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Low CR Piston to wall Clearence VQ35DE - turbo build

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Hi, I'm building my second engine for 350Z with forged internals for turbo.

I did buy Cosworth pistons second hand (supposed to be in the engine for less than 200km) and they are 8,8:1 and should be for 96mm bore. I have no original box from them. From the same guy I did buy a block. It is bored to 96mm. Also, I have my old High compression Wiseco pistons for 96mm bore, which I'm comparing it to.

When we put it into the block, immediately they looked small. I will attach video, but they bang on the side. I did watch the video of HP academy with similar banging of the sides, but still I'm concered if I should continue. The piston, if I measured it correctly is 95,3mm on the skirt, I have to remeasure it, I did not pay that much attention to it before, to have the exact 90 degress and measure in the exact point from the top. Above the first ring it has 94,9mm.

For me it looks like a lot to have a 1mm difference. I also looked online and phoned up Cosworth, if they have done pistons which would be for 95mm bore and they did not, the guy on the phone could not answer me the questions, what size should be the piston for 95,5mm and for 96mm, that they are selling. (Also, on the web you can choose just 95,5 rings or 96mm ring, not the actual pistons, but I think, its just a web issue)

So now, if I put the piston into the block, its banging from side to side on the top. I can't feel any movement on the bottom, but who knows. I need you help, if I should continue or not. If it's going to expand under heat from the turbocharing and run allright, or will just blow up in the garage.

Thank you a lot.

Attached Files

What did cosworth recommend for minimum / maximum cylinder wall clearance? That is the information you need along with the measured sizes of your pistons and cylinders (all of them!) Then you can decide if the combination is with the spec. Take multiple measurements to make sure you've found the correct value. If you don't have the tools to measure, consider taking the parts to a machine shop so they can measure it for you and confirm.

It seems like you're a little confused by how to measure the piston, and the allowance made for thermal expansion in the running engine as the piston heats up unevenly.

The piston is normally measured at 90 degrees to the 'pin, around 25mm from the bottom of the skirt (it may vary a little), and this will be the largest diameter part of the piston. Depending on the piston design, structure, and alloy used there can be quite a big difference in cold clearance and part of this is there can be quite a lot of clearance at the top of the piston - even enough to see the top ring clearly - which may allow the piston to 'bang' in the bore, this is usually referred to as 'piston slap' and is one of the most annoying things about 'race' pistons in 'street' cars. Another factor that can affect the 'rocking' you show is the length and design of the skirt, as some race pistons are quite short and show more than other brands that are longer. As the piston is heated in use, it expands and should take up just about all the gap.

That said, you think you have 0.7mm nominal clearance from your measurements, assuming they were done correctly, and bore is actually 96mm, that would be ~0.027" which is WAY too much! It would be close for a 0.020/0.5mm overbore, though, especially if you were slightly off on the measuring!

What I'm concerned about it just how accurate the story about the parts is - worst case is the wrong pistons were mistakenly sold to you that were for a smaller bore - which means sorting out the correct ones with the seller, buying a new, correct size set, or buying another stock block that can be correctly bored and clearanced for the pistons you have.

With the rings, there may be 95.5mm overbore pistons those rings are for, but sometimes file-to-fit rings aren't available, and one may need to buy the next size up to file for the correct clearance.

As David said, you REALLY need to take all the pistons, and the block, to a reputable machine shop and have them measure the bores and the pistons, so you know exactly what you're dealing with. In the meantime, have a very careful look over the pistons for any markings or numbers that will help you identify the pistons.

Hi, thanks a lot for all suggestions.

I'm going to the garage today with a guy from machine shop to do the measurements.

My concern is, that those pistons will be for 95,5mm bore and not the 96mm. I know that there are some numbers on the pistons, so I will try to get them and Google it or call Cosworth to be sure, what exactly they are. Also, I will try to contact them, to get the sizes and clearence from them.

The pistons to be fair looks like they have never been used, no markings, no scuffs, anything that would tell me, that they have been used. Which is a good and bad thing for me right now.