute. The extra oil capacity just means it takes longer for the oil to heat up initially.

These numbers are for example only, I don't know the flow capacity of the stock oil pump, or the oil cooler. The pump may have sufficient excess capacity to offset the oil cooler, I just don't know, but I think it should be considered.

I suggest putting an oil pressure gauge on it and checking the oil pressure when it's hot and at cruise rpm, then decide if you think you can take the chance of lowering it.

The pump may have sufficient reserve capacity to handle the loss to the cooler, but the only way to find out is to install it and monitor the pressure.

I'd be interested in seeing the results myself

I have corresponded with a fellow that races a 750 twin, and he told me that he had a problem with wiping out number 1 rod bearing, it appears that there is an internal crank oil passage that delivers oil to the starter clutch and that was bleeding off enough pressure to starve number one rod of oil. His solution was to block that oil passage and it solved the problem.

My concern is that if the stock setup is unable to deliver enough volume to compensate for the oil to the starter clutch under racing conditions, the oil cooler may do the same thing under normal use.

Better safe than sorry, I guess

steell wrote:

....it appears that there is an internal crank oil passage that delivers oil to the starter clutch and that was bleeding off enough pressure to starve number one rod of oil. His solution was to block that oil passage and it solved the problem....

Sorry for side-tracking the thread, but I thought I'd comment...

Another way, for us non-racers, is to leave the starter sprockets and chain in when you remove the starter clutch. The oil journal is under the sprocket on the crank, and leaving the sprocket there will give it just that extra tid-bit of back pressure.
But, if you leave that sprocket there, you may as well leave the other sprocket (and starter) and chain to keep it in line....

(Said like a news anchor...) Back to you, Steell...

steell wrote:

If your oil pump moves 2 gallons per minute, and the oil cooler passes 1 gallon per minute back to the oil pan, that leaves you with 1 gallon per minute of flow through the motor, a reduction of 1 gallon per minute. The extra oil capacity just means it takes longer for the oil to heat up initially.

These numbers are for example only, I don't know the flow capacity of the stock oil pump, or the oil cooler. The pump may have sufficient excess capacity to offset the oil cooler, I just don't know, but I think it should be considered.

I suggest putting an oil pressure gauge on it and checking the oil pressure when it's hot and at cruise rpm, then decide if you think you can take the chance of lowering it.

The pump may have sufficient reserve capacity to handle the loss to the cooler, but the only way to find out is to install it and monitor the pressure.

I'd be interested in seeing the results myself

I have corresponded with a fellow that races a 750 twin, and he told me that he had a problem with wiping out number 1 rod bearing, it appears that there is an internal crank oil passage that delivers oil to the starter clutch and that was bleeding off enough pressure to starve number one rod of oil. His solution was to block that oil passage and it solved the problem.

My concern is that if the stock setup is unable to deliver enough volume to compensate for the oil to the starter clutch under racing conditions, the oil cooler may do the same thing under normal use.

Better safe than sorry, I guess

I don't really know. I remember they used to make high volume oil pumps for chrysler engines that pumped a higher volume of oil at the desired pressure.


I do know on the output side after the pump on my 750 ther is a steel ball bearing with a spring that functions as a "pop off valve" when the internal pressure gets too high. On the low end, I know mine would drop to about 20 psi in the summer when very hot at idle. I would not want to drop it any lower.

The thing about the oil in the cooler circuit, it's only about a quart as opposed to the 4.5 quarts in the engine. To achieve a slight decrease in oil temp (maybe 20 - 30 degrees) would not require you run that "cooler flow" real fast if the radiator is reasonably efficient.

bountyhunter wrote:

I don't really know. I remember they used to make high volume oil pumps for chrysler engines that pumped a higher volume of oil at the desired pressure.

Yep, did, and still do make them for Ford, GM, and Chrysler.

I do know on the output side after the pump on my 750 ther is a steel ball bearing with a spring that functions as a "pop off valve" when the internal pressure gets too high. On the low end, I know mine would drop to about 20 psi in the summer when very hot at idle. I would not want to drop it any lower.

Pressure relief valve, I forget what the opening pressure is, but it's in the manual.

The thing about the oil in the cooler circuit, it's only about a quart as opposed to the 4.5 quarts in the engine. To achieve a slight decrease in oil temp (maybe 20 - 30 degrees) would not require you run that "cooler flow" real fast if the radiator is reasonably efficient.

Calculating that is a whole lot more work than I am interested in doing

If you want a bypass cooler, just tee off the pressure switch and put a fitting in the pan.

Run a pressure gauge and let us know how it goes.

steell wrote:

If you want a bypass cooler, just tee off the pressure switch and put a fitting in the pan.

Run a pressure gauge and let us know how it goes.

I can't add a bypass cooler because I don't think there are any aftermarket coolers available for 750 twins.

That's the whole point of the thread, to find out if anybody knows if there is an aftermarket cooler for a 750 twin. I've never seen one, so I won't be doing any tests.

You can use pretty much any cooler that you like, then run lines as mentioned above.
The only hard part is attaching a bung to the oil pan, and you can just take the pan off and have one welded in.
Making up your own "Oil cooler kit" is no big deal, lot's of folks on here have done it, just not on the 750 twins. 750 twin owners are a minority here after all