Wilsons Sandbox

Someone asked me a question tonight and I'm stumped, I need some help. With all the talk of compression ratio vs bore size and the dangers in over doing it , where are the dangers in over milled heads? Many of these cars have been "shaved " or trued during rebuild to the point of compression ratios that that even avgas won't handle. Where are the points of common sense and where do we stop and shop for a new head. Thanx, cheers Fig

FiG: What follows is part of a letter I sent to Dave Tinker in Feb, 2001. It concerns the XPAG head and a Laystall head. Blake ******************** I found the following data in a booklet written by WKF Wood in 1968. The chamber size is for all four chambers. Remove Head Total Chamber Ratio Finished Depth Size 1&1/4 L. 1&1/2 L. Std. 76.75 mm (3.022 in.) 200 cc 7.25 8.33 1/16 in. 75.16 mm (2.959 in.) 175 cc 8.1 9.0 3/32 74.37 mm (2.928 in.) 165 cc 8.6 9.9 1/8 in. 73.58 mm (2.898 in.) 150 cc 9.3 10.7 I checked this against an original factory tunning booklet "Special Tuning of the MG Midget type XPAG (As fitted to Series TB and TC Cars)" Issue 1, June 1949. It shows several stages of tune also. They list the chamber size as 45.5cc and the head gasket C.C. as being 4.5 c.c compressed. Adding these together and multiplying by 4 you get the total head cc as 200 which is the same as in the chart above. Using MGs data, the chart would be like this. The chamber size in this chart is for one cylinder. Head Chamber Size Ratio Remove Finished Depth With Gasket 1&1/4 Litre Std. 76.75 mm 50.0 cc 7.25 3/32 74.37 mm 8.6 1/8 73.575 mm 9.3 Checking the data in the first chart by recalculating seems to verify that the data is correct. For instance, multiplying 76.75 mm by the conversion factor 0.03937 you get 3.0216475 in. for the original head thickness. The head thickness of 73.575 converts to 2.897 using the same process. The cylinder capacity can be calculated by taking 66.5 dividing it by two, squaring, multiplying by pi (3.14156) and mulitply by the bore of 90 mm which gives an individual cylinder capacity of 312.59 cc for one cylinder and 1250.36 for all four. Using the head size of 50 cc and the cylinder capacity of 312.59, the Compression Ratio can be found by adding the 50 to 312.59 and dividing by 50. Doing that gives a CR of 7.2518. Using 1250cc for the engine capacity and 200cc for the total head capacity the result is exactly 7.25. We can check the 9.3 CR by adding 150 to 1250 and dividing by 150. (1250+150)/150=9.333... So The CC capacities listed for the head in the first chart must be very accurate. Using that data, I found that every 1/64 inch (0.0625 in.) removed from the head's thickness reduces the cylinder head's chambers about 6.25 cc. This assumes that the chambers walls remain more or less vertical to the heads deck. This data could be used to get an approximate CR of a milled Laystall head's IF you know the original thickness. Attached is a poor copy of a picture of a standard head and the Laystall. The shape of the chamber is slightly different. It appears that removing the same amount of thickness from it will leave the finished chamber size slightly larger than that for a standard iron head. If you can cc an original Laystall head and measure the head's thickness you could complete this partial chart for Laystall head which would probably be very close. The article on the Laystall head I sent yesterday was done on a TD (1250 cc). If you happen to have the head on a 1500 cc XPEG the results would be about what's in the chart below. The minus sign indicates the decrease in the heads thickness or the decrease in the chambers capacity. The ~ (tilde) sign is the best I can do on the keyboard for the mathematical symbol "is approximately equal too." Head Chamber Size Ratio Remove Finished Depth With Gasket 1&1/4 L. 1&1/2 L. Std. ? ? 9.3 ~10.7 1/64 in. -0.015625 in. -6.25 cc ~9.7 ~11.4 1/32 in. -0.03125 in. -12.5 cc ~10.1 ~11.9 3/64 in. -0.046875 in. -18.75 cc ~10.5 ~12.4 1/16 in. -0.125 in. -25.0 cc ~11.0 ~13.0 This is the best that I can do with the books I have. Sincerely, Blake J. Urban