xbgs351

What is the minimum recomended piston/head clearance for a 351 cleveland, factory crank, 302 rods, hypatec hypereutectic pistons and a 6500 rpm shift point?

russxr67

I would be aiming to get zero deck height or as close as possible to it..it works equally well on either open or closed heads.The head gasket itself is usually enough clearance(around 40 thou) in itself.Have your machinist zero deck your block when its in the shop.p.s check what is required with ring end gap on the hyper pistons as these generally require larger gaps than conventional pistons.

xbgs351

The Felpro O - Ring head gaskets are 0.041" thick. This would be the clearance with a zero deck. I have a understanding that you can run 0.008" to 0.010" less than this. This will improve quench and raise my compression ratio.

russxr67

do you mean 8-10 thou below the deck or 8-10 piston to head clearance?

xbgs351

Pistons proud of deck by 8-10 thou, giving 33 to 35 thou piston to head clearance.

shaynet

I have just rebuilt my 351 using same crank, rod & piston combo as you and finished with zero piston to deck height. I don't know if the block has been decked before to end up with this result.

Now I'm not an engine builder but wouldn't you want to keep the piston wholly within the bore, especially with the rings being closer to the top of the piston on this setup?

If you want more compression just shave the head but obviously valve to piston clearance must be a consideration. Sounds like a dry run with parts assembled and plasticine on the piston tops to measure clearance is in order. That's if you have any clearance to measure!

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russxr67

Whilst it can be done I have personally never seen or heard of this procedure with a clevo.I have heard of other engine builders doing this(big block 460) and i would assume the extra 8- 10 thou would give you a slight comp increase at what expense?Zero deck height is a proven combination. Anything sticking above the deck is uncharted ground.Who knows ,may just work.Who knows how much a rod will stretch at 6500 rpm.?

davis

Unless you have domed pistons, you probably won't have a clearance problem. What you need to check is more likely going to be piston to valve clearance or simply P-V. If you were building a "chamber follower" piston trying to eek out the most possible compression, you'd want to leave at least the thermal expansion characteristics of your pistons. I've never built an engine with anything but forged pistons, so I can't say what that dimension is for your pistons. Then you'd want to add a bit, say 8-10 thousandths to be safe. You'll probably want to have a total of about .045-.055" clearance, but please check with the piston manufacturer and don't take my word for it. Here is a link to a site that says that .040"-.045" quench distance works best.

Piston Info

Note that if you use a .040" compressed head gasket, that you should be very close to or at zero deck. My 393, for example, has what are basically aluminum copies (A3 cylinder heads) of the factory 4V CC combustion chambers. I didn't find any way that they are really very different except for a bit more space around the valves, but they were also right at 66cc, too! My pistons are .008" below zero with a .040" gasket giving .048" quench.

However, you're more likely to find that you have plenty of clearance around the piston in everywhere but the valves. Checking your P-V you'll want to maintain at least .100" with a solid lifter cam and .125" with a hydraulic lifter cam.

You probably have flat tops with dual valve reliefs...just guessing (and not familiar at all with Aussie 302C rods/351C piston choices). Be sure to use the recommended springs for your camshaft and ensure that they are installed to the correct installed height as listed for proper spring open/closed pressures.

:davis:

xbgs351

I know that David Vizard recomends 28 thou piston to head clearance for a small block chevrolet.

It has since occured to me that the reason you may run less clearance with a small block chevrolet is due to greater piston/valve clearance as the chevrolet valves are not canted and have a lower rocker ratio.

I believe the Hypatec pistons have one relief for the inlet valve. The relief is 3 CC. I don't know where I can obtain the relevent specs and recomendations for these pistons.

At this stage I am inclined to use zero deck height. I will wait until I have my heads flowed and choose a suitable solid camshaft prior to commencing machining.

Another concern regarding deck height is that if I remove too much material I won't be able to square the deck for a future rebuild.

clevo400

Agree with russxr67 zero deck height. rod bolts stretch and valves float. the piston crown might slap the head with a closed chamber but open might be OK. KB and Speed Pro/TRW should have a piston to suit your comp ratio.

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regards
Stephen