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Cooling Oil Circuit Thermostat.

John Warner

Cruisin' Guzzisti
GT Contributor
Oct 14, 2015
South Bucks UK
I think I've mentioned on here before, that I'm planning on adding a Thermostat to the Cooling Oil Circuit on my Stelvio.

Most, if not all, of the CARC Models are badly over-cooled.
Mine would struggle to get the Oil above 70°C once the Ambient Temps dipped below around 10°C.
I've had my Cooler fully covered, with a cheap Mouse-Mat, Oil Temps are around 115-120°C in the low teen Temps ,105-110°C when it's around freezing.

The Mat is simply tucked under the top end of my home-made Engine 'mudguard', I can adjust to suit the Ambient Temps by sliding it up or down, it stays put at any speed.

Started on the Thermostat build in earnest now.
Stripped the Tank and Plastics off, plus the Forks/Yokes (I'm fitting a Super-Moto style M-Guard to go with the Griso Front Wheel I have), and stripped the Rear (adding a Hugger, and maybe swapping Shocks).

I was unsure where the Thermostat Unit would go (I'm making it myself, using a Mishimoto 93°C Wax-Stat).
I thought I'd have to replace the original Oil Lines, starting from 'scratch' as it were, but after playing with some Hoses yesterday, I found I could keep the originals.
It'll be mounted down behind the Sump-Guard, I'm making up a new one anyway, as the original is badly cracked around the Mounting Holes (common fault), so I can make room for it if needed, plus add a 'Tab' to mount it to.

I have most of the parts needed, just waiting on a Tap for the Hose Unions in the Body of the Unit, and a couple of Right-Angle Hose fittings.

I've gone with AN-6 Hose Fittings, 8.5mm bore (0.38"), 9/16UNF Threads.
All the Oil Lines use the common 14mm x 1.5mm (Pitch) threads for the Banjos, and 16 x 1.5 for the Cooler Inlet & Outlet, which is common to a lot of car items I believe, I have an old Mitsubishi Pajero Cooler that I bought for another Project, and that has the same threads.
Plenty of Converter Fittings available too, for instance to connect the AN-6 Hose-Ends to the 16mm Cooler Threads.

As the Cooler Circuit is Low-Pressure, you could go for Push-Fit parts, just use decent O-Clips for the Hose Connections.
O Clip Pliers
Not much progress yet, Christmas got in the way!
Still waiting on a couple of 90° Hose Fittings to arrive as well.

Started turning down the huge bar of Alloy I'm making the Housing from, will continue when I'm back at Work (much better Lathe there than mine!).
Originally planned it as a single-piece Housing, with a removable 'Cap' at one end, but I've decided a two-piece will be easier to machine, and 'tweak' if necessary, and still only one main joint to seal.
Made a bit of progress with my Thermostat now.
Had previously turned down a big Bar of Alloy roughly to size.

Now I've got it down to the right size, drilled the through-holes for the Bolts that will hold the two halves together, parted it into the two halves, and machined a shallow Boss on each end, to serve as the mountings.

I've bored out one side to accept the Wax-Stat unit, which is slipped into an Alloy Housing, which will act as the 'Valve' to restrict/divert the Oil flow, other side still to be done.
Thermo1a Thermo1b
Still a bit of 'tweaking' to do with the size/shape of the Housings.

Next, I'll Mill flats on the sides where the Unions will be located, top ones are the Guzzi originals from the Cooler, they'll be the inlet and outlet connections on the unit, using the original Pipes.
The black ones are for the new Cooler pipes, all the non-original Fittings are AN-6.
I have Adapters to connect the AN-6 Pipe Fittings to the 16mm x 1.5mm Cooler bores.

When that's done, I can Mill away all the surplus 'bulk' from the two pieces.

The basic design . . .
'Return' Spring not shown, it'll go underneath the Wax-Stat unit.
Sorry for the delay . . .
Nearly done now.
ThermoI ThermoII ThermoIII
Need to make the End-Cap now (to go in the threaded hole).
I decided to make it slightly adjustable, via different sized packing pieces (still to make) that will fit in the cap, and form the location for the Rod of the WaxStat unit.

The larger (16mm) Unions are the Guzzi originals, the original Oil Lines will connect to them.
The smaller (14mm) ones are for the new Cooler Lines.
Bit more progress on the Thermostat project . . .

WaxStat unit sits in the 'Shuttle', the Push-Rod locates in the blind hole in the Spacer, which in turn sits in the End Cap.
Different sized Spacers will give me some 'fine-tuning'.

Component parts of the 'Valve' Assembly.

Top-End assembled . . .

Might turn out to be at the Bottom once mounted, depending which way I decide's best for the Oil-flow.
Milled away as much surplus 'meat' as I could, now Primed ready for Painting.

Was going to seal with an O-Ring, but I didn't leave enough space with the Bolts, So I've made up a Gasket instead.
I've also opened up the Oil Channel in the top section a little.

