Thorneycroft T90 1.5 and 1.8 ..... or just re-engine?

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mattnj

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Is the T90 (1500) and T108 (1800) the same size engine does anyone know?
I'm wondering if it is the same block etc, just a different stroke/bore.

I have a T90 which works ok, but is in a bit of a state and leaks oil and has other serious cosmetic issues, it needs pulling out and sorting/painting etc, but as it is a bit under powered anyway for the boat, I am wondering if I can find an 1800, rebuild that at home then swap them out. Are the mountings/gearbox mounts all the same?

Alternatively I may go for a new MarinePower unit, but that's a load of work when there isn't really anything wrong with mine (other than a slight lack of power for the boat and oil leak)

Or can I buy a recon/exchange T90 or T108 somewhere?
 
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You don't say what the boat is however I would stick with the engine you have and before you do anything find out the internal condition of the engine before you spend money.
I was going to buy a new engine for the boat I have just bought, (I also have a Thornycroft) Perhaps I am fortunate in that when these BMC Diesels were being made (1970-80') I was selling BMC J4 Diesel vans with hundreds of thousands of miles on them with these engines in them (Marinised as a Thornycroft) and I very rarely had a guarantee claim so when I bought my present boat (as a non runner) although I budgeted for a New Chinese Engine I had the engine "Compression Tested and the compressions were excellent and only needed new injectors and glow plugs.
With your engine I would have a Compression Test and if good have the engine out and new gaskets to fix the oil leak,
And when you say "lack of power" many boats are underpowered although my 24ft boat has the Thornycroft 50hp engine. Also a BMC 2500 Diesel.
 
Jim@sea said:
You don't say what the boat is however I would stick with the engine you have and before you do anything find out the internal condition of the engine before you spend money.
I was going to buy a new engine for the boat I have just bought, (I also have a Thornycroft) Perhaps I am fortunate in that when these BMC Diesels were being made (1970-80') I was selling BMC J4 Diesel vans with hundreds of thousands of miles on them with these engines in them (Marinised as a Thornycroft) and I very rarely had a guarantee claim so when I bought my present boat (as a non runner) although I budgeted for a New Chinese Engine I had the engine "Compression Tested and the compressions were excellent and only needed new injectors and glow plugs.
With your engine I would have a Compression Test and if good have the engine out and new gaskets to fix the oil leak,
And when you say "lack of power" many boats are underpowered although my 24ft boat has the Thornycroft 50hp engine. Also a BMC 2500 Diesel.
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Moody 36....it could do with the extra hp from the 1800 really....hence why im asking....if im going to all the hassle of taking it out....thought i could get some more hp too.....plus if i source a new one...i can take my time and keep using what i have whilst i rebuild in a warm garage!
 
They are great engines and I'd be surprised if you couldn't find a nicely rebuilt unit without the ancillaries or gearbox for well under 2k. But use the spelling Thornycroft when googling, otherwise you'll get results for a district in Southampton ;)
 
mattnj said:
its the oil leak that's really annoying....in the bilges, such a mess!
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Where is the leak coming from?
The worst culprit with the T90 on my last boat were the oil filter O rings which I could never get the seat properly. It was the old style with a paper element.
I bought a modern filter housing adaptor that takes spin-on cardridges and never looked back.
The tappet box gaskets are an easy fix. The only other source of escaping oil is the breather or the sump gasket, both of which would be more challenging.
 
Gordonmc said:
Where is the leak coming from?
The worst culprit with the T90 on my last boat were the oil filter O rings which I could never get the seat properly. It was the old style with a paper element.
I bought a modern filter housing adaptor that takes spin-on cardridges and never looked back.
The tappet box gaskets are an easy fix. The only other source of escaping oil is the breather or the sump gasket, both of which would be more challenging.
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Its in a right state anyway....needs to come out for some love.
Does anyone know if the 1800 is the same....that would be idea....refit it with a few more HP.
 
