26 replies to this topic

[JonnyAlpha]

Hi;

I have searched the topic and found some info but none complete hence the questions.

My 1275 A+ Metro block has just been bored to +40 (+20 was not enough to clean up cylinder one).
I am fitting Nural P21253 +40 pistons, that are on the way from Mini Spares, had to exchange the +20's that I purchased.

As I understand it a 1275 +40 is 1311cc (2.821"/71.65mm) so that should be what the machinist has bored it to? He did the machining based on his long standing experience of many years and knowing that we would be using Nural Pistrons (we had a +20 set at the time) we did not have the +40s to hand?

I don't have the pistons yet but have been trying to do some calculations (just out of interest) but nothing is making sense. The Nural Catalogue on pg 88 lists the bore size for the 1.3 pistons as 70.62mm, however if the diameter of a standard 1275 piston is 70.50 that would give a gap of 0.12mm (just under 5 thou?) Anway I'll wait until I actually get the pistons.

The Crank has been ground -10 thou on the mains and the journals and I have ordered some +10 oversize bearings to fit (just arrived as I write).

So the next step after the rebore I understand is the dry build, but I would also like to check clearances on the bore as well. Before doing so can I get some advice please on the following:

1. Do I do the dry build without the piston rings?

2. I have read that it is best practice to use linished down piston pins for the dry build, I asked the machinist who said why not just press the pistons on the actual pins as they won't be coming off?

3. Do I need to fit all four pistons to calculate check the deck height, or just the one?

4. Do I use the Engine Build Lube for the dry build or will Engine Oil Do? (I have Torco Engine Build Lube)?

5. If the answer to question 1 is no, then when and how do you check the piston ring gap?

6. For a 1310 using P21253 pistons what should the bore size be for the recommended gap of 0.003"

7. I have read that when working out the deck height I should aim to get the piston top flush with the deck, however I have also read that setting the piston to 0.005 below deck height will help if ever a later rebore to 1380 is considered? Comments would be appreciated?

Many thanks for any input

P.S. Seems strange that, considering how long the A Series engine has been modified, there is no complete guide to engine building? Or am I looking in the wrong places. If anyone can point me there it would be great. I have the Bill Sollis DVD but he skips this bit

Edited by JonnyAlpha, 29 August 2019 - 09:50 PM.

[mini13]

measure the piston "A" dimension,  this is at the bottom of the skirt 90 degrees to the pin, google it. measure the bore, and calculate the clearance. if its ok accordiding to the piston manufacturer specs then you can go ahead and fit the rods to the pistons, and check the deck height & crank clearances. once youve checked all that then you can put the rings in the bores and check the end gaps, then fit the rings to the pistons, and assemble accourding to the manual

 

watch out for things mike mains caps not being snug on the dowels. if the dowel;s are suspect look at the swiftune billet ones, they are SNUG and pretty cheap.

[grizzler73]

This is what I think, in RED

 

1. Do I do the dry build without the piston rings? Yes, as mini13 said check the clearances carefully and get the block bored if nessersary, you should always buy the pistons and then get the block bored to suit.

 

2. I have read that it is best practice to use linished down piston rings for the dry build, I asked the machinist who said why not just press the pistons on the actual pins as they won't be coming off? AC Dodd will sell you a set of dry build pins.

 

3. Do I need to fit all four pistons to calculate check the deck height, or just the one? All, then you might need to swap them about with the rods to get the deck gaps evened out. Measure at the pin ends not perpendicular to them

 

 

4. Do I use the Engine Build Lube for the dry build or will Engine Oil Do? (I have Torco Engine Build Lube)? Oil is fine, Lube can't hurt if you already have it.

 

5. If the answer to question 1 is no, then when and how do you check the piston ring gap?

 

6. For a 1310 using P21253 pistons what should the bore size be for the recommended gap of 0.003" Measure the pistons properly, it's the only way

 

7. I have read that when working out the deck height I should aim to get the piston top flush with the deck, however I have also read that setting the piston to 0.005 below deck height will help if ever a later rebore to 1380 is considered? Comments would be appreciated? It all depends on your desired CR, chamber size etc. there is no simple answer to this.

Edited by grizzler73, 29 August 2019 - 01:46 PM.

[GraemeC]

My thought below.  Ultimately depends just what you want from the engine - optimising it will be best but takes longer and costs more.

 

1. Do I do the dry build without the piston rings? Yes.

 

2. I have read that it is best practice to use linished down piston rings pins for the dry build, I asked the machinist who said why not just press the pistons on the actual pins as they won't be coming off? Depends how good you intend your dry build to be.  If you intend to mix and match pistons & rods to get the most even piston heights in then you need to use dummy pins.  If you are working to factory build tolerances then press them straight on.

