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Old 12-11-2017, 11:09 PM   #1
SKVang
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Default A and B piston questions

I will be upgrading to 95.5mm forged pistons but had a question. I was reading and there are A and B sized pistons for the ej25. Does that mean when I buy forged pistons, I have to get "A and B" pistons as well or do the aftermarket forged pistons just come one size and I just hone?
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Old 12-12-2017, 09:00 AM   #2
Loyale93
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Have you asked the vendor you're buying from?

Who's doing the installation on your motor? Have you asked them?
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Old 12-12-2017, 04:37 PM   #3
SKVang
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Originally Posted by Loyale93 View Post
Have you asked the vendor you're buying from?

Who's doing the installation on your motor? Have you asked them?
I will be having a person i know who's reputable or a shop do it. I've been looking around online and the aftermarket forged pistons don't say anything about A and B's so I wanted to make sure I ask and research before I do any shopping.
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Old 12-12-2017, 10:01 PM   #4
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One does NOT buy pistons willy nilly. One has a dialogue with their builder to determine which piston best suits their need. If you think you can show up to a shop with a box of parts and say, "Build this", you are in for a world of hurt.
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Old 12-12-2017, 10:48 PM   #5
monkeyposeur
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A and B are OEM designations stamped on the two block halves which correspond to two factory bore sizes. Iirc Manley made A & B size 'drop in' pistons.

Normally aftermarket pistons are slightly oversize from the OEM bore for rebuilds. For example the bore on my EJ22T is 96.5 (I think, it's been a while) from the factory. When I bought my pistons from Wiseco they were listed as .02" OS. OS = oversize, meaning the pistons are .02" larger than the factory bore. So the block needs to be bored out .02" in order for them to fit plus the PTW (piston to wall) clearance required to allow the piston to expand upon heating.

If let's say I needed to rebuild the engine a second time and needed new pistons I could get some .03" OS pistons and get the block bored again. Of course there is only so much material that can be removed without compromising the strength of the sleeves.

Sometimes there is a range offered by the piston manufacturer. The builder/machinist measures the bore of the engine and determines the minimum amount of material that needs to be removed to get the bore back in round and then orders the correct OS piston to match the desired application.

Your builder can help you determine what size pistons you will need based on the above information unless you want to buy some mics and dial bore gauges and do it yourself, lol.

Assembling the shortblock is fairly straightforward. What takes skill is measuring and machining everything. But nothing wrong with searching for info and researching piston manufacturers. Lots of info on this site including the piston FAQ.

It would help if you stated in your first post why you think you need new pistons, what your budget is, and what you hope to do with the engine afterwards e.g power goals.
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Old 12-13-2017, 09:24 AM   #6
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Quote:
Originally Posted by SKVang View Post
I will be upgrading to 95.5mm forged pistons but had a question. I was reading and there are A and B sized pistons for the ej25. Does that mean when I buy forged pistons, I have to get "A and B" pistons as well or do the aftermarket forged pistons just come one size and I just hone?
One of the best things you can do to help yourself is talk to your builder first.
Builders prefer to use the pistons, and other parts, they are most familiar with.
They usually stick to specific brands because:
1. They have the most experience installing said brand
2. They have the least amount of issues with said brand
3. They have a good relationship with said brand manufacturer(s) which includes great Customer Service if there is an issue with said parts

The other thing you should consider, AFTER discussing your goals with the builder, is let the builder buy all the appropriate parts, instead of you showing up with all your own parts. Why?

Because in the event of an issue, the builder will usually handle all issues with parts replacement, warranty, etc. on your behalf.
If you bring in a box of parts, you are typically solely responsible.

Yes, many people say, "I want to guy my own parts to save money".
If the builder buys the wrong parts, or the parts fail due to manufacturing issues, they will handle it all and you are typically paying once.

If you buy the parts, and something goes wrong due to part failure from manufacturing issues, you will be paying twice because it will all fall on you, not the builder.

A reputable builder will not "rip you off" on parts but just remember they need to eat too.
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Old 12-14-2017, 11:50 AM   #7
SKVang
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Quote:
Originally Posted by monkeyposeur View Post
It would help if you stated in your first post why you think you need new pistons, what your budget is, and what you hope to do with the engine afterwards e.g power goals.
I don't plan on big power but I had planned to get my sti tuned onto e85 and 91 for reliability so I did a leak down test at the tuning shop. Second cylinder's result didn't come back good and they said it was leaking past the rings and that it was most certainly ringland. So they said that I could buy a new shortblock and they could do it or drop in new pistons if the walls are still good. Hearing that, my plans as of now, is to get it tore down and check the pistons and walls. If the walls are still good, I was planing on getting 99.5mm wiseco pistons (not 100% sure what brand I want to go with) and honing the walls. If the walls end up being bad, I'm thinking bore and hone to bigger size pistons or if anything just buy new shortblock.
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Old 12-14-2017, 12:06 PM   #8
rtv900
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The A and B are what they use to give themselves a tolerance. If they shoot for 95mm, when they a 94mm they call it A, when they get 96mm they call it B. (I'm making up numbers)
So basically they machine your block, see what they ended up with, and then call it A or B and their piston shop makes two piston sizes so the assemblers can pick the best fit.

If you are rebuilding your motor it would be crazy to just go and buy pistons before checking your bores, so no need to worry now, waste of time.
If your bores are good measure them and get pistons to fit.
If they suck bore it out to a size that matches available pistons.
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