What is the impact of 0.030 pistons for Yamato racing

[GrandpaRacer]

I posted this on Stock’s Facebook page but got no response. Here are some facts for you to consider especially if you are running a stock bore. If you want to check my math displacement = (bore/2)squared, times Pi, times Stroke, times 2.

A stock 302 is 399.3ccs, a 20 over engine is 405.5ccs, and now a 30 over is 408.6 ccs. This makes the newly allowed 30 over motor 2.3% bigger! In racing this is huge. When you are straining to reach the first turn first would you rather have a 408cc engine or a 399cc engine? All things being equal the 408cc engine will be faster. Did the commission consider this huge difference they created? It seems a simple solution without causing this disparity would be to find a source for sleeves for the 302, such as LA Sleeve.

What are your thoughts?
John Adams

[ZUL8TR]

Using stock bore = 2.598" and stroke = 2.283 from manual I get numbers on volume a bit lower but same 2.3% greater at 30 over. What about piston weight increase for high revs at 30 over for such a small increase in volume? Maybe a wash?

http://yamatoracing.com/y302manual.pdf

Pete

[GrandpaRacer]

No Comments so far tells me racers don't care or they don't understand. The 30 motor being bigger will develop more power and be faster. Your standard bore motor will be obsolete when racing against the 30 over motor. It will cost you about $400 to 500 to make it a 30 over motor and when it wears out it is junk unless they allow 40 over bores next. Right now the old long time teams has some worn out 20 over motors that they would like to bring out of the junk pile. A beter solution to this problem is to resleeve them but sleeves are hard to get from Yamato. They can be purchased from LA Sleeve which is a company making it their business to provide aftermarket sleeves. They are just like Weisco making after market pistons. The Solution involing resleeving with aftermarket sleeves is a permanent solution and doesn't upset the current status quo in the class. Each time your motor gets tired and warn out it can be resleeved rather than becoming junk until the SORC authorizes a new bore and new pistons. 2021 is a long time from now and this decision can be change if you speak up.

[GrandpaRacer]

Pete, is 2.598" the piston size? I used Bore size in the APBA spec of 2.605 (Spec less 0.020) for the standard bore. I checked a std bore to confirm this. The Stroke spec was 2.286 +/- 0.010 but it changed a couple years ago to 2.276 to 2.291. I am not sure which is nominal 2.286 or 2.283, but as you say the % increase in size is the same.
John Adams

[GrandpaRacer]

Pete, the current APBA spec is 2.625 max, this is the 0.020 over size spec. So the standard bore I used was 20 less. 2.605. The 30 over is 2.635.

[ZUL8TR]

John

Seems weird, if the 302 spec says 2.625 max bore diameter than how can that be exceeded?

Pete

[GrandpaRacer]

The current APBA spec is the 2.625 inch maximum bore. This spec allows one to use a 0.020 larger piston. Right now that spec can not be exceeded. But as the stock commission has announced in 2021 they will change change that max spec to 2.635 which will be 0.030 over. The standard pistons and bores are in the 2.602-2.605 range. The one I measured was 2.605 but it all depends on how much hone work has been done and the piston to cylinder clearance has been chosen. It seems each mechanic has a difference secret number.

[modhydro]

John,

In the statistics you list, you don't mention that it is only .76% more that the .020" overbore that was previously allowed. If the boring thing was truly that big of an advantage everyone would already be at that so incrementally the change is small. I hear what you are saying, but it isn't like they just made a .030" change like the post seems to infer.

While there are aftermaket places like LA sleeves, most won't even talk to you unless you are going to order a substantial amount to make it worthwhile. Do you have any $$ figures for a set of sleeves to put into a worn out Yamato? Just curious to what they would cost. We are such a niche sport/hobby that firms aren't exactly lining up to support us.

Steve

[ZUL8TR]

Steve

Actually 1.5% larger volume at +20 using John's numbers I get the same with mine above.

Pete

[modhydro]

Pete,

I was talking about the delta from .020 to .030.

1.5% plus .76% = John's 2.3%

[ZUL8TR]

Ok that explains it.

[GrandpaRacer]

Pete if the piston were cast like the Yamato ones 30 over should be heavier. Weisco pistons are forged and probably a different aluminum so they could be heavier or lighter and expand differently. I know on my OMC they had ring positions that were different than Stock and didn't meet APBA specs but that may have been fixed.
John Adams

[ZUL8TR]

John

Yes the weight difference could be lighter or heavier I only have experience with the Merc 25ss +15 pistons and they were heavier than the stock ones. Both Merc brand, but alloys could be different and other shaping?

Pete

[pav225]

No impact. I plan to race my standard bore Yamato until it’s scored bad enough to need to bore. Then I’ll go 20 over. If I ever score that bad, I’ll go to 30 over.

[GrandpaRacer]

Steve, thanks for checking my math. 2.635/2.625^2 is 0.76% bigger in area compared to the 20 over so less of an issue, but compared to the standard bore is is 2.3% bigger. And this is a lot.

I am not sure how much it cost to make a run but it will be cheaper than one off. I know they will make a one off copy of a sleeve. My son had a one off made for $200 for his Konig. They have made them for 102s but not 302s yet. Our motorcycle shop bought several from them years ago.

[ZUL8TR]

John
At +20 bore increase in diameter your calc should be (2.625 + 0.020/2.625)^2 = 1.0153 or about 1.5% greater but you used the Max bore as base in the APBA spec and increased and bore cannot be larger than 2.625" max?

Using 302 manual bore this would be (2.598 + 0.020/ 2.598)^2 = 1.0155 or about 1.5%

Pete

[GrandpaRacer]

Mike, Your boats are so good you don't need the extra 1.5% you could have now at 20 over.
John Adams

[ricmontoya]

Who is going to do all the prep work to make the Japanese Stadium Raced motors into not so stock APBA Stock Outboard motors?
What is the cost of all the prep work to make a Yamato Stadium Raced motor into a APBA Stock Outboard motor?

Let's exclude the DIYers that have been around this sport for years and think about the new guys.

[deeougee]

I’m with Mike on this one, I will stay with the stock bore until it’s absolutely necessary to bore. The gains are negligible without doing other work such as port work. If the ports tapered into the bore then we’d have something to talk because the port opening would be bigger. We checked out all this stuff along time ago at Bud Parker’s shop. Now I’m all for new sleeves if we could get them. Rick, even for the DIYer it costs money and more importantly a lot of time. For new guys they would be better served going to an engine clinic and doing some research and doing it themselves and then they could graduate into MOD. For people who don’t want to do any of that, 300SSH is their calling or one of the SW engines.

[fcliff]

In the Mod applications I stay with stock bore as long as the holes are good. When bore condition requires I go to the 20 over. I find no noticeable change in performance when enlarging. I think the quality of the bore is the number one issue. I am glad there will be the option of 30 over when my 20 year old over sized block needs a new bore

[mercguy]

I agree 100% with you Cliff!!

[David German]

I agree with Darren

[guedo499]

Note: I’ve been looking for a block with a few specific measurements and have picked up about 8 stadium raced blocks in the last year - buying stadium raced blocks, standard bore is extremely simple and they are available. Sleeves aren’t available and more expensive than the block with the sleeves in them, even if you could get them, go figure.