The Dowty (Bonded Rubber) seals I ordered for the Bolts have arrived (in case of slight leakage past the Gasket edge), Just about ready to fit now.
Sorry for the lack of updates Guys, so many other things to do in the last year and a bit!!
Been running the Thermostat since April 2017, just over 6000miles ago (yeah, I know, I need to ride it more!!).
Works great.
Faster warm-up, and the Oil Temp settles at around 110-115°C in all weathers.
Still drops a bit when it rains, as the Unit is directly in the line of fire of the spray from the Front Wheel.
I need to make a little guard for it.

Thermo2 2
John, another good idea that I am going have to copy. Went for ride today (cool weather) & was surprised just how long it took the bike to reach 100c. Think it will be my winter project.
In any temps below around 15°C, mine would never get to 100°, it usually hung around 75-85°.

Ordered all my parts last night. Looks like the season may be just about over here so a good time to get it done. Last ride was 17C, & yeah, took forever to reach temp.
John, I liked it so much I had to do it. Went pretty well. Just had a hard time deciding where to mount the T-stat. All in all I'm pleased with the way this went.

DSCN2569 DSCN2570 DSCN2571
John, I liked it so much I had to do it. Went pretty well. Just had a hard time deciding where to mount the T-stat. All in all I'm pleased with the way this went.

View attachment 15928 View attachment 15929 View attachment 15930
Nice looking fit. Did you reuse both the original pipes, but extended to the thermostat? I've just bought a Mishimotor stat, and trying to decide the best place to fit it. I guess picking up the bolts on the transmission housing to mount a bracket would work best.
I only reused the hose that went to the heads but did end up changing the fitting on it. The stock fittings at the cooler are not AN & will leak even with some adapters. So they were all changed.
Sump Spacer holds the Thermostat in the 2V Engines.
Have you done this swap, or seen it done?

I don't think the 2V Sump Spacers would be the same as the 4V version.
Does it have the Cooling Oil Circuit Pump in those for example, and the associated components?
Sump Spacer holds the Thermostat in the 2V Engines.
Have you done this swap, or seen it done?

I don't think the 2V Sump Spacers would be the same as the 4V version.
Does it have the Cooling Oil Circuit Pump in those for example, and the associated components?

I have both that I bought for projects. The 2v and 4v sumps are identical except the 4v has no thermostat behind the thermostat housing and has a twin oil pump in place of the 2v single pump. The pick up for the cooling circuit pump hangs down into the sump and is a plastic strainer affair. Castings for both sumps are identical. Pressure relief valves appear to be shimmed the same.
And yes to use correct terminology the sump spacer holds the thermostat.
To clarify. The sump and sump spacer are identical on 2v and 4 v models except for the items I mentioned in my post above.

Updated……thinking about this a little more. I believe the oil cooler on the Stelvio is for the head cooling circuit. Due to the heat produced up top I’m sure full time non t stat controlled oil would be preferable.

That would explain the 8v engines not running a t stat. There’s nowhere for bypassed lubrication circuit oil to go. The oil cooler is already busy with head oil circuit.

I’ll modify my previous statement and say it’s not a simple swap to use a 2v sump spacer on a 4v motor.

Another option though would be to use the 2 v t stat and direct the lubrication oil through the cooler and just run the head cooling oil directly from the existing side port to the head feeds and not through the cooler. But maybe that would be dumb ?

Or run 2 coolers for the 2 circuits but that would be a lot of plumbing.

Hence the existing system. Cooling the heads takes precedence and it most riding conditions the lubrication oil eventually warms to optimum temperature.
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Thanks for the info.

As I explained in my original post, the Oil doesn't usually get up to a 'decent' temperature in cooler, wetter Climates.

The Cooling Pump does indeed just feed Oil into the Heads, via the Cooler.
It's not a Pressure Feed like the one for the Main & Big-End Bearings, it simply floods the areas around the Exhaust Valves, purely for cooling.
The Thermostat doesn't need a 'bypass', it simply directs the Oil destined for the Cooler straight to the Heads, until it reaches the set temp (93°C in mine), when it starts to modulate the flow through the Cooler, before it goes into the Heads.
The Thermostat will be constantly modulating it's position, dependant on the temp of the Oil going through it.

The purpose of the Thermostat in this Circuit, like the Thermostat in a Water-Cooled Engine (and like the Air-Flap system in the old Air-Cooled VW' Ducting), is for a faster warm-up, and a more stable temperature range during running.

A too cool running Engine will wear out much quicker than one that gets up to temp fast, and then stays there all the time it's running.

I used to get a lot of 'Mayo' (Emulsified Oil) in my Valve Covers before I fitted my Thermostat.
You really don't want that in any Engine . . .
I fitted an oil thermostat to my Ducati years ago. On a ride a couple weeks ago, the thermostat started pissing oil. I need to figure out if I can fix it, or simply remove the thermostat and put the circuit back to stock.

On my V11 Sport, I solved the problem another way: I removed the oil cooler altogether (which was necessary for fitting a deep-V sump anyway).

I would think it just needs a Seal/Gasket replacing.

Before fitting my Thermostat, I would cover the Cooler when the ambient temps dropped below 14°C or so.
I fitted an Oil Temp Sensor and Gauge soon after getting my Stelvio.

I used a thick Neoprene Mouse Mat, tucked behind the Guard I'd made to keep crap off the front of the Engine.