Moved Into The Garage
Posted 24 February 2019 - 06:21 PM
Ok so where do I start?
While by no means complete, it may give an appreciation for some of the processes;-
http://www.theminifo...gine-machining/
There are many ways of adjusting the CR, and while you can end up with the same end (static) numbers, the actual results at the flywheel can be quite different.
Up Into Fourth
Posted 24 February 2019 - 07:54 PM
You need to see what the combination you have is. So you need to know the head chamber volume and the deck height. Then I can work out a combination that will work for your application.
Ac
Ok noob here and a little confused, may I assume that before any work is done whatsovever and before I choose the pistons I need to calculate the head chamber volume and measure the deck height, (i.e the height of the piston crowns in relation to the deck height at TDC).
Ok so where do I start?
While by no means complete, it may give an appreciation for some of the processes;-
http://www.theminifo...gine-machining/
There are many ways of adjusting the CR, and while you can end up with the same end (static) numbers, the actual results at the flywheel can be quite different.
Looks great. ill take a look.
Up Into Fourth
Posted 25 February 2019 - 08:43 AM
Mokespider i had a read of your link, very comprehensive, should be a sticky, quick question though, what are the Spot Facings you mentioned on the Con Rods?
Also is it necessary to remove the oil gallery plugs and dowels. Bill Sollis does mention getting the Oil Gallery Plugs tapped on his Ultimate Mini DVD.
Not 100% sure about the process before machining but I think I need to measure everything up before getting any work done.
- Piston Crown to Block Deck Height (On the old pistons? how does this equate when you are changing the pistons)
- Cyl Head Chamber Capacity
- Calculate the piston to bore gap? Before the rebore to confirm the bore size after rebore and honing
Seems I have a lot of questions for the machine shop so I’d better get my act together. It would be far easier sending it to a Mini Specialist but that adds shipping and labour (for the dry build).
Edited by JonnyAlpha, 25 February 2019 - 09:33 AM.
Moved Into The Garage
Posted 25 February 2019 - 09:34 AM
Mokespider i had a read of your link, very comprehensive, should be a sticky, quick question though, what are the Spot Facings you mentioned on the Con Rods?
Seems I have a lot of questions for the machine shop so I’d better get my act together.
I think I need to measure everything up before getting any work done.
Thanks for the kind words, but I only wrote that post up as a bit of an awareness type of thing, it's not really 'comprehensive'.
There are loads and loads of all small detail items in any engine that all need equal attention, no matter how small and never make any assumptions.
Con Rod Spot Facings is just one such 'small detail'.
Sorry, I don't seem to have a file photo for this.
The Bolts or Studs used to fasten a cap on a Con Rod are a high grade tensile type. They need to be to survive the many millions of high loading cycles they do. While Tensile Fasteners are very strong in tension, one thing they don't like at all is trying to be bent, in this regards, they are not all all strong and will crack.
The Spot Facings on the Con Rods are the Facings that the Head of the Bolt or Stud (and it's mating Nut) Land on. If these are worn or not perpendicular to the hole or not chamfered enough, there's a good chance it will lead to a Rod Bolt failure.
For a Mini Engine, this usually results in everything under the Cylinder Head being written off. They usually let go at higher RPMs and that Rod that's now floating about with be clobbered by the Crank, giving you a 'leg out of bed' (a Con Rod though the side of the block) and this usually occurs around the sump line, so, that's the gearbox too. The Crank usually ends up damaged beyond repair and often the Cam will get in it all too.
All for the sake of a small, but critical machining operation that costs peanuts to carry out. This is just one good illustration of how something very small, can very quickly, turn in to something big.
As a matter of course, without fail, I always recondition the Con Rods, in which the Spot Facings are including in.
Likewise, the Main Bearing caps, while usually OK, need to at the very least, have the spot facings checked.
< Edit - found a Photo
You can see the dressed Spot Facings under the nuts.
You can also see that the Bearing Tunnel has been Honed. This is all part of the Reconditioning process for Con Rods.
and yes, they can let go
End Edit >
Edited by Moke Spider, 25 February 2019 - 09:45 AM.
add pictures
Up Into Fourth
Posted 25 February 2019 - 06:09 PM
You need to see what the combination you have is. So you need to know the head chamber volume and the deck height. Then I can work out a combination that will work for your application.
Ac
Ac, bit of a noob when it comes to engines I' afraid, head chamber volume I get but do I work out the deck height with the old pistons in? If so why when they will be changed?
Sorry if this is a dumb question.
Up Into Fourth
Posted 25 February 2019 - 06:47 PM
Up Into Fourth
Posted 25 February 2019 - 07:02 PM
The 21253 pistons will be ideal. However, the only way to achieve the CR you want is by accurate measuring and calculation.
