Dry-building a Guy Croft StIII race unit

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Dry-building a Guy Croft StIII race unit

Post by Guy Croft » June 15th, 2012, 10:17 am

Part 1:

Details of the dry-build work on Jon Cummins' big valve 2 liter Fiat Strada engine. My term 'dry-build' covers all the checks necessary to ensure that:

1. Valves do not hit each other
2. Pistons do not hit the head
3. Valves do not bit the pistons.

If you don't execute this process there is a good chance your engine will smash itself to pieces rather quickly.

JC_block assy (6).JPG
first check is to offer up the head with valves loose-fitted and Plasticine in the valve reliefs. Valves are coated with silicone spray (the stuff used to stop door and window seals from squeaking) to prevent them lifting the clay off the pistons. The pistons are set at TDC. I think I will have plenty in hand on this check - which is to ascertain the radial clearance betw the valve head and edge of the VR (valve relief) but if you think it might be a bit borderline then pack the piston central in the bores to stop the piston rocking over.
JC_block assy (6).JPG (106.2 KiB) Viewed 6898 times
JC_block assy (7).JPG
this is the head with big inserts - KS sintered ex and Trojan inlet, 45/40 one-piece 21-4n stainless NHRA spec valves with long tips to help with shimming.
JC_block assy (7).JPG (107.58 KiB) Viewed 6898 times
JC_block assy (8).JPG
the head is carefuly offered-up using two dummy bolts, a spare head gasket and locating dowels. Rubber bands stop the valves falling out. No need for any head bolts, just press each valve in turn into the clay.
JC_block assy (8).JPG (112.07 KiB) Viewed 6898 times
JC_block assy (9).JPG
here are the 'footprints' of the valves. With well-designed pistons (these are GC.C.A8 with offset VRs on pistons 1&4) it is only necessary to do one valve.
JC_block assy (9).JPG (111.03 KiB) Viewed 6898 times
JC_block assy (10).JPG
lift the head off and carefully slit the clay with a thin sharp blade. Peel off half of the clay leaving the rest in place in case you mess up the op and measure the radial clearance using vernier as a comparator. By that I mean set the calliper at the width youthink it is anc compare - if you try to to measure the clay directly he calliper will just 'dig' into the clay and you won't know what you are measuring. If the clearance is over 2mm - given that the piston may have rocked over then you are safe. The piston can rock both ways can't it? So you could get a reading that looks OK but during running could be some 0.5mm or more less. You must have 1.5mm - or more.
JC_block assy (10).JPG (99.78 KiB) Viewed 6898 times
Guy Croft, owner


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