Bi pass pipe

[fsk658]

Can anyone tell me the thread size of the metal Bi pass hose that is threaded into the cylinder head. Thanks John

[bmcecosse]

It may be 5/8" UNF. But why do you ask ? It will be VERY difficult to get an old/rusted by-pass pipe out of a cylinder head! You will probably have to shell it out with a drill.

[fsk658]

I am just reconditioning a marina 1275 engine, I have a 1300 GT metro head I aquired years ago which has larger valves and runs on unleaded, that I want to fit to the marina engine. I will drill and tap the head to fit the metal Bi pass pipe so I can connect the water pump bi pass pipe as normal. I have done this before with a 1098 mini / metro head but can`t remember the size tap I used because I was then at work and had access to various taps.

[Matt]

None of the factory 1275 unleaded heads had large valves - so its either a small valve head or has been converted...

[fsk658]

hi its realy the tap size for the bi pass pipe I realy want. I have both heads on my bench and the metro head has definately got larger inlet valves. Also David Vizards book quotes the MG metro head inlet at 1.4in and the marina at 1 5/16in

[bmcecosse]

As above - large valve heads are NOT 'unleaded' - but the good news is - it doesn't matter! Just run the engine on unleaded anyway (no additives) - just set the exhaust valves to 15 thou gap and check/reset every 3000 miles. As for the by-pass - you are FAR better without it! Don't drill /tap the head - just fit a pump with no bypass - or - as I have done, just put a plug in the bypass hose. The heater will warm up MUCH more quickly and will be much more effective.

[katy]

Can't you just measure the thread on the new nipple?

[Declan_Burns]

It' a strange thread 5/8" x 16 UNS not UNF. The S refers to special and if you look at the cost of a UNS tap you will soon realize that you could buy a cylinder head for the same money! I doubt if you have such a tap lying around your workshop. Removing the old stub can be difficult and I doubt if you can get it out in a re-usable condition. When I fitted my 940 head I ran into exactly the same problem. To remove the old stud we welded a 10mm rod with a 90° bend onto the stub and then heated the head slowly with a blow lamp and with a pipe 10mm inner as an extension on the rod, -off it came. Some of the stubs being sold have the wrong thread! I bought a stainless steel one and it was OK. As far as I can remember it has 16tpi and not 18tpi (UNF). Blocking it off as Roy suggested is not a bad idea.
Regards
Declan

[bmcecosse]

I gather the head has no by-pass pipe Declan. And I suggest that's a GOOD thing!

[fsk658]

Thanks for the info guys, bit disapointed that its not an unleaded head! I was informed years ago that the later metro heads could take unleaded, bad info. Never mind might put back the marina head.

[bmcecosse]

If the new head has the larger inlet valve - it will give more power and is worth having. Don't worry about it not being 'unleaded' - the engine will be fine - just set the 15 thou exhaust valve gaps and check every 3000 miles.

[Declan_Burns]

I gather the head has no by-pass pipe Declan. And I suggest that's a GOOD thing!

Roy,
I must admit I have fitted the stainless steel stub with a silicone hose and no problems up to now but if there's any sign of problems I will block it off.
Regards
Declan

[bmcecosse]

It's not a case of 'problems' - but with no bypass the heater starts working much more quickly - and much more water is pumped around the heater circuit!

[Declan_Burns]

Sure, I agree but I have a mini heater and it works really well-do you want to see me roasted??
Regards
Declan

[Stig]

Well, I've got a Mini heater and it's still not up to the job (yes, I flushed everything really well when I fitted it). So a question about this bypass hose, surely it's there to stop the water pump working into a dead load when the heater valve is closed and the thermostat's not open. If I were to block it off (which sounds like a good idea this time of year) surely I'd have to do something different in the summer unless I want to leave the heater on and roast?

I'm asking this as a sort-of rhetorical question but accept that my understanding might be flawed, so have I missed something?

[bmcecosse]

True enough - but the pump is not a 'fixed displacement' pump - so it would just stall. The official answer is to drill 2 X 1/8" holes in the rim of the thermostat and then you can shut the heater supply valve without worrying too much. But for winter use - best to use an 88 degree stat with NO holes - swap to a 74 degree stat with the holes for summer.....

[fsk658]

As stig said the bi pass must be there for a reason. I thought it was to stop damage to the pump impellors when the thermostat is closed otherwise causing cavitation wihich will eventually damage the pump. I have the round heater in my car and it has always been very hot which I believe is unusual for this type. John

[katy]

As stig said the bi pass must be there for a reason.

The by-pass is there to avoid hot spots in the block, by allowing the coolant to circulate in the block untill the thermostat opens.

[bmcecosse]

Until Rover realised after many years it wasn't necessary - and eliminated it! 2 holes in the stat rim is all that's required - but as long as the heater valve is open - these aren't necessary.

[IslipMinor]

Until River realised after many years it wasn't necessary - and eliminated it!

Roy,

Which engine did they do that on?