First of all have the machining done and do a 'dummy build'. Measure how far down from the block deck the piston crowns are at Top Dead Centre (TDC). Then strip it all down and have the block deck machined down until the piston crowns come to between 0.005" and 0.002" of the block deck at TDC.
As a guide, for every 0.010" of distance of piston crown down the block, the capacity above the piston increases by 1 cc. Thus at -.005" down the bore, there is 0.5 cc plus the piston dish volume above the piston.
Allow 3.7 cc for gasket volume.
You can then calculate the ideal volume for the combustion chambers in the head. It is all on here somewhere, just do a search for how to do the calculations.
Once you know the desired volume, assemble the head and measure how much needs to be skimmed from the head surface. After this is done, re-assemble the head and do the final assembly of the engine.10.3
It may sound a long job, but it is all necessary to get it right. That is why engine building is not a cheap job if done by a professional building a good, reliable and powerful engine.
Sorry missed this post which seems to explain the process.
So get the block bored to 1293, but what about the honing to achieve the correct piston / bore gap, should this be done after the dry build?
Once the piston to deck height has been measured I can calculate the CR.
Where I am getting confused is everyone mentions Piston Crown when calculating piston / deck hieght, is this the flat edge of the piston or the centre of the piston?
I also thought the idea was to get the edge of pistons flush with the deck?
With regards to Pistons
The Minispares P21253 here don't state the dish cc but do mention a possible compression ratio of 10.3:1. According to AC Dodd the 1293 needs something a little lower so I presume this can be adjusted by altering the amount to be skimmed off of the head.
These are the same pistons that they have fitted to all of their Bill Richards engines.
Up Into Fourth
Posted 25 February 2019 - 07:08 PM
If we know what the deck height is with the old pistons, then if we know what the Piston is we will know if the new one will give you a working combination. If you block has already been skimmed, a new piston if the wrong type may pop out.of the block at tdc and give you a CR which is much to high. Do the checks and calcs now, before you buy a piston.
Another way of looking at it is how can you buy a piston if you don't know what dish are deck height you need to work with the head and cam?
Ac
Ok got it. Best order the DTI gauge and stand. These seem to be the most common stands but the Americans always go one better.
Up Into Fourth
Posted 25 February 2019 - 07:08 PM
Up Into Fourth
Posted 25 February 2019 - 07:21 PM
Up Into Fourth
Posted 25 February 2019 - 07:38 PM
Up Into Fourth
Posted 25 February 2019 - 08:48 PM
The rim around the edge of a piston dish is the top of the crown.
Ac
Up Into Fourth
Posted 25 February 2019 - 09:30 PM
Just for info here is a shot of one of the existing cylinders:
And the cylinder head:
Up Into Fourth
Posted 10 March 2019 - 02:56 PM
If we know what the deck height is with the old pistons, then if we know what the Piston is we will know if the new one will give you a working combination. If you block has already been skimmed, a new piston if the wrong type may pop out.of the block at tdc and give you a CR which is much to high. Do the checks and calcs now, before you buy a piston.
Another way of looking at it is how can you buy a piston if you don't know what dish are deck height you need to work with the head and cam?
Ac
Ok so I have posted this info on my other thread 1275 Rebore.
Using the DTI Gauge I found TDC and then set the gauge on the block face and zero'd it. Then gently releasing the magnet I span it around over the piston crown and re-engaged the magnet. Doing this a few times and on 3 of the 4 pistons I found the piston to deck height to be 15 - 15.5 thou.
These are 21268 pistons.
As far as the head is concerned I need to measure it all up an work out the chamber CC just to be safe. Here is an old thread where I posted up some measurements but I only measured the valves.
http://www.theminifo...rts/?hl= 12g940
Just been out and measured the thickness of the Cylinder Head and it is 69.4mm
Edited by JonnyAlpha, 10 March 2019 - 03:44 PM.
Uncle Cooperman, Voted Mr TMF 2011
Posted 10 March 2019 - 03:53 PM
I normally bring a piston to TDC, then put a straight edge across the bore and use a set of feeler gauges to measure the gap. Of course, it the piston top to deck level is over 0.025" that doesn't work and I then use a vernier calliper to measure the depth by taking it from the top edge of the straight edge to the piston and subtracting the height of the straight edge.
In terms of capacity it is almost exactly 1 cc for each 10 thou down the bore for a 1275 engine.
The 21253 piston has an 8.3 cc dish and gasket volume is normally considered as 3.7 cc to 4 cc.
The best power results are achieved when the piston comes to the top of the bore, or within a few thou of the top.
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