If it's in a moody 33 then you may have difficulty in getting a bigger prop in and if you can't get the prop to fit you can't use the extra power
You may not even be able to use what you have
I believe they have direct drive low shaft angle and minimum clearance !
 
mattnj said:
Its in a right state anyway....needs to come out for some love.
Does anyone know if the 1800 is the same....that would be idea....refit it with a few more HP.
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Just adding more power will not necessarily improve performance. As scottie says a major limitation of the 33/36 hull shape is the lack of clearance between the shaft and the hull which limits the size of propeller you can swing. It is the prop that moves the boat and more power needs a bigger prop, preferably larger diameter. The nominal 35hp of the T90 is enough to achieve hull speed comfortably and you are unlikely to see any improvement with a more powerful engine, although you may be able to cruise at lower revs for the same speed.
 
Tranona said:
Just adding more power will not necessarily improve performance. As scottie says a major limitation of the 33/36 hull shape is the lack of clearance between the shaft and the hull which limits the size of propeller you can swing. It is the prop that moves the boat and more power needs a bigger prop, preferably larger diameter. The nominal 35hp of the T90 is enough to achieve hull speed comfortably and you are unlikely to see any improvement with a more powerful engine, although you may be able to cruise at lower revs for the same speed.
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We are only talking a few more hp....with a kiwi prop so i can tweak the blade angle...not talking about putting 500hp in....just 36hp instead of 30hp at 3000rpm
As im taking out and sorting then if an 1800.fits....then i dont see a disadvantage.
 
Extra power is not necessarily for extra speed but the ability to maintain cruising speed in adverse wind and wave conditions. Even an extra half knot is useful if punching a strong adverse tide.

I had a 35hp engine in my Westerly Berwick which was ideal so I consider 35hp too much power for a 33/34ft boat. In fact I would prefer a little more.
 
Many canal boats use these engines, so it may pay you to talk to someone from that world.

See http://www.canaljunction.com/boat/engines.htm

Re swapping a 1.5 for a 1.8. This can be done but from [ distant ] memory the rear crankshaft oil seal is different so your backplate may require some machining. I did this so as to be able to use 1.5D J4 gearbox on a MGB engine.
 
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1.5 BMC block has a pukka rear oil seal.
some 1.8's do not,have an oil throw instead.Important only if engine is mounted inclined aft,when it would make a mess!Is the Moody engine mounted level?
 
TQA said:
Many canal boats use these engines, so it may pay you to talk to someone from that world.

See http://www.canaljunction.com/boat/engines.htm

Re swapping a 1.5 for a 1.8. This can be done but from [ distant ] memory the rear crankshaft oil seal is different so your backplate may require some machining. I did this so as to be able to use 1.5D J4 gearbox on a MGB engine.
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IIRC, the 1.5 has a 3 bearing crank and the 1800 5 bearings. Also, heard several reports of heads cracking on the 1800.
 
mattnj said:
We are only talking a few more hp....with a kiwi prop so i can tweak the blade angle...not talking about putting 500hp in....just 36hp instead of 30hp at 3000rpm
As im taking out and sorting then if an 1800.fits....then i dont see a disadvantage.
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Pretty sure you won't get a Kiwi prop in. They are 15" diameter+ and I think your limit is 11 or 12" because of the shallow angle of the shaft and the flat bottom of the hull.

You need to start from the size (diameter) of prop you can fit and work back to the shaft speed and hp to drive the prop. Measure the distance between the centre of the shaft at the prop end, multiply by 0.85 and double it - that will give you the maximum diameter you can fit. So, if the distance is 7" the maximum diameter is 2*(7*.85) = 13" approx. Ideally for you size boat displacing 6700kg and a LWL of 30' you need 35 hp, a 2:1 reduction and a 15" prop. So ideally the distance you need is 9.75" to swing a 15" prop. However, pretty sure you will find you have a 1:1 box with an 11 or 12" prop so you will need to increase the pitch to absorb the additional power, but there will be no noticeable difference in maximum speed, just the engine will run at lower revs to produce the same power as your existing engine. The amount of power a diesel engine produces at given revs is a function of the load placed on it by the prop, not what it is capable of producing at those revs.

Just to put it in perspective although the potential of the bigger engine is small in absolute terms it is a 20% increase, which is only of value if you can fit a prop that will use it.
 
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