| Author | Message | ||
     Bill Vatter Experienced User Username: bill_vatter Post Number: 131 Registered: 09-2004 |
I am beginning this thread after reading a curious post here where a comment was made about seating new rings. http://au.rrforums.net/forum/messages/16999/1356.html I searched to find an existing Thread where I might reopen discussion and I came up with this one in miscellaneous, where the discussion seems to relate to V8 engines, but has partial applicability to EPW engines. http://au.rrforums.net/forum/messages/17004/16430.html There are some other discussions that address this subject. Perhaps David Gore will move this post to one of those and delete these introductory words, which would be fine with me. Piston rings seat by wearing to EXACTLY match the cylinder bore and thus effectively seal both oil below and combustion above. This wear is enhanced by creating a rough surface on the cylinder wall by honing it and moving the hone up and down the bore while the hone is running to create a cross-hatch pattern. The diagonal scratches cut the ring surface but do not catch the ring, which could possibly break it, as might occur if the scratches were instead straight around the bore. The trick in this process is that the scratches themselves also wear and the cylinder wall becomes smooth, or "glazed," and after seating, the piston ring wear ceases. Selecting the proper grit for the hone results in the piston rings and the cylinder walls arriving at their ideal surfaces simultaneously. Experience has proven that the above process cannot work if the cylinder bore is not virtually perfectly round. Therefore, if the cylinder bore has more than an extremely small amount of ovality, a re-bore and new pistons is indicated. I would personally not fit new rings in a worn bore that was anywhere more than 0.001 inch out of perfectly round. When you compare the cost of new pistons and a rebore with the effort of doing the job over when piston rings fail to seat, it isn't really worth taking a chance on fitting new rings in an old bore. | ||
| Trevor Pickering Experienced User Username: commander1 Post Number: 163 Registered: 06-2012 |
I always thought that the main purpose of the cross hatching caused by honing was an aid to retaining the oil film in the bore. You live and learn! | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 2389 Registered: 09-2004 |
Trevor you are 100% correct, the honing is indeed for the purpose of lubricating the rings and for the modern engines of less running in period. I have seen engines that have huge mileages that have regular oil changes and have not suffered heating problems that have the bores with the honing still showing like new. (This wear is enhanced by creating a rough surface on the cylinder wall by honing) Total rubbish! | ||
| Trevor Pickering Experienced User Username: commander1 Post Number: 164 Registered: 06-2012 |
As part of my apprentice training I spent a year working in the engine recon.shop and spent many hours using the boring bar and then honing bores lubricated with paraffin. Some of the engines we worked on did not require a rebore but just a light hone to get rid of the glazing on the bore and restore the cross hatching for oil retention. I am sure that there is also some truth in what Bill says about the cross hatching enhancing ring bedding in. | ||
| Christopher Carnley Unregistered guest Posted From: 5.62.43.138 |
Trevor, you are correct. The factory had to develop 2 boring operations and 2 honing stages to obtain the required finish such that the edges of the rings were not worn. The rings on A & B series cars wore quite quickly. From "The Merlin in Perspective" by Alec Harvey-Bailey,p 71. On lack of oil retention on cylinder walls. "It was Sunbeam Talbot who,using a special lapping powder (Goldsworthy,s 6X) and a sort of reciprocating spiral action lavatory brush,produced a satin finish which held oil. Somewhat refined it became the standard liner finish and with high Brinell liners (550 hardness)was the final cure." (Message approved by david_gore) |
