To Bore Or Not To Bore

[21FtFreedom]

Hi guys

I'm quite the newbee when it comes to engines. I'm just figuring this out as I go along, so appreciate everyone's patience and advice.

A couple of years ago, a friend and myself bought this 'project boat' for next to nothing - a 21ft Shetland 640.

Some compression testing showed that we had lost compression on all four cylinders, to varying degrees.

Two years - and a global pandemic - later, I've finally stripped her down.

What did I find inside? Black gunk and a sort of carbon residue all over the top of the cylinders / pistons. No wonder we weren't getting any compression!!

I've taken all the pistons out. And now I'm trying to figure out my next step.

Should I have them bored? Can I get away with some kind of home-brewed honing method? Or should I just buy a refurbished block with everything already fixed?

I've attached some photographs of the cylinders. Running my hand down the surface, I'm some what confident that the discolouration you can see is only staining.

Right at the top though, there's definitely a lot of pitting. You can see the black / brown sections. But I wonder how important this is, since the seals on the pistons don't seem to rise this high at any point?

Any advice anyone can give me is much appreciated. Especially if you've had any experience tackling something like this before. Anecdotal advise is always useful.

Or if you have advise on how much I should be paying for re-boring.

Many thanks in advance.

 

[Rappey]

How old and what hp is the engine ?

 

[21FtFreedom]

How old and what hp is the engine ?

Hi Rappey

I forget the exact HP, but something around 37. It's a Thornycraft BMC 1.5

I think the boat is around 1975-77 or thereabouts. So I guess the engine is as old as that. Unless it was swapped out at some point; which is more than possible.

Thanks

 

[21FtFreedom]

I've seen a lot of videos of people doing things like this:

Measuring the bore to see how 'out-of-round' it is.

Is this kind of thing really necessary? Or are these people like just really anal about squeezing every last bit of compression out of their engine?

Thanks

 

[waynes world]

see what the bore size is and then see what the next size up piston rings are you can get. yoiu may be able to hone then out, if not a rebore is called for.

 

[Restoration man]

I would pop round to an engine reconditioning place most are very helpful and will measure the bore for you wile you’re there , and will advise you of the best corse of action for your engine

 

[21FtFreedom]

see what the bore size is and then see what the next size up piston rings are you can get. yoiu may be able to hone then out, if not a rebore is called for.

Thanks @waynes world - I'll have a look

I'm noticing that there's a ridge at the top of each of the cylinders. I thought it was just part of the design initially, until I came across some forums where they talk about how it's formed over time.

Anyone got any tips for removing this ridge?

I see a lot of Forums and videos from the US, using something called a 'ridge reamer'. Cheapest I've found for one of these is about £50, which is a bit pricey for a tool I'll probably only ever use once. So if anyone's got any other ways to remove this, I'm all ears!

 

[Rappey]

Omg.. I wrongly presumed it was an outboard engine on a shetland...
If it were an old outboard probably not worth the time and money as an old one still eats petrol for breakfast
But since its a bmc.. Rebuild it ! Once rebuilt it can last longer than if you were to buy a brand new diesel engine today !
28 yrs ago when i bought my boat project my bmc pistons were seized in the bored due to rust.. Having no money back then i beat the pistons out with a block of wood, used a hammer and screwdriver to knock the seized rings out of the piston grooves, used emery cloth to de rust the bores and piston grooves , bought new rings, big and small end bearings. This was a cheap and temporary measure but 28 yrs later and a 55 year old engine and its still running perfectly.
The beauty of a bmc is that all parts are crazy cheap and available for the 1.5 as there are so many of them still around .
injectors can be bought for £50 each, a reconed injector pump is around £300 ..
Asap supplies has most parts in stock , calcutt and a few others specialise in these engines..

As restoration man suggests, take it to a engine reconditioning place and see what they say.

 

[lynallbel]

Top and tail it, hone the bores best as possible, but do not remove any more material than needed does not have to be perfect ( it is not a racing car engine!) new rings/shells/seals and see how she goes.

