Jump to content

Model T compression reading should be: ???


Guest
 Share

Recommended Posts

I recently ground the valves and installed new rings in my 1924 T Speedster, which has been on the road for nine years and several thousand miles. <P>During restoration, the block was overbored to 0.030 for new aluminum pistons and rings, and new valves, lifters, cam bearings, etc were installed. I used the higher-compresssion Ford cylinder head. For years the engine ran strong and smooth, capable of a steady 45 mph all day long.<P>The performance and smooth idle have largely disappeared the last couple years. Time for a valve job (at least!)<P>Compression test readings prior to beginning work... Dry: low forties, wet: one or two lb more. Definitely sounds like valves.<P>I hand ground the valves, several of which were quite poor, obtaining some contact area all the way around on every one. Back together, adjusted the lifters, runs no better than before.<P>Compression test readings after the valve job... Dry: low forties, wet: one or two lb more. <P>So maybe it's the rings ? pulled out the piston only to find ring-end gaps of 0.050-0.065 on the compression rings and 0.090 to 0.220(!) on the oil rings. Several printed authorities recommend as little as 0.006 to as much as 0.012. Radial thickness varied from 0.010-0.030 undersize compared with new rings. Strangely, there was no blue smoke in the exhaust, and no indication of blowby.<P>Glazebreaker on the cylinder walls and new 0.030 rings installed. Adjusted the big end bearings, fired it up, and it ran almost worse than before.<P>Compression test readings after the ring job... Dry: low-to-mid forties, wet: one or two lb more. <P>During the course of all this, I had the carburetor off, set the float, and made sure the needle/seat looked OK. In operation, the mixture adjustment seems best slightly less than 1/2 turn from closed -- small adjustments either way will either starve or flood the engine, so I'm assuming the carb is at least reasonably good. It has gravity feed to the carb, and the tank is nearly full.<P>As well, I installed a new timer cover and roller. The spark seems strong and steady, with a 90+ degree duration -- the timing adjustment spans from 55 degrees BTDC, to about 10 degrees after at full retard. It runs smoothest when the ignition kicks in at about 30-40 degrees BTDC (and lasts another 90 degrees). Hand-cranking and careful observation indicates all four cylinders seem to be timed about the same. I start the car on battery and switch over to mag for more power and smoother running.<P>All cylinders are definitely firing, although shorting out the #1 plug at idle does not slow the engine down as much as do each of the other three. Once started, and with a fair bit of throttle, the engine will continue to run on any one of the cylinders, with a regular, rhythmic chugging, just like my one-cylinder Orient.<P>In fact, it seems to run best with a fairly wide open throttle at relatively low speeds, such as accelerating in high gear or climbing a hill. And, thankfully, it has continued to be a good starter even with all its other troubles.<P>This post has grown from a question about the normal range of compression readings for a Model T to a full-fledged plea for help!<P>*What compression readings should I expect from a used Model T with fresh rings and hand-ground valves? *And/Or what other faults could lead to this rough idle and lack of power?

Link to comment
Share on other sites

Forget the compression test and do a leak down, if it's over 7-10% you have a serious problem. <P>Also those end gaps are way out of line even for an engine that old, rule of thumb 0.004" per inch of bore on the compression rings. The oil control rings you can't do anything about so don't worry about it.<P>You don't mention ring to groove side clearance, this should only be 0.002"-0.004". If your at 0.006" you need new rings and probably pistons. What about bore taper and roundness? Hand ground valves?, that's lapped and can't be counted on to get a good seal on worn seats and valve faces. Regrind both and lap to get a good seal. <P>Use liquid lighter fluid to check for a good seal, it should not leak by your valve seat. I use a vacuum pump on the intake and exhaust ports while squirting lighter fluid on the valve head if any gets sucked in, I do it again. <P>You should be able to draw and hold a vacuum if the valve face has a light film of oil on the contact face. Make a plate with a vacuum fitting and a glued rubber face to fit your intake/exhaust port.<P>The Old timers on this site can probably give you a better idea what a good compression reading is for that motor, I think that 70-90 psi is a better spec. I could be wrong on that. At any rate a leakdown test is what you need to do, it is a much better test of your engines health.<P>It kind of sounds like to me that the last rebuild was a half a__ attempt at a rebuild and now your finding the results of the carelessness.<P>Hope this helps.

