HELP request with A112 head plumbing

Frank L. Di Gioia

True Classic
I Bolted a head from a 903/965cc A112 on my 903 engine. I can't remember the correct way to run the temp gauge and thermostat bypass water lines. I'm using an after market gauge with a 1/8" NPT connection. The A112 temp gauge is a tapered thread NOT NPT and it's ohm impedance will not work with my gauge nor can the banjo be fitted. I've never really understood the function of the water bypass banjo. POSSIBLY the temp sensor read a lower temperature if the water level dropped below the sensor tip. The bypass would insure there was actual fluid going to it. ?
My solution is to:
1) drill out A112 temp gauge and fit 1/8" NPT sensor to top of block.
2) route water bypass directly from thermostat base to water pump with a restrictor.
3) plug outlet formerly for heater at front of block. Again it's NOT a NPT sitting but is tapered.
Any suggestions would be appreciated.
 
the thread in the head is a straight thread, 14 x 1.5 from memory but don't hold me to that could be smaller on the little OHV than a SOHC, so just find an adapter that goes from the head to the thread you want to use. loose the banjo bypass thingo, and yes I think it was to read a "mean" temp of the system and keep water to the sender even if the level dropped so it kept reading... and if you want to clean the thermostat housing end up so you can loose the small nipple on that too then use the lower housing from a 127 / a112... though I guess keeping some form of circulation bypass to the water pump is a good thing... the A112 / 127 still had a small hose going there, but it was fed from a different spot the heater metal pipe that attaches to the spigot on the front of the head... so I would think the best spot for water re-circulation and bleeding out trapped air would actually be via the heater fitting at the front of the head (found on 127/a112 heads) if you found a way to plug / reduce the size of the fitting down with another screw in right angled nipple to the small 1/4 or 5/16 inch bypass circulation hose size and route that to the top of the water pump from there, with no restriction, and you've copied the factory later setup

SteveC
 
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the thread in the head is a straight thread, 14 x 1.5 from memory but don't hold me to that could be smaller on the little OHV than a SOHC, so just find an adapter that goes from the head to the thread you want to use. loose the banjo bypass thingo, and yes I think it was to read a "mean" temp of the system and keep water to the sender even if the level dropped so it kept reading... and if you want to clean the thermostat housing end up so you can loose the small nipple on that too then use the lower housing from a 127 / a112... though I guess keeping some form of circulation bypass to the water pump is a good thing... the A112 / 127 still had a small hose going there, but it was fed from a different spot the heater metal pipe that attaches to the spigot on the front of the head... so I would think the best spot for water re-circulation and bleeding out trapped air would actually be via the heater fitting at the front of the head (found on 127/a112 heads) if you found a way to plug / reduce the size of the fitting down with another screw in right angled nipple to the small 1/4 or 5/16 inch bypass circulation hose size and route that to the top of the water pump from there, with no restriction, and you've copied the factory later setup

SteveC

Just an opinion here but on the 850 engines the bypass was to stop the water pump from air locking. The pump was a local "high spot" in the system so air tended to be trapped there. On older well used and not maintained 850 engines the port on top of the pump would plug up and eventually cause overheating.
The 2 hoses used on the 850 was most likely a "design development" of the single hose on the 600, possibly to vent a high spot in the head.
I lost a cylinder head when the water pump vent port plugged up and it suddenly overheated. Took me a while to find the cause, but cleaning the port solved the problem.
Anyway, I think that is how all this plumbing came about.
Edit/Additional guess: Possibly the double banjo was to keep air from the pump from getting to the cylinder head, maybe there was a problem with routing it through the head to the radiator on the 600s.
 
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Those hoses were to ensure there is always some flow through the head.

Starting at the thermostat housing, to the bypass sensor bypass and then to the water pump return side and back through the block to the thermostat until it opens..

Without the bypass the thermostat won’t be able to work properly, it pulls coolant past the base of the thermostat ensuring it heats up properly and opens. Once open, the primary and easy flow through the rest of the system minimizes the effect of the minor flow of the bypass from the thermostat housing.
 
Those hoses were to ensure there is always some flow through the head.

Starting at the thermostat housing, to the bypass sensor bypass and then to the water pump return side and back through the block to the thermostat until it opens..

Without the bypass the thermostat won’t be able to work properly, it pulls coolant past the base of the thermostat ensuring it heats up properly and opens. Once open, the primary and easy flow through the rest of the system minimizes the effect of the minor flow of the bypass from the thermostat housing.

I have to agree, I have been looking at this totally wrong. Not sure now why the plugged bypass caused my problem.
Confusing...
 
Thanks guys,
I think everyone is partly right and Karl is the most so. I started the motor yesterday and ran it static for several minutes till the thermostat opened so it does work without the banjo fitting. NOT having lower pans fabricated yet I shut it down before it overheated. In the past I've always just connected everything back as factory. I can't remember water route with center outlet PBS head though. In Gene's case having the clogged bypass maybe left a "cold" pocket below thermostat long enough to cause enough overheating pressure to kill the head? Again, I think it's a combination of all of the above.
 
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