 

[21FtFreedom]

Omg.. I wrongly presumed it was an outboard engine on a shetland...
If it were an old outboard probably not worth the time and money as an old one still eats petrol for breakfast
But since its a bmc.. Rebuild it ! Once rebuilt it can last longer than if you were to buy a brand new diesel engine today !
28 yrs ago when i bought my boat project my bmc pistons were seized in the bored due to rust.. Having no money back then i beat the pistons out with a block of wood, used a hammer and screwdriver to knock the seized rings out of the piston grooves, used emery cloth to de rust the bores and piston grooves , bought new rings, big and small end bearings. This was a cheap and temporary measure but 28 yrs later and a 55 year old engine and its still running perfectly.
The beauty of a bmc is that all parts are crazy cheap and available for the 1.5 as there are so many of them still around .
injectors can be bought for £50 each, a reconed injector pump is around £300 ..
Asap supplies has most parts in stock , calcutt and a few others specialise in these engines..

As restoration man suggests, take it to a engine reconditioning place and see what they say.

Hi @Rappey
I loved your response. Definitely gave me some hope when I read your own story.
I too would prefer to have this old diesel inboard engine. Apart from anything they make a lovely sound and remind me of childhood trips to the local Island.
The rings on my pistons were not seized, fortunately. They move above and come out very easily.
I think I'm going to give the honing a try!

 

[21FtFreedom]

Top and tail it, hone the bores best as possible, but do not remove any more material than needed does not have to be perfect ( it is not a racing car engine!) new rings/shells/seals and see how she goes.

Hi @lynallbel

Thanks for your response. When you say "top and tail it', are you referring to the ridge that appears to be at the top of the cylinders? (see attached photo)

My understanding is that this is the original cylinder diameter; with the rest of the cylinder being 'worn larger' over time. Is that correct?

I don't see one at the bottom.

So far, the only thing I've seen to remove this is a dedicated 'ridge reamer' tool. But this costs around £135!! (Engine Cylinder Ridge / Step Removal Tool Ridge Reamer | eBay). Or I can pay £70 and wait for it to come from the States. Seems a lot for what will probably be a single-use tool.

Do you have any tips on other ways to remove this ridge? I'm guessing this is what you meant by "top and tail"?

Many thanks!

 

[21FtFreedom]

I would pop round to an engine reconditioning place most are very helpful and will measure the bore for you wile you’re there , and will advise you of the best corse of action for your engine

Thanks @Restoration man

I'll definitely consider this. One of the difficulties I've got the at moment is how I would actually get it out out to a machine shop! The bloody thing's so big and heavy.

But now that I'm down to just the block, maybe it's possible.

Cheers

 

[ARE]

Hi guys

I'm quite the newbee when it comes to engines. I'm just figuring this out as I go along, so appreciate everyone's patience and advice.

A couple of years ago, a friend and myself bought this 'project boat' for next to nothing - a 21ft Shetland 640.

Some compression testing showed that we had lost compression on all four cylinders, to varying degrees.

Two years - and a global pandemic - later, I've finally stripped her down.

What did I find inside? Black gunk and a sort of carbon residue all over the top of the cylinders / pistons. No wonder we weren't getting any compression!!

I've taken all the pistons out. And now I'm trying to figure out my next step.

Should I have them bored? Can I get away with some kind of home-brewed honing method? Or should I just buy a refurbished block with everything already fixed?

I've attached some photographs of the cylinders. Running my hand down the surface, I'm some what confident that the discolouration you can see is only staining.

Right at the top though, there's definitely a lot of pitting. You can see the black / brown sections. But I wonder how important this is, since the seals on the pistons don't seem to rise this high at any point?

Any advice anyone can give me is much appreciated. Especially if you've had any experience tackling something like this before. Anecdotal advise is always useful.

Or if you have advise on how much I should be paying for re-boring.

