We get back into the mechanical side of things this issue with a detailed look at the block machining process.
The SR20 block is an interesting piece of equipment. The alloy block is extremely light, so light that the machinist moved it from the boring machine to the honing machine by simply lifting it up. However don’t assume that this is a sign of weakness. Considering its size and thickness the SR is quite a durable block. Being a sand cast block has dictated a reasonable wall thickness is necessary to allow for shifting of the blanks. Basically sand casting is when a block shape is carved out of the sand and then molten aluminium is poured in, creating a cast. This technique is great for creating weird and wonderful shapes, but can leave imperfections, hence the need for the thicker walls. Interestingly the SR20 block is not one single piece. There are thin cylinder sleeves in the block that have been created separately from the rest of it.
When the block was first dropped off at HM Gem Jason Leckie had the block chemically cleaned to remove the general grim of the workshop combined with over 100,000km on the road. Being aluminium the block is treated differently to cast iron, and the mix is not caustic. A mix of softer cleaning agents heated to 65-70 degrees Celsius and baked for 8 hrs in swirl pool left the block looking spick and span. Once the block was dry it was time for the first process, the boring of the cylinders.
Boring:
Boring the cylinders of the block entails removing a thin layer of the walls to increase capacity slightly and make room for oversized pistons. The ACL Race Series forged pistons we are going with are 86.5mm in size. In other words, 0.5mm oversized from the stock SR20 86mm pistons. The first piece of prep work is to fit the timing chain case so that it is the same height as the block after the milling is done. At this point the sump rail is filed down gently so it is smooth and sits flush on the machine floor.
Once the block is bolted into place the parameters are all set up and checked on the machines computer. The four corners of the block are checked with a gauge that is accurate to 0.0000196mm to make sure it is square to the machine. A dummy run is then performed by the machine to ensure that everything is running smooth and all the parameters and setting are accurate.
The Rottler machine centres the spinning device with three feeler arms before it starts to spin. The cutting of the actual bore is carried out by a carbide cutting tip. It is actually quite small in size but it cuts the meat in the cylinder cleanly and evenly.
For the machinist to do an accurate job he needs the pistons present with him. This is for a number of reasons, mainly to inspect the quality, weight and shape of them as well as the size and fitting of the piston. The ACL Race Series for the SR20 are actually quite impressive, with their weighting all very even and a good shape too. Each one was measured individually by our machinist to ensure that they were indeed well matched. It truly is a testament to the quality of the actual pistons when they are so consistent. Now the real test will be how they handle everything the Garrett turbo can throw at them.
The pistons needed a 0.1mm clearance for the job. One thing I became aware of quite quickly as I watched all this work happening is that there is a hell of a lot of measuring and re-measuring and double checking necessary for a machining job to be done properly.
Once the machine was properly set it was time to start. The first cylinder bore lining proved to be surprisingly hard! Once again the SR is showing a little spirit. Unfortunately the process had to be halted and another more suitable cutting tool had to be fashioned. About thirty minutes later we were ready to roll again. This time the spinning speed was slowed down giving a surer cut and finer finish to the walls. As you can see from the pictures the bores turned out great.
The last process here before the milling can start is to cut a little tapering groove that is wider than the bore size. Without it, putting the piston rings in would be almost impossible.
Milling:
Milling the top of the block is the next stage. The same machine performs this task but with a different cutting tool. A large spinning piece with a carbide cutting blade on each side is fitted in preparation for the job. Milling is performed to smooth out the top surface of the block so any warping and variations are ironed out and there is greater chance for an even seal with the gasket and cylinder head. The better the seal the better the efficiency and power.
Having the timing chain case on for the milling process is important as it needs to be even with the block. Interestingly the first three cuts at 0.0254mm consistently shaved the timing chain case but barely touched the block. So from the factory the timing chain case was 0.0762mm off and this could have caused a problem with sealing. Evidence of this can be seen on the face of the block where markings that indicate water pockets have developed.
In the end, to clean the top of the block and ensure an even and consistent finish 0.127mm was taken off the top. The milling seemed to be the fastest of the three machining techniques used, but it still requires just as much patience, accuracy and finicky inspection.