 

3. Do I need to fit all four pistons to calculate check the deck height, or just the one?  Ideally Yes.  Whether you've optimised the pistons to the rods or not (see above) - how do you know if there is any variance and which one you should use for your deck height calcs if you don't fit all 4?

 

4. Do I use the Engine Build Lube for the dry build or will Engine Oil Do? (I have Torco Engine Build Lube)? Build lube.  You're only putting it on the shells at dry build stage as you're not fitting the rings (see 1).  When final build comes use engine oil on the rings (and toi fit the piston/rod assemblies and build lube everywhere else (build lube won't allow the rings to bed in properly).

 

5. If the answer to question 1 is no, then when and how do you check the piston ring gap? Just prior to final assembly

 

6. For a 1310 using P21253 pistons what should the bore size be for the recommended gap of 0.003"?  Measure the pistons as described by mini13

 

7. I have read that when working out the deck height I should aim to get the piston top flush with the deck, however I have also read that setting the piston to 0.005 below deck height will help if ever a later rebore to 1380 is considered? Comments would be appreciated?  Ideally flush for best performance, but 5 thou down allows a bit of fail safe if the deck ever needed surfacing later.  You might lose about 2 bhp.

[DeadSquare]

As this is your first time, buy a pair of piston ring plies.

[JonnyAlpha]

measure the piston "A" dimension,  this is at the bottom of the skirt 90 degrees to the pin, google it. measure the bore, and calculate the clearance. if its ok accordiding to the piston manufacturer specs then you can go ahead and fit the rods to the pistons, and check the deck height & crank clearances. once youve checked all that then you can put the rings in the bores and check the end gaps, then fit the rings to the pistons, and assemble accourding to the manual

 

watch out for things mike mains caps not being snug on the dowels. if the dowel;s are suspect look at the swiftune billet ones, they are SNUG and pretty cheap.

 

Piston Clearance Sorry where did you get the 'A' dimension from? Anyway yes I recall reading that pistons are slightly oval and wider at the base hence measuring the skirt 90 degrees to the gudgeon pin.

 

Ring Gap Just watched all four of the Janspeed DIY Mini videos on Youtube (made in the 80's),  measuring the piston ring gap is shown in detail, I hadn't even thought of putting the ring in the bore after taking it off of the piston.

 

Main Cap Dowels  Didn't know these came out - thanks

[JonnyAlpha]

As this is your first time, buy a pair of piston ring plies.

 

Good call - again never seen this tool, not expensive. Two main types, so I watched a video, one supports the ring and the other doesn't.

 

Amazon Link

[JonnyAlpha]

 

This is what I think, in RED

 

1. Do I do the dry build without the piston rings? Yes, as mini13 said check the clearances carefully and get the block bored if nessersary, you should always buy the pistons and then get the block bored to suit.

 

That was the original intention, we bought the +20s based on the original plan as endorsed by the machinist. He then started cylinder one and called me to say +20 was not enough. We had agreed that I would take a look first and conform that I could exchange the pistons. When I popped up to see him,  I found that he had done all four cylinders to +40 before I had even ordered the new pistons. Lets hop he has not taken them out too far!!

 

2. I have read that it is best practice to use linished down piston rings for the dry build, I asked the machinist who said why not just press the pistons on the actual pins as they won't be coming off? AC Dodd will sell you a set of dry build pins.

 

Understood - may need to invest in a set.

 

3. Do I need to fit all four pistons to calculate check the deck height, or just the one? All, then you might need to swap them about with the rods to get the deck gaps evened out. Measure at the pin ends not perpendicular to them

 

I thought it was imperative to keep the Con Rods in the original order. Not quite sure I follow this, are you saying that if one piston is higher (or lower) than the rest moving it to a different position could change its height. Is this due to rod stretch from new? When you say measure at the pin end do you mean place the DTI gauge on the crown above the gudgeon pin? 

 

4. Do I use the Engine Build Lube for the dry build or will Engine Oil Do? (I have Torco Engine Build Lube)? Oil is fine, Lube can't hurt if you already have it.

 

5. If the answer to question 1 is no, then when and how do you check the piston ring gap? Sorted

 

6. For a 1310 using P21253 pistons what should the bore size be for the recommended gap of 0.003" Measure the pistons properly, it's the only way

 

Happy

 

7. I have read that when working out the deck height I should aim to get the piston top flush with the deck, however I have also read that setting the piston to 0.005 below deck height will help if ever a later rebore to 1380 is considered? Comments would be appreciated? It all depends on your desired CR, chamber size etc. there is no simple answer to this.

