<html><body>Guys-this is a very lively topic and one of sometimes great mystery and confusion. The unit is complex but the end result is working well or not functioning at all.<div><br></div><div>On the subject of pressure required to operate the shift into overdrive there are many schools of thought. Some people replace or stretch springs or add washers to increase the pressure needed. The health of the 8 springs that keep the unit in direct drive is important to keep the clutch from slipping. The pressure supplied to overcome the strength of these 8 spring , and effect overdrive, relies on adequate hydraulic pressure. The change from direct to overdrive needs to be smooth, not harsh, and if pressures are high the unit will snap into and out of OD rather than the smooth interchange we are used to while driving.</div><div><br></div><div>When applied to the Austin Healey 100, the Laycock overdrive was initially a 32% reduction unit. It was soon found out in these early Warwick cars that 32% was too much reduction. DHMC swapped units three times in the original DMH tour car until they finally found a combination that produced the smooth shift and complimentary gearing desired.</div><div><br></div><div>It is difficult, today, to find any information on this original 32% gearbox, however if you examine the booklet (attached) supplied by Laycock for the early cars, you will notice that the casting allowed for a large accumulator piston (seen on p6 in brochure). After some research and modified engineering, the company came up with this alloy "sleeve" called the piston housing (photo) which partially filled the chamber of the original large accumulator casting and allowed for a smaller accumulator piston that we see in all the later 28% reduction units. That size piston, in proper condition, produced from 350-450 lbs of needed hydraulic pressure to overcome the springs influence.</div><div><br></div><div>I'm confident that the remark made by Geoff Healey in his writings about the early transmissions "harsh shifting" revolved around this huge accumulator piston producing too much pressure. Additionally, there were three various accumulator springs supplied which adjusted the pressure. The early boxes produced about 350lbs and the later about 450.</div><div><br></div><div>Today, Overdrive Spares supplies that sleeve housing with only one rubber O-ring as it is basically a filler part and not under extreme pressure. The accumulator piston within is under that pressure. They supply a modified piston using O-ring technology. So, it is not unusual to get good pressures over the normal 450 lbs required. If it works well don't fix it. The only concern is if the pressure does not release when overdrive is not called for. A clean unit, with proper 30W oil, will allow that to happen. All this some old theory and history to ponder. Hank</div></body></html>