Honing:
In what may be described as the most tedious and slowest of all machining processes mentioned so far, honing involves going over the bored cylinder walls with a rough stone and then a smoother stone to create a cross hatch pattern. The rough stone are 280 grit and the finishing stones are 400 grit and not only do they spin in a circle to grind the walls, they also move up and down in a pumping motion to leave the cross hatching imprint in the walls.
The cross hatching of the cylinder walls is very important as it gives the piston rings something to run on as they traverse up and down the bore. It also allows the oil to travel through but also hang around so that the cylinder wall and piston rings are all well lubricated during the combustion process.
Smoothing out the insides of the bore is a necessary process but there is more to it than just a finer finish. For a start the cross hatching needs to be retained so it is not as simple as going for a totally smooth finish. Also the machinist has to ensure that the bores are still kept even. The shape and science of the cross hatching is also important. More and more machinists are coming to the conclusion that a plateau finish is best.
Honing leaves a series of microscopic valleys and peaks along the bore. The peaks don’t provide much support for the ring and so when the engine is started the rings move up and down, shaving the highest peaks and wearing them down gradually. Eventually the cylinder walls will be relatively smooth (except for the valleys of course).
Breaking in an engine produces a plateau finish anyway, but during that time the piston rings and cylinders will experience unnecessary wear and tear and increased blow by and oil consumption. A plateau finish straight from the honing will give maximum compression from the word go and eliminates more ring seating and sealing problems.
Honing is quite a tedious process because after just a few spins of the grinder the machinist has to measure the cylinder walls to see how much has been taken off to ensure he has gone far enough without taking more than he wants. This was done with a measuring tool (name of the tool?) that accurately read the diameter of the bore. Measurements were taken at the top, middle and bottom of the bore to ensure uniformity.
After this experience it seems that machining truly consists of constant checking and measuring. With all clearances needing to be accurate to within such minute tolerances this is one job where rushing does not pay off. As you can see there is a fair bit of work that goes into the block alone to have it prepped for a rebuild and that isn’t even including the cylinder head and the crankshaft. Stay tuned to see exactly what goes into both the crankshaft and the cylinder head when locked in a room with an enthusiastic machinist and a bunch of fancy machinery.
HM Gem Machining
One look at the size and sheer amount of machinery at the HM Gem Sydney base in Ingleburn and you can tell these guys are serious about engines and machining. Through the garage door at the side of their modern premises there is a warehouse full of heavy machinery, most of which are carrying price tags well into the tens and even hundreds of thousands of dollars. Having previously resided in Chipping Norton, Jason Leckie who runs the Sydney operations informed us that a move was necessary to keep up with the work demand and space requirements of the ever expanding business.
A 40 year history in engine machining, HM Gem has always maintained a leading edge by staying at the forefront of new technology and machining techniques. This has kept them in good stead with the automotive industry in Australasia. Machining jobs on anything from semi trailer, prime mover and tractor motors right down to passenger vehicles and even performance machining, HM Gem have done it all and no job is too big or small.
But not only do HM Gem do machining, they also remanufacture replacement engines for pretty much anything you can think of. Even old engines are floating around the warehouse looking pristine and wrapped up in plastic ready to go. This means that purchasing a remanufactured engine from HM Gem is a drive in drive out affair with your old engine taken out and replaced all at the workshop. The machining done to the SR20 block was amazing to witness and I want to thanks all the boys at the workshop and in particular Jason and (name) for guiding myself and Dean through the process. It was a fascinating insight into the part of the engine building process that seems to often get forgotten and overlooked.
SR20 Brawler Affiliates
HM Gem
Nation Wide Machining
www.hmgem.com.au
General Enquiries (03) 9797 1100
Engine Builder:
Indy So
IS Motor Racing
Unit 5, 5-7 Bermill St
Rockdale 2216
NSW Australia
Ph: (02) 9599 9918
Fax: (02) 9599 9981
Turbo:
Garrett
Honeywell Turbo Technologies
www.turbobygarrett.com.au
Check the website for your local state distributor.
Clutch:
Exedy Australia
www.exedy.com.au
Check the website for your local state distributor.
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