 

Yes there appear to be different trains of thought. Down the deck for better burn and potential later re-bore, flush for better performance?

 

 

My responses in blue

Edited by JonnyAlpha, 30 August 2019 - 04:13 PM.

[JonnyAlpha]

My thought below. Ultimately depends just what you want from the engine - optimising it will be best but takes longer and costs more.

1. Do I do the dry build without the piston rings? Yes.

Sorted tks

2. I have read that it is best practice to use linished down piston rings pins for the dry build, I asked the machinist who said why not just press the pistons on the actual pins as they won't be coming off? Depends how good you intend your dry build to be. If you intend to mix and match pistons & rods to get the most even piston heights in then you need to use dummy pins. If you are working to factory build tolerances then press them straight on.

So if this was a new engine / new rods etc no need to mess around, but as the rods are used, although I thought rods should be retained in their original positions, them mixing and matching may be required?

3. Do I need to fit all four pistons to calculate check the deck height, or just the one? Ideally Yes. Whether you've optimised the pistons to the rods or not (see above) - how do you know if there is any variance and which one you should use for your deck height calcs if you don't fit all 4?

Understood

4. Do I use the Engine Build Lube for the dry build or will Engine Oil Do? (I have Torco Engine Build Lube)? Build lube. You're only putting it on the shells at dry build stage as you're not fitting the rings (see 1). When final build comes use engine oil on the rings (and toi fit the piston/rod assemblies and build lube everywhere else (build lube won't allow the rings to bed in properly).

Understood, I was not intending on using build lube on the pistons Just seen the Janspeed DIY Mini videos on Youtube, the mechanic is pouring oil on straight out of a Castrol GTX bottle

5. If the answer to question 1 is no, then when and how do you check the piston ring gap? Just prior to final assembly

OK

6. For a 1310 using P21253 pistons what should the bore size be for the recommended gap of 0.003"? Measure the pistons as described by mini13

Happy

7. I have read that when working out the deck height I should aim to get the piston top flush with the deck, however I have also read that setting the piston to 0.005 below deck height will help if ever a later rebore to 1380 is considered? Comments would be appreciated? Ideally flush for best performance, but 5 thou down allows a bit of fail safe if the deck ever needed surfacing later. You might lose about 2 bhp.

Understood - not planning on a rebore but who knows

Edited by JonnyAlpha, 30 August 2019 - 04:20 PM.

[JonnyAlpha]

Ok so I took the newly received +40 pistons to the machine shop, today and had a few jobs done. Whilst there I asked the machinist to measure the bores and the pistons so I could see and confirm the gap.

 

He only measured one bore and one piston but the results were as follows:

 

Bore - 2.8205" 71.64mm

Piston - 2.8185" 71.59

Gap - 0.002"

 

When I mentioned that the gap I have been advised is 0.0025" - 0.003" he said that if it were his engine he'd be happy with it and that it was within the Nural recommended tolerances. 

He said it could be increased but would be at my risk.

 

Advice?

 

He pressed off my old pistons, and ran the block and my rods through the engine wash, so I am nearly ready to start the trial build 

[grizzler73]

Get the rod small ends checked for interference fit ( should be no more than 0.8112"), they can be too loose then the rods are scrap. Also, there is no harm in swapping the rods around, the factory machining tolerances mean that some rods are longer than others.

[mini13]

for a standard(ish) build i'd go with the 2 thou unless it was in a hot climate, if its performance I'd look at getthing them to run the hone through it to take it out a bit.

[JonnyAlpha]

for a standard(ish) build i'd go with the 2 thou unless it was in a hot climate, if its performance I'd look at getthing them to run the hone through it to take it out a bit.

 

Planning on an AC RS Cam or MD266, Stage One Kit and some mild modifications to the head, can't afford Stage 3 head work at the mo.

 

If I were able to go with a better head I would'nt want to have to rebore again?

[DeadSquare]

You are not going circuit racing.

 

The reason to have extra clearance over the manufacturer's bore diameter is that there is a little bit less friction, and when competing, every little bit might counts.

 

A club racer's engine might do practice and race twice on 30 week ends a year, at the most 2,000 miles

 

Boring a road engine with .002" extra clearance just save the pistons having to wear that much more metal away before the engine needs a rebore.

[mini13]

I probably should have said "road use" basially 2 thou should be ok unless its goign to be given stick for a sustancial amont of time, ie track days, racing, pasitng it up the motorway at 80 for a coupe of hours at 4500rpm... as a for example, i ran 2 though in my 1380 turbo lump with forged pistons.... BUT! mostly road use, and I did have oil jets coolling the pistons.

 

more important to have adequate ring gaps