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Fitting Pistons into Bores - A recent experience


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I post this as others may find it helpful ...

Assembling the pistons into the block is a job I have done many times over the years. I use a band type clamping tool to compress the rings which is a common type. It’s basically a spiral wrapped tube of thin steel which can be tightened round the piston to compress the rings. The rings on my current pistons are very thin and I managed to break 2 on my first 2 attempts. This was puzzling because I had never had this problem before. I managed to solve it as follows. First. I  drizzled (sorry, been watching too many cookery programmes !) oil between the thin steel leaves of the tool so that they could slide over each other more easily and thereby close down tighter on the piston. Secondly, I ditched the quarter inch cross section L shaped winding handle supplied with the tool and used a ratchet from my socket set (frustratingly this would only fit into the tool at one position so be aware if you try this). Using my ratchet also enabled me to tighten the tool further. Having got the tool tightened on the piston and the piston resting in the bore ready for insertion, I then turned the tool a few degrees backwards and forwards to settle the piston rings in their grooves. Having done all this the pistons went into the bores very easily. I think that because I had not used the tool for some time, the rubbing faces had dried out and this prevented the tool working efficiently.

It is possible to buy/make a machined sleeve of the correct diameter and these are supposedly much better but cost and size availability are a problem ….

Keep your tool oiled!

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If you generally only work on a couple of types of engines (therefore not needing to cope with lots of different sized pistons)  it would be fairly easy to turn a tube of the correct diameter with an internal chamfer near the end to help get the rings started.  If I was doing this I'd want to start with a large, thick-walled pipe.  

Edit: ^ That's a better idea.

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It’s worth turning a reverse chamfer on the bottom of the inserter to lock into the chamfer on the top of the liner on the K, this eases the ring transition, the beauty of the inserter is that it does not grip the pistons like a band clamp and it is perfectly round (unlike a band clamp). This means that minimal force is needed to insert the piston to the bore, any resistance is then easily felt and the risk of ring breakage is virtually zero. I have made them in various sizes from old liners, they been trusty aids for the last 20 odd years. Another useful tool is a rod guide made from an old big end bolt soldered to a length of central heating tube to guide the rod onto the crank pin as the rod is deeply recessed in the K series and hard to reach.


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Roger, I suspect from your description that your ring compressor is the same as mine and I must admit to having occasionally felt it jumping as I’ve tightened it on the rings ... really good advice to give it a good oiling :)

Many many years ago I rebuilt a Mini Cooper engine using a ring compressor made from a baked bean tin and some jubilee clips and it worked perfectly, strange that now having the proper tools brings a whole new set of problems!



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I have used both methods recently.

I've always used the steel band type in the past and no particular problems with it, but I do always oil it very liberally. The trick seems to be to tap around the top of the band first to make sure it's fully settled flat on the top of the liner, then press it down firmly while tapping the liner through to make sure the rings can't pop out as they pass from the band into the liner.

For my most recent build, Dave/Oilyhands kindly lent me one of his tapered inserters and it was certainly very easy to use and I can see it stood a lot less chance of scuffing the piston or damaging the rings. I think need to get myself one for the future. It's definitely a better way to do it if you can get / make / borrow an inserter when you need it.

As for the rod guide; I tend to insert my pistons with the engine as accurately vertical as I can get it on an engine stand and unusually put the bottom end together with the crank pins vertically top and bottom (i.e. No 1 at TDC) which gives clear access to all the big end bolts from underneath. I'm also careful to orient the pistons correctly and I've never had a problem getting the big ends to just settle and seat naturally onto the crank pins, especially if everything is liberally libricated. I'm always very careful to tap them very lightly to make sure I never end up knocking a corner of the rod into a crank pin.

I guess the big problem with this method is it doesn't really work when the engine is installed in the car as it is canted over at an angle and there's not much you can do about it. In these cases, with the use of liberal lube of the piston and liner, I've managed to tap the pistons into the two cylinders where the crank pins are at the bottom, tapping them half way down the bore then gone under the car and pulled them down into engagement from below with my fingers. The ones at the bottom are readily accessible. After fitting the big end caps I've then turned the engine through half a turn as described below and done the other two. A little tedious maybe but safe.

Once I've done up the big ends I then nip a head down (either the head to go on the engine without the cams fitted, or I have a scrap old soft head I keep just for the purpose) using an old gasket to make sure the load is spread appropriately, which frees the crank and allows me to turn it to safe position before reinstalling the assembled head. You only need to tighten the bolts to say the initial 20Nm to get the crank to free up. I usually just do them "reasonably tight" by hand in sequence.

As well as making sure whatever you use is well oiled it's important to ensure that it's absolutely and immaculately clean. Building an engine recently I ended up with some fine scratches on the bores which didn't look right, so I took the pistons out again and washed them in a jug of solvent. There were some fine particles left behind in the bottom of the jug. These were freshly built up pistons with new rings. The liners had been thoroughly washed and scrubbed in hot soapy water after honing and wiped down with solvents before lubricating with clean engine oil to fit the pistons. I can only think that after being so careful to ensure that all my parts and tools were scrupulously clean I'd casually fitted the scrap head to allow me to turn the engine without thinking that it had been sitting in my garage gathering dirt and dust and some had got into the cylinders. Lesson learned!

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