Link to comment
Share on other sites

Terry and Chuck, thanks both for your replies and suggestions.<P>Terry, the ring gaps with the new rings range from 0.012 to 0.025. Intake manifold leak is likely still a possibility, although the old trick of aquirting oil around the manifold-to-block connection to temporarily fill any void seemed to have no effect. As well, a manifold leak can't explain low compression, in that it is upstream of the valves.<P>Chuck, your comment about this being a half-assed attempt at a rebuild is pretty much dead on, but there's a purpose to this "sloppy workmanship"...<P>Of my several vintage rides, my Model T is the one most realistically treated as it would have been owned/driven/maintained some 50 to 75 years ago. That means I built it (as a speedster, from 2 pickup loads of junk) in my garage using mostly hand tools, and<P>I drive it regularly, year round ? including to work and shopping, driving Junior to school, pleasure trips, and so on. The car is no stranger to gravel, dirt and fields. It gets washed every few months whether it needs it or not. I leave it for hours in parking lots and on the street (and it has never once been messed with). Dad and I drove it 900 miles through the Rockies in 1995 with all our gear piled on, a variety of tools and parts, and no chase vehicle. (On this journey, we lost the #1 rod bearing climbing a long steep hill ? Dad and I wrapped the bearing with oil-soaked leather, squeezed it back together and not only completed the trip, but drove it like that for over a year!)<P>My point, finally, is that Henry designed and expected these cars to withstand regular abuse, and hamfisted repairs by inexperienced owners. I feel I should be able to get this car back to reasonable running condition without a machine shop, and by following the instructions in Ford's own period service manual. I prefer to use just simple tools, basic diagnostic figurin', and clean work habits on this car.<P>Obviously, I didn't measure the bores for taper and wear, but there was only the slightest indication of a ridge at the top ? to me, and according to Ford's service manual, this should indicate that wear and taper is no particular problem.<P>Even if the valves are not seating perfectly, I know they are much, much better than before. Same for the rings ? not a perfect job I'm sure, but a huge improvement over what it had previously.<P>**So why are the compression readings virtually unchanged?** For example, assuming the rings are sealing better, the unchanged compression numbers must mean that something else got worse, no?<P>By the way Chuck, Dykes 18th Edition, revised 1937, quotes typical compression test readings as 60-75 psi ? I presume compression ratios were higher then than in the Model T era, so your estimate of 70-90 psi is probably high.<P>Does anyone reading this have a good running electic-start Model T in the garage? If so, I'd sure appreciate you slipping out there for a few minutes, throwing a compression tester on a few cylinders, and letting me know what you find. Thank you!

Link to comment
Share on other sites

I love your savoir faire, your absolutely right in how these old cars were meant to be driven. You have my respect and I admire your independance, anyone who pulls a bottom end to put a leather "sleeve" in for a rod bearing to continue a trip means you are one McGivering SOB!<P>That being said it still leaves us with your basic problem, the fact that all the cylinders are all low leads me to believe that there is nothing wrong with your motor. The tolerance stack from all those worn components are just conspiring to give you a low reading, hell it still runs doesn't it!<P>It would be interesting to see how far you could run an engine like this before compression would get too low for it start. If you are not getting an overheating problem, the oil and coolant is not disappearing and the blowby is not severe then I say keep drivin it!<P>Although a nice precision rebuild would sure wake her up and give you some power, I don't think there is anything basically wrong with your engine. She's just tired is all.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...