Wilsons Sandbox

In a message dated 1/11/01 14:27:50, EDeanButler@compuserve.com wrote to the Vint. Racer's list To wit:: supercharger_services@yahoo.co.uk> and communicate directly with John on technical matters. It is John who is finishing up a batch of Marshall blower copies, both the small one for J/P series cars and the large one for T-series. I believe all but two of the T-series superchargers are now sold. Dean P.S. For what it is worth, I would personally love to see more of this sort of information shared with the "list." A lot or you guys out there must have a lot to share! -------------Forwarded Message----------------- From: "John Bibby", INTERNET:supercharger_services@yahoo.co.uk To: [unknown], INTERNET:DHDARBY@aol.com CC: "Zakira'sGarage", INTERNET:zakiras@one.net "E. Dean Butler", EDeanButler Date: 08-01-01 16:09 PM RE: Re: cams/tappets Dear All, To put all this in perspective we must start by remembering that the MG XPAG started life as a 1100cc Morris 10 unit rated at about 38 bhp at 4000 rpm. with valve gear to suit. Much of the sound original design can handle the demands some of us now make ((( like 150bhp at 6500 with rev limit of 7200 ))) but the valve gear is probably the "weakest link" in the whole plot. . I cannot fault any of the foregoing text by Dean and Chris so I'll attempt to fill in the gaps. 1. The lobes are too narrow and even with the larger base circle now used marginal. The original cam lobes were always parallel / flat ground with matching flat followers.The cams were steel the followers chilled cast iron. With the exception of Crane units and some made recently in UK all replacement cams have been flat ground including those sold by BMC in the 1950s/60s. Re the followers , this is pure speculation but it seems likely that back in the 60s CRANE did make matching radius-ground followers but dropped them.( I may be wrong on this, but I think they dropped the cams for some time and reinstated them) It is my belief that the best cams available today are the Crane units, which I use.(and I pay a lot more for them) 2. Until about 10 years ago the only source of cam followers was Clancys who are a very big UK specialist cast cam and follower OEM supplier. ALL retailers sourced from them as did BMC (I understand they made the originals also) ENTER MARKET FORCES About 10 years ago Moss or B&G or both together changed to SRN, I suppose as the best deal .My understanding is that they manufacture in Israel or India. These units continued to be flat-ground .The problem is that the economic batch size for chill-cast followers is around 8000 +. The state of play some 4 yrs ago was that you had to live with the miss-match . There were tricks, such as ensuring lots of clearance in the block to reduce resistance and also allow the follower to tilt to conform to the angle on the cam,and avoid the line contact on the high end of the lobe, this did help to promote spin but strong race springs worked against you in this quest whilst increasing the forces which was doing the damage.There had to be a better way. I located a sub-con shop with a CNC grinder and had a batch of new SRN followers re-ground to give radius as recommended by Crane.( which whilst doubling the cost price gave me the best solution at the time) I still have some of these in stock at 13.00 each. Recently B&G have started supplying modified MG 'C' followers (which are radiused) but as these are of the deep inverted bucket type they come as a kit with longer push-rods. I have not used these yet. 3. I would like to be able to summarise with a list of points to follow which would ensure success, I have seen it quoted that engineering is all about managing compromises, well here we are. An XPAG even with a full-race cam will not produce power above 6500 unless supercharged. Do not make the mistake of assuming that the best springs are the strongest. If the cam in this engine is a 1/2 race unit ( Crane only make two grinds road and 1/2 race ) and if the springs are of the B&G Cosworth type,then the fitted length should be set to give 70 / 75 lbs seated this should give a rev limit around 6500.. with T F size valves fitted. However this assumes that the rest of the valve gear has been lightened as much as possible that the valve guides are not too tight, if you are using valves with chrome stems and bronze guides I would advise 0.0010-0.0015 clearance on inlets and 0.0020-0.0025 exhaust , and if a road engine stem seals on inlet only and if a race engine no seals at all. Re binding in valve gear. during their production life these engines were fitted with two different lengths of rocker tappet screw and different length push-rods it is possible to arrive at a situation when at full lift ,that the edge of cup on the push-rod hits the underside of the rocker arm. Re bedding in yes 1500-2000 for 30mins at first start - up. Oil is important, running-in oil or straight low grade oil without additives is important for bedding-in, then change to a high grade mineral, Valvoline racing 20/50 or high quality fully synthetic. To return to rpm, with the above set-up idle speed should be kept as high as possible at least 1200rpm and if bedding - in on the road do not cruise below 2000rpm. If a low idle speed is preferred on the track, then a hand throttle can be fitted for paddock use. JOHN G. B. >>