Got the shortblock assy. back from the engine shop last week, so I thought today before I started to finish assembling the engine I would check a few things out. I put #1 piston on TDC and just looking at it there seemed to be a fair amount of clearance between the top of the piston and the deck of the block. The pistons I used are Clevite #TC1949 which are supposed to be the stock replacement for this year ('75) of engine. The Clevite web site lists the following specs. for this piston: -compression height (center of pin to top of piston) 1.805" -recess (dish) 3.05" X .215" deep Taking the above 1.805" compression height and doing the math I come up with what should be the total TDC piston height: 1.805"-piston compression height 6.387"-con rod length 1.925"-half of 350 Buick stroke of 3.85" The sum of the above numbers is 10.117" (from the crank center line) at the top of the piston @ TDC. Subtract 10.117" from the 10.18" deck height of a 350 (which we confirmed here) you should end up with an approx. deck clearance of .063" right?? Wrong!!!! :af: Using stacked feeler gauges I came up with an approx. deck clearance of .104", .041" more than it should be. That would mean that the pistons must be only about 1.76" or there abouts compression distance. I measured the depth of the dish and it is .230" deep. The dish is 3.05" wide at the top and 2.44" wide at the bottom. The stock '75 heads that I am using is casting# 1243452 which are from what I have read have a chamber size of 58cc's. The block is bored .020" over, the composite head gaskets will be .040" thick and the gasket bore measures 3.89". I figure that the displacement of the piston dish is about 1.38 cubic inches. Taking all these numbers and going to Silvolite's site and using their CR calculator: http://kb-silvolite.com/calc.php?action=comp I come up with a compression ratio of approx. 7.7:1 . Using the figure for the deck clearance I should have (.063) it would be 8.2:1 . So my question is, what gives here? Why would my deck clearance be so much? Are actual piston heights not the same as advertised? I never measured the pistons before I took them to the shop, and they didn't seem to have either before they installed them. I now think I have a lower CR than what the motor came with so there goes my efficency :rant: . Perhaps I should get the chambers measured, then have the heads shaved a bit. How much can these heads be shaved down, what is considered safe? Thanks, Greg
Update, kinda Thanks Rob for the info on shaving the heads. I talked to the shop today and the guy that put it together didn't notice the lower clearance height. He also doesn't remember comparing the old and the new pistons though he usually does. He said resizing the rods can change the height about .005", but that is it. I found out from Clevite that these pistons are made by Silvolite, the Silvolite p/n is 1746. I phoned UEM (Silvolite) and talked to a helpful fellow named Glen in their tech dept. I told him my woes and give him all the figures, he came up with the same CR as I did. Glen told me that he would guarantee that the compression height of the pistons would be the published 1.805" as each one is checked before it leaves their factory. He said that the problem has to be else where, though with the piston so deep in the hole I won't have any detonation problems. So I am still confused . Has anyone ever seen this before?? I may have to pull one piston out and measure the center to center of the rod to see what gives. Any help on this would be greatly appreciated. Thanks, Greg
Greg I havn't checked the 350's but I know that on the 455's the deck height on the later years (after about 73) started to grow from the factory. My personal opinion would be to zero deck the block and use a composite gasket. This would help in the quench area. Good luck. Mike
There are so many possibilities for where the problem is! First guess: The pistons really are what they're advertised to be. Next guesses: The rods are NOT what they're supposed to be. That'll be doubly true if you had them re-sized. And there's nothing that says they're all the same, either. The crank may or may not be the stroke it's supposed to be, and there's nothing that says the four rod pins are equal, either. So the stroke "could" vary from one rod pair to another. The block machining may be inaccurate--deck height may be different side-to-side, or end-to-end, most likely both. There is NO WAY to know until you either trial-assemble everything, or do lots of precision measuring before assembly.
Well I did some measuring with my steel rule and vernier calipers and it seems that the deck height on this motor is approx .040" higher than I thought it should be. So instead of being 10.18" deck height it is about 10.22 so I guess that it explains it :grin: . I finished putting the top end together on the weekend and everything went together just fine. Next time I will look into a different piston and get the block decked down if I use a later block that has a taller deck height like this one has. Thanks to all that replied for your help. Greg
Good to hear you've got that figuered out! It looks like with the later blocks I could use the flat top pistons and still stay under 10:1, I'll have to make some measurements.....
I always have to take .030 - .040 off the decks when I build my 350 buick race motors. I also stroke the crank .013. It seems that the blocks are alittle tall. Dan
I use .060 over trw 10.25 pistons with deck of .011. 50.5 cc head chamber. 041head gasket. yealds about 11 to 1. Not sure if that is exact. If some one has the formula to figure exact compresson we can find out exact numbers. These pistons are no longer available. Dan
I'm hoping that we are going to start getting some aftermarket help. These are really good motors but they get no respect from the general car crowd. I hope thats startng to change. Thanks for the info.
i was talking to dave hemker and steve (blue '72 gsx ) at the pure stock drags and they had told me the same thing that this is normal and that you can deck a 350 block almost .100" .i'm presently running a 350 in my 72 that was supose to be built to 70 hi compresion spec ed
I knew that you know, that was just an FYI to the people that didn't. Sometimes when these dead threads get resurrected someone tries to help solve the OP concern which can be funny but most of the time, sad. Of coarse you knew, you had to go WAY back in time with the search function or back close to the beginning threads of this website to find this thread.