Many thanks in advance.

That needs a rebore, I would not be happy with just a glaze bust on this. But like others say, any good engine reconditioning company would advise better.

 

[Rappey]

You need to find out what size the bore is and what size the rings are, and if they are within tolerances or needing a rebore and the next size up in rings..
Its certainly worth doing as once done the engine should be good for a long time..
Plan B could be this?
Buy Canal Boat Engines from Calcutt Boats

 

[QBhoy]

Personally...I’d put an engine in it, more like what was originally in it from new. Treat her to a nice used 3.0 mercruiser or VP gm engine. Let that lovely hull do what it was meant to do !

 

[lynallbel]

Hi @lynallbel

Thanks for your response. When you say "top and tail it', are you referring to the ridge that appears to be at the top of the cylinders? (see attached photo)

My understanding is that this is the original cylinder diameter; with the rest of the cylinder being 'worn larger' over time. Is that correct?

I don't see one at the bottom.

So far, the only thing I've seen to remove this is a dedicated 'ridge reamer' tool. But this costs around £135!! (Engine Cylinder Ridge / Step Removal Tool Ridge Reamer | eBay). Or I can pay £70 and wait for it to come from the States. Seems a lot for what will probably be a single-use tool.

Do you have any tips on other ways to remove this ridge? I'm guessing this is what you meant by "top and tail"?

Many thanks!

Top and tail it is a term from years gone by when engines needed rebulding/decoking all the time, bascially means replace most wearing parts.
You could even get stepped piston rings to take into account the wear step at the top of the bore.

 

[Bran]

Hard to say from the pictures but I would hone the cylinders first, see what they look like then. You can measure the bores and compare the diameters against service manual data, also check for ovality. If in spec for diameters, carefully remove a piston ring and put this into the bore and check the end gap, this will confirm if you need new rings. Generally the ridge should not be an issue, as mentioned above you can use stepped rings.

 

[john_morris_uk]

Stepped bore, bore surfaces scratched and scored? IMHO honing and stepped rings leave a real danger of the engine not lasting long and burning oil at the least.
A rebore shouldn’t be more that £120 or so and with new pistons and rings it’ll last for years. Check the main journals and big ends while it’s all in bits. New bearing shells are not expensive. If the head is good with valves nicely ground in and no leaky valve stems it just a matter of how skilled the OP is at cleaning everything and putting it together.
(I’ve just paid £50 for a specialist engine place to put a gearbox casing and ancillaries through their hot wash and acid dip to make them like new.)
Try looking at homemade engine hot wash videos on YouTube and other clips about how clean engine parts need to be before reassembly.
My apologies if I’m teaching grannies to suck eggs, but I remember having a few failures and disappointments as a callow youth when I was starting to build engines.

 

[PCUK]

Top and tail it, hone the bores best as possible, but do not remove any more material than needed does not have to be perfect ( it is not a racing car engine!) new rings/shells/seals and see how she goes.

I totally agree. You can buy a proper bore hone for a few quid. It's basically a great little engine that will see you out!

 

[PCUK]

Stepped bore, bore surfaces scratched and scored? IMHO honing and stepped rings leave a real danger of the engine not lasting long and burning oil at the least.
A rebore shouldn’t be more that £120 or so and with new pistons and rings it’ll last for years. Check the main journals and big ends while it’s all in bits. New bearing shells are not expensive. If the head is good with valves nicely ground in and no leaky valve stems it just a matter of how skilled the OP is at cleaning everything and putting it together.
(I’ve just paid £50 for a specialist engine place to put a gearbox casing and ancillaries through their hot wash and acid dip to make them like new.)
Try looking at homemade engine hot wash videos on YouTube and other clips about how clean engine parts need to be before reassembly.
My apologies if I’m teaching grannies to suck eggs, but I remember having a few failures and disappointments as a callow youth when I was starting to build engines.

That is also good advice. Depends on your budget.