R75/6 engine / Oil temp

Andey
Posts: 89
Joined: Wed Sep 02, 2020 12:45 am

R75/6 engine / Oil temp

Post by Andey »

Hi all,

Andey here again, it’s been a while since logging in.

Straight to the point, how hot should the engine (looking more at the crankcase, rather than cylinders) be running at? I know we cant gauge the engine itself, so should I be looking at the oil temp?

I might be imagining it, but the crankcase seems to be a lot warmer than the cylinders at a medium powered run out… nothing like 80mph.

Of course the cylinders have the fins, but I always remember it being around the same temp as the crankcase and vice versa…

I’ve been reading that it should be around 220-260F, or is that just everyone says?

The bikes been riding great, slowly taking her further out, but still need to overhaul the gearbox at some point, most likely will happen in the next 2 months or so and will redo a compression test.

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schrader7032
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Location: San Antonio, TX

Re: R75/6 engine / Oil temp

Post by schrader7032 »

I think your number is in the ball park...maybe closer to 200-230F would be a good operating temperature and I'm speaking of the oil...not sure about the case. I suppose you could get one of those cheap infrared temp readers and take some spot readings around the bike just to get a baseline.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

Andey
Posts: 89
Joined: Wed Sep 02, 2020 12:45 am

Re: R75/6 engine / Oil temp

Post by Andey »

Good to hear from you Schrader, hope all’s well!!

I’ll take it for a few more rides and see if anything changes…

I’ll shoot it with a temp sensor after the ride to see if there’s any insights that way…

I did get a cam down the pistons and I’m wondering if the cylinders are passing oil past the rings…

Is that too much carbon build up on the exhaust valve side? It’s been about 800kms since I had the heads off
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schrader7032
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Location: San Antonio, TX

Re: R75/6 engine / Oil temp

Post by schrader7032 »

I'm sure someone has more experience in reading the tops of pistons that I do. There is going to be some oil getting by the rings if you're still in the break-in process, which at 800kms sounds like it...I'm assuming you replaced rings then. Rings have to seat plus there's a way to orient the three rings to minimize seepage. But I don't think it's an exponential build up in the chamber...I'm sure it stabilizes as some point unless you have way too much oil usage which you would soon know if you see smoke from the engine and/or you have to put a lot of oil in.

Here's a picture of my left cylinder after about 90K miles. It's certainly covered in build up but I don't think it was excessive. If you do get excessive build up you'll begin to hear pinging when you had not before.
Attachments
LeftGasket.jpg
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

jimcarrr100rs
Posts: 37
Joined: Sun Oct 13, 2019 2:47 pm

Re: R75/6 engine / Oil temp

Post by jimcarrr100rs »

Your piston looks wet in the picture. It should be dry like the other picture.
77 & 83 R100RS, 56 R69

Andey
Posts: 89
Joined: Wed Sep 02, 2020 12:45 am

Re: R75/6 engine / Oil temp

Post by Andey »

It’s wet from the oil I think… but there’s no smoke

I’m wondering if the carb is set rich, but I’ve had no running issues.

Oh and Schrader, I think the head gasket is on backwards. The pushrod holes don’t line up with the gasket.
Last edited by Andey on Sun Jun 19, 2022 5:51 am, edited 1 time in total.

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srankin
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Location: Spencerport, NY USA

Re: R75/6 engine / Oil temp

Post by srankin »

So start up the engine, get it good and warmed up, take a long ride with little idling and when you get home pull straight in and shut down the engine ASAP. Pull the spark plugs, if they are black, you are running a rich mixture or burning oil, if they are wet and black, same thing.

A engine in proper tune with no oil consumption will have light tan colored insulators on the spark end of the plugs. It is a lot easier to pull plugs and check them than pulling the top of the engine off to check.

Did you do a compression check? Shot rings show up real fast as a problem with low compression.

No matter how clean you get the tops of the pistons after riding a bit you will still get carbon built up on them. Kurt's picture shows a normal piston.

All engines burn some oil, it is deposited there as normal, if there were no oil on the inside of the cylinder, the rings would seize to the wall of the cylinder, the big issue is how much oil being burned is too much? St.
Owner of a 84, R80RT and 78, R100RS

Andey
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Re: R75/6 engine / Oil temp

Post by Andey »

Let me have another look inside the cylinder.

See if I can take another picture.

And I’ll check the spark plugs too.

Andey
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Re: R75/6 engine / Oil temp

Post by Andey »

The plugs seem ok.

Some soot around the edge which I think will be the oily situation I have.

Again. No smoke and seems to be running well.

But I definitely think the oil is passing the rings…
There’s a small pool of oil at the bottom of the cylinder. The amount, I imagine is the cylinder walls collecting the oil as the piston isn’t TDC.

I’ll do a leak down test as soon as weather clears up here.
Attachments
But an oily glistening head, same on both sides…
But an oily glistening head, same on both sides…
Left plug
Left plug
Right plug
Right plug

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srankin
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Location: Spencerport, NY USA

Re: R75/6 engine / Oil temp

Post by srankin »

The picture of your plugs if you did what I suggested show a well tuned no oil burning bike. Perhaps a touch lean but I would run it like it is.

What were the results of the compression test? Low numbers mean the rings are shot or not seated and oil can get into the combustion area.

Like I said, there is always oil in the combustion area because of the wiping action of the rings, if there was NO oil, the rings would seize up. Tighter rings and better machining mean less oil burned but there is always some burned.

So, if you have the rings installed properly, and the cylinders are in good cylindrical shape, have good compression, the spark plugs are tan, what more can a person ask for? Ride it. St.
Owner of a 84, R80RT and 78, R100RS

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