To Bore Or Not To Bore

I also have a Shetland 640 with the bmc 1.5, and an enfield outdrive.
I fully rebuilt my engine with a rebore and new pistons (+30thou)
Many of the bearings etc are common with early MG engines.
valve guides are common to MGB
Since I have the same boat, feel free to PM me for any info you need.
 
I rebuilt a mark I Escort engine in my youth with my Dad - I remember hours lapping the valves by hand (There is a tool you can get for a drill these days to make it easier) but we never bothered with that little ridge, I did run some ultra fine emery over it (dipped in a tiny bit of oil to prevent too much scoring) because it is where the old rings stopped travelling up the bore - as long as you aren't changing the stroke at all, then the new rings (if you use new rings) will also stop in the same place it shouldn't be an issue.

However if you can afford it, a re-bore and new rings shouldn't be that dear and should ensure the thing runs better. One tip (and it's heresay no evidence at all) but when I re-built that old engine, I smeared all the bearings with "Slick 50" before I reassembled them - the thinking being it contains PTFE which is supposed to bond to the metal when it all gets hot (but my logic was that it might just help lube it during first start-up.

I don't know if it made any difference at all, but once re-built and re-started, the engine forever seemed to run better and smoother than it ever had before......... May be a bit of "wishful thinking" may be just my imagination, but it certainly seemed to make a difference.....
 
Looks like pitting on some of the bores from corrosion where the pistons were sitting. Quite far down so not so critical. You have three options, do it properly which means re boring and new pistons. Measure the crank journals and if in spec new shells, if not re grind. Strip the head and check valve guide clearance etc . Doing this properly will cost but give you a like new engine.
Hone the bores, only replace the shells if they show much wear and stick it back together. Minimal cost and it may work out ok. You may also want to strip the head.
Buy a used, running, engine.
 
Thanks for everyone's input! This is such a great site and great community of boat enthusiasts.

@gordmac - yes, these look like my three options. Being an engineer myself, I'd rather do it properly. So I'm pricing up re-boring.

So far I've had one quote which was £220+VAT. How does that price sound to people?

One question: if they're re-bored, my understanding was that I can just use oversized rings. But some places I've read that I also need to use larger pistons? For example, looks like you got larger pistons @Hunterlees ?

By the way @Hunterlees, that's very useful to know we have the same boat. I think our stern drive is a different model. Do you have any images of your 640? Ours has lots of potential for renovations :)

Some great tips there @ontheplane. I loved your story about the old Escort.

@john_morris_uk some good advice I think there. No I'm definitely no veteran in this area. A complete newbee and still quite youthful in my ways, so I appreciate your experiences. Unfortunately re-boring seeming more pricey than this. But might be worth it, as you say. Yes, the hot wash videos are interesting. That's the next thing I'm thinking about is how to clean her up best. I'm in an indoor workshop with this, so I can't use a pressure washer.

Thanks everyone! I will update.
 
21FtFreedom said:
Thanks for everyone's input! This is such a great site and great community of boat enthusiasts.

@gordmac - yes, these look like my three options. Being an engineer myself, I'd rather do it properly. So I'm pricing up re-boring.

So far I've had one quote which was £220+VAT. How does that price sound to people?

One question: if they're re-bored, my understanding was that I can just use oversized rings. But some places I've read that I also need to use larger pistons? For example, looks like you got larger pistons @Hunterlees ?

By the way @Hunterlees, that's very useful to know we have the same boat. I think our stern drive is a different model. Do you have any images of your 640? Ours has lots of potential for renovations :)

Some great tips there @ontheplane. I loved your story about the old Escort.

@john_morris_uk some good advice I think there. No I'm definitely no veteran in this area. A complete newbee and still quite youthful in my ways, so I appreciate your experiences. Unfortunately re-boring seeming more pricey than this. But might be worth it, as you say. Yes, the hot wash videos are interesting. That's the next thing I'm thinking about is how to clean her up best. I'm in an indoor workshop with this, so I can't use a pressure washer.

Thanks everyone! I will update.
Click to expand...
You definitely can't use oversize rings in standard pistons in a bored out engine. I believe you're get a lot of piston slap and the engine wouldn't last very long.

That sounds pricey for a rebore. Try shopping around. Whereabouts in the country are you? I wouldn't necessarily go for the cheapest option though. There are some real cowboy engineering shops about.

The cheapest oversize pistons I've found are £82.25 plus VAT each. Farm engineering supplies seems be a good source. (That's nearly half the price ASAP are charging!)
 
Last edited:
john_morris_uk said:
You definitely can't use oversize rings in standard pistons in a bored out engine. I believe you're get a lot of piston slap and the engine wouldn't last very long.

That sounds pricey for a rebore. Try shopping around. Whereabouts in the country are you? I wouldn't necessarily go for the cheapest option though. There are some real cowboy engineering shops about.

The cheapest oversize pistons I've found are £82.25 plus VAT each. Farm engineering supplies seems be a good source. (That's nearly half the price ASAP are charging!)
Click to expand...

Hi @john_morris_uk this is good to know. And definitely adds to the equation.

Let's say I can get the re-bore done for £200
Plus £100 (inc VAT) per oversized piston

So, we're talking £600, before I even look at anything else. At which point, buying a refurbed engine looks more and more appealing.

However, the honing tool I bought was £30. And, because of this stupid pandemic, I have a lot more spare time on my hands. So, I think it's well worthwhile giving the honing a try and seeing what kind of compression I can get on a cold crank test.

What d'ya think?
 
ontheplane said:
I smeared all the bearings with "Slick 50"
Click to expand...
Many years ago I was nearly taken in with slick 50 claims, until I Googled it.
Turns out their "miracle" product clogged many oil filters as the particles were bigger than the holes in the filter and were sued numerous times .
Nasa decided to test their claims along with dupont (inventors of Teflon? ) and they stated they could see no benefit of adding Teflon to an engine whatsoever !
 
Second hand engines always sound attractive because you dream of trouble free power, but you always have to ask why it’s being sold.
I assume you’ll try and clean up the rings and pistons and hone the bores and use the old head gasket just to try out the compression (I would be wary of throwing money at new head gaskets just to try it out!?)
How are you going to measure the compression? You need to borrow a Diesel engine compression tester as the cheap and cheerful petrol ones you hold in the spark plug hole are no where near up to the job. I’m afraid I sold my Diesel engine compression tester last week!
Just asking...
 
I suppose you could compare how hard it is to turn with and without the head. I suspect if it will start cold it will have enough compression to use.
 
The engine stopped production some 40 years ago so I would be surprised if any parts are "original"
They certainly are not original on your link.
A lot of bmc replacement parts are manufactured in India.
 
Rappey said:
The engine stopped production some 40 years ago so I would be surprised if any parts are "original"
They certainly are not original on your link.
A lot of bmc replacement parts are manufactured in India.
Click to expand...
You may well be right.
My own oversized pistons were sourced from Funchal after a long internet search - I got two sets still sealed in their original Hepolite boxes, and sold one set on Ebay for more than the two sets cost me.
 
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