Richard71 Posted May 27, 2018 Report Share Posted May 27, 2018 Hello All, I don't have my Brown Bible to hand (a friend has borrowed it). I recall the cylinder diameter is 74.7mm (I think?), does anyone know if there is a maximum permissible wear level before over-boring must be carried out? Regards, Richard. Quote Link to post Share on other sites
Ragtag Posted May 28, 2018 Report Share Posted May 28, 2018 I might be showing my ignorance here but if the 1st oversize piston is +20 thou, that is only 0.5mm Quote Link to post Share on other sites
Richard71 Posted May 28, 2018 Author Report Share Posted May 28, 2018 You're absolutely right Chris, I'm hoping someone can point me in the direction of oversized piston rings to counter only very minor bore wear.....I'm trying to keep an engine block standard. Richard. Quote Link to post Share on other sites
astontr6 Posted May 28, 2018 Report Share Posted May 28, 2018 You're absolutely right Chris, I'm hoping someone can point me in the direction of oversized piston rings to counter only very minor bore wear.....I'm trying to keep an engine block standard. Richard. Hi Richard! There is usually a ridge at the top of the bore and some people call it the finger nail test not very accurate .But if it can stop your finger nail some people say it needs a re-bore. The correct way is to use a telescopic bore gauge. The bore does not wear round in shape but oval and off centre at the top and bottom. If you get a reading of + 0.010" wear, it should be re-bored. If fitting new rings in old bores, the bores s/b glaze busted to help them bed in. Bruce, Quote Link to post Share on other sites
TriumphV8 Posted May 28, 2018 Report Share Posted May 28, 2018 It is just a question what you are willing to tolerate. I swapped to a modern hypereutectic piston with only 0.04mm clearance. No blow by and no oil burning! If you increase clearance the engine burns oil and pumps burnt air out of the valve cover. This is also an indication of loosing power. A former limit was to lock the tube for the air to the air filter box and open the filler cap and drop it on the hole. If the cap starts dancing when you pull the throttle it is time to refresh. The official way ist that a clearance extension of 0.1mm is a limit and the clearance of rings should not exceed 0.5mm But why doing something on the engine if the owner feels fine with it? Burning oil, especially when engine is pushed, is often related to valve guides. I would have an eye on the oil pressure and often our TR6 have worn big end bearings. I would not hesitate to change them to extend the lifetime of the engine. That must be done before they fail and a slightly hammering noise after start before the oil pressure comes up is also an indication. Quote Link to post Share on other sites
Nick Jones Posted May 28, 2018 Report Share Posted May 28, 2018 You don't really need oversized rings as the ring gap is not that critical (more critical that it's big enough). However, one trick is to buy a +0.020" set and gap them to suit the bore. Bores tend to wear oval (more wear on the thrust side) and the degree of ovality is at least as important as the overall wear. If just replacing rings then (as already mentioned) the bores need to be properly honed so they bed in and any wear lip dressed to reduce the risk of it breaking the top ring. Though if there is an appreciable wear lip you should probably be re-boring anyway. Nick Quote Link to post Share on other sites
RogerH Posted May 28, 2018 Report Share Posted May 28, 2018 The correct way is to use a telescopic bore It is easy enough to knock up a bore gauege. 2 x 1/4 unf bolts 1 x 1/2 steel round bar - drill and tap this for the 1/4unf bolts Cut the heads off the bolts. Insert the bolts fully into the round steel bar Cut the bolts so that they just fit into the bore (a 1/8 " gap would be fine. Unscrew one of the bolts to fit nicely against the bore wall. Measure with a 6" vernier gauge. It would be usefull to have lock nuts on the bolts Round off the ends of the bolts. Roger Quote Link to post Share on other sites
Waldi Posted May 28, 2018 Report Share Posted May 28, 2018 Per recommendations on this forum, I bought a bore gauge from Dasqua, 80 euros. Perfect instrument at a 3rd of the price of a similar Mitutoyo. Off course you need a micrometer to set the bore gauge. Recommended. Waldi Quote Link to post Share on other sites
RogerH Posted May 28, 2018 Report Share Posted May 28, 2018 (edited) Here is my home brew As per Waldi - you need a big Mic' or vernier to take the measurement This will do 80mm to 100mm Roger Edited May 28, 2018 by RogerH Quote Link to post Share on other sites
BlueTR3A-5EKT Posted May 28, 2018 Report Share Posted May 28, 2018 Give it a quick go with a flex hone, then re ring.? Peter W Quote Link to post Share on other sites
Waldi Posted May 28, 2018 Report Share Posted May 28, 2018 I measured my bores (std size) and pistons. The clearances were just within the limits. I decided to order bew piston rings, do a hone. I had no edge in the bores. Be sure to do everything by the book, so check ring gaps (at tge bottom, where it is narrower) verical clearance in the grooves, etc. Waldi Quote Link to post Share on other sites
Richard71 Posted May 30, 2018 Author Report Share Posted May 30, 2018 Many thanks for the replies, I've left the block with a machine shop, they're unusually busy for this time of year, it'll be 3 weeks before I'll get a report back. Richard. Quote Link to post Share on other sites
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