ENGINE REBUILDERS PTY LTD
7 BARRETT STREET, FLEMINGTON VIC 3031,
TEL: (03) 9372 3111, FAX: (03) 9372 3122
ACN: 006 798 492 ABN: 74 006 798 492
MARCH 2OO3
TECHNICAL NEWSLETTER SUBJECT: REBORE/HONE FROM A TORQUE PLATE
The cheapest gain in horsepower and reliability is obtained from detailed attention to the steps needed to get effective piston ring sealing.
The first step is to use a torque plate for the final machine honing of the bores.The machinist assembles main bearing caps to the block using engine oil on the threads, or moly as required by the stud manufacturer, and tensions the caps to the procedure recommended by the manufacturer.
The torque plate is then fitted and again tensioned up to manufacturer’s procedure with oil/moly as required. This has stretched the block casting into the shape that it will be when running down the track and it is in this stretched position that machining proceeds.
When the machining has been completed, and the torque plate and main bearing caps removed for cleaning, it is pointless to attempt measurement checks at this stage because the block is now relaxed and dimensions will be all over the place.
The only time that you will be able to assess the results of your machinist’s and assembler’s skills will be at the first strip down after the engine gets it’s first freshen up. This is discussed below.
The machinist supplies the block with both the tensions used and the tensioning sequence taken in pre-stressing the block for machining. The assembler has to repeat this procedure for best results.
All piston skirts must be deburred around the lower edges of their skirts so that the edges are not operating as oil scrapers. We want the oil to get between the piston skirts and cylinder walls: for obvious reasons’ Oil control is the task of the oil and oil scraper rings.
The assembler’s attention to detail in cleaning the cylinder walls is critical to the success of the ring-sealing task we are discussing. Hot soapy water followed by white rag and engine oil: wiped until the white rag shows nothing of the honing abrasive remains in the honed surface of the cylinder walls. The grey colouring on a white rag is abrasive material from the hone stones.
For emphasis: the assembler has to repeat the tensioning procedure used by the machinist.
At the first time that the head/s are removed, evidence of the machinist & assembler’s work will appear at the top compression ring contact at TDC.
The best engine block, usually one without core shift or other manufacturing anomalies, machined and assembled with great care and tuned properly, will show a very high percentage of full circle ring to cylinder wall contact. Full contact on 4 out of 6 cylinders is good; 6 out of 6 is exceptional and usually relates to a engine that has run beyond expectations.