BN1 Tyre Diameter

[zblu]

I know this has been asked before, but can someone tell me the original diameter?

From memeory around 26"?

another question, when changing out the flexible oil hose from block to capillary pipe to oil pressure gauge on dash does the tubing have to be bled all the way to the gauge?

thanks in advance

 

[Bob_Spidell]

re: "From memeory around 26"?

Pretty close; maybe a little more.

re: "another question, when changing out the flexible oil hose from block to capillary pipe to oil pressure gauge on dash does the tubing have to be bled all the way to the gauge?"

Some think it's required, but where does the oil in the hose/tube go when the engine stops and pressure goes to zero (hint: there isn't a check valve in the hose). Short answer: no.

 

[Keith_M]

re: "From memeory around 26"?

Some think it's required, but where does the oil in the hose/tube go when the engine stops and pressure goes to zero (hint: there isn't a check valve in the hose). Short answer: no.

This is an interesting question. What is actually in that hose??? You definitely don't need to bleed the hose, but I have always assumed that the hose full of air, not oil. Air should be able to transmit the pressure to the gauge just fine, and if it's full of oil, and where did the air go if the hose was full of air to begin with?

 

[Bob_Spidell]

This is an interesting question. What is actually in that hose??? You definitely don't need to bleed the hose, but I have always assumed that the hose full of air, not oil. Air should be able to transmit the pressure to the gauge just fine, and if it's full of oil, and where did the air go if the hose was full of air to begin with?

My assumption: the fitting for the flexible hose is tapped into an oil gallery. When the engine stops, all oil drains into the pan (that's why it takes a couple seconds for pressure to register on the gauge when the engine starts). At engine start, the gallery fills and is pressurized and oil is pushed up into the flex line. Since the line is sealed, any air in the line gets compressed, to exactly the pressure of the oil (it 'equilibrates'). So, what you're actually measuring in the gauge is a bubble of compressed air, at the same pressure as the oil. No point in priming the line, the oil drains back when the pump stops anyway.

 

[Keith_M]

My assumption: the fitting for the flexible hose is tapped into an oil gallery. When the engine stops, all oil drains into the pan (that's why it takes a couple seconds for pressure to register on the gauge when the engine starts). At engine start, the gallery fills and is pressurized and oil is pushed up into the flex line. Since the line is sealed, any air in the line gets compressed, to exactly the pressure of the oil (it 'equilibrates'). So, what you're actually measuring in the gauge is a bubble of compressed air, at the same pressure as the oil. No point in priming the line, the oil drains back when the pump stops anyway.

I think you're probably right. So... the ideal gas law states that PV=nRT. Assuming 1 atm of pressure in the line when assembled, when you have 60 psi of oil pressure (almost exactly 4 atm), the air in the line should be compressed to about 1/4 of it's original volume, so really most of the line (3/4) is full of oil. I live at 4500 ft where the atmospheric pressure is about .85 atm. Assuming this was the starting pressure in the line shows that about 4/5 of the line is actually full of oil.

I can't imagine how this information could be useful, but at least I'm not curious about it anymore.

Keith

 

[Simon TR4a]

My 165/15 tyres measure 25 inches in diameter- I think this is the same size as your Healey.

 

[Rob Glasgow]

Moss Motors has an article on their site that discusses modern tires for our cars. The 100 originally came with 5.90 x 15 tires with a diameter of 25.62". The closest modern equivalent is either 165/80R15 at 25.39" or 175/80R15 at 26.02. The 175 size is very difficult to impossible to find today.

 

[Bob_Spidell]

Got an interesting email from Denis Welch Co. the other day. Seems Avon has reclassified their 185/70 tire as a 175/70 tire. Same tire, just now called a different size. I suspect a 'large' 175/70 tire is close in size to a 'normal' 175/80.

 

[zblu]

Hi All,

Thanks for the replies, all reassured me that what I had done was correct.

Started motor and cracked junction of flex and copper tube so as to bleed (old flex had oil in it) so as to duplicate what was there before.

Gauge works same as before.

Thanks for pointing out the Moss page on tyres Rob

Regards All