Maruha motors

Maruha Original Crank Shaft For BP/85mm・87mm Stroke


Prices and details

Crankshaft for BP 85mm stroke
BP用 85mm 専用クランクシャフト  85mmストローク
code:mar20110085 For NA8C NB8C
Crankshaft for BP 87mm stroke
BP用 87mm 専用クランクシャフト  87mmストローク
code:mar20110087 For NA8C NB8C

Overseas customers (non-residence in Japan) don’t need to pay any domestic tax in Japan. Please refer to Japanese prices shown in parenthesis.

The crankshaft is the spine


If you could compare a crankshaft to our body parts, it’s like a backbone. In case of a tree, it’s like a trunk.
A good engine consists of multiple balancing including cams, pistons, valves, and other parts.
The main component, so to speak, core must be a crankshaft. Engine maintenance cannot be made without a stable crankshaft.

Genuine crankshaft is discontinued


Now, let’s check out the inventory of Mazda genuine parts record history for NA8 and NB8.
As of January 2021, item no. BP06-11-300D, Mazda genuine crankshaft, (the final price was JPY130,800) is discontinued. It is not in stock at Mazda anymore.
That is, no more brand-new crankshaft from Mazda is available.
Therefore, engine overhaul under a crankshaft problem must be difficult.It means that there is no more chance to overhaul using a brand-new genuine crankshaft.

Reuse genuine crankshaft


Overhaul without any consideration of engine milage (travel distance) with reusing crankshaft is not a good idea.
The contents of overhaul should be decided comprehensively depending on the owner’s intention and budget, but it seems to be difficult for only owner’s knowledge to decide replacing or reusing of crankshaft.As a result, owners generally respect and follow their shop’s opinion.

Here is a question for you.
After 100,000 miles driving, in case that the owner asks to overhaul for enjoying the next ten years or more 50,000 miles driving, reusing crankshaft should be recommended? Most of answers might be vaguely no problem or not recommended.
So far, reusing of crankshaft is naturally decided without sweeping away any doubts.If it is broken, then it should be taken care of … while thinking about a used engine, the contents of overhaul is consequently decided.

Market has changed. Nowadays well-conditioned used engines for NA8 and NB8 are traded at higher prices.Recently getting used engines is also not so easy. The rise of used engine cost will be accelerating rapidly.
No more counting on used engines.Needles to say, there is no way dealing for a used single crankshaft unit individually.

Weaknesses of genuine crankshafts

There seems to be no specific problem on the crankshaft taken from an overhaul apparently, but there is a weak point on it originally.

The length at the snought of genuine crankshaft is not enough.
The oil pump, the timing gear, and the pulley boss are attached to the tip and locked by the center bolt.
The problem is the remaining space after installing the timing gear.
This remaining space is used for attaching the pulley boss, but in the case of genuine crankshaft, it is only 6mm.

The heavy front pulley is attached to the pulley boss attached unsteadily to the only 6mm space. The belts for the alternator and the air conditioning & the power steering system are tensioned from right and left sides and attached to the front pulley.
The car must be run with this load for thousands of miles. All of this burden has to be held by the only 6mm space.

Before long, this load will cause friction between the tip of crankshaft and the pulley boss and lead to a big trouble.
It’s the only 6mm space. It is the only 6mm space and there must be also tolerance there. In case that turning pulley is moving thrust direction unsteadily, there must be almost certainly uneven wear occurred between the crankshaft and the pulley boss.

The pulley boss can be replaced with the new one, but it’s not going to work out that way for the crankshaft.
As mentioned, the brand-new genuine crankshaft was already discontinued, and furthermore replacing the crankshaft is a kind of heavy overhaul which is the same as engine overhaul.

Maruha original crankshaft has enough length to keep 14mm space for the pulley boss to be stable.Therefore, the connection of pulley boss is stable. As a result, a prolonged lifetime can be expected.


The modification from the genuine crankshaft is not only the tip .
As weight saving, the Maruha original crankshaft is over 1kg lighter than the genuine one. Improvement of response is expected.

85mm、87mm Stroke

The stroke of genuine crankshaft is 85mm.The length from the center of crankshaft to the pin journal is 42.5mm.Therefore, the stroke distance of round trip between up and down becomes 85mm.
The stroke length of 87m (43.5mm x 2) Maruha original crankshaft is also available.
If you need standard overhaul, we recommend you use 85mm type of Maruha original crankshaft as same as the genuine stroke.
If you need exhaust displacement increasing, 87mm typed Maruha original crankshaft is recommended.

How to use 87mm strokes

The radius of gyration with 87mm typed Maruha original crankshaft is 1mm longer than the genuine one.
In case of using 87mm typed Maruha original crankshaft with the genuine pistons and connecting rods, the top position of piston (top dead center timing) is 1mm protruded than the genuine one.

The genuine gasket (85mm) is used. Green in the above illustration : the genuine gasket is used.
+1mm thickness typed gasket The above illustration in blue: +1mm thickness typed gasket (87mm)

Using the +1mm thicker gasket is easily adapted to this 1mm protrusion of the top position of piston.
Maruha would like to try to increase types of the original gasket for achieving this method and utilizing the Maruha original 87mm crankshaft with various occasions as many as possible.

Metal head gasket for 87mm crank bore83.5mm t1.8mm
メタルガスケット ビードタイプ(for 87mm crank)(2023年1月〜) 内径83.5mm 厚み1.8mm
code:mar21032000 For NA8C NB8C
Metal head gasket for 87mm crank bore84.5mm t1.8mm
メタルガスケット ビードタイプ(for 87mm crank)(2023年1月〜) 内径84.5mm 厚み1.8mm
code:mar21061600 For NA8C NB8C
Metal head gasket for 87mm crankbore86mm t1.8mm
メタルガスケット ビードタイプ(for 87mm crank)(2023年1月〜) 内径86mm 厚み1.8mm
code:mar21032001 For NA8C NB8C

In other words, depending on ideas of head-gasket, various kinds of pistons and connecting rods enable you to try several engine menus.
Not only each size of pistons made by HAHLE, but other manufactures’ products can be also applied.

Use cases of crankshafts


MAHLE 85.5mm/1mm Short Pin Height

マーレーピストン製85.5mm 1mmショート

Other than the method for absorbing the +1mm caused by 87mm crankshaft at the head-gasket, the new design piston 85.5mm (for the exclusive BP) made by MAHLE has been established.
The profile is the same as conventional type, but the pin height is shortened by 1mm to adjust.
The 0.8mm or 1.0mm thickness type of head-gasket is available as usual.

Mahle Custom Piston(-1mm PH) BP 85.5mm for 87mm crank
Mahle パワーピストン (1mm PH ショートタイプ) MAHLE SPECIAL 85.5mm(2023年11月〜)
code:mar21020610 For BP

Click here for details

Journal overlap with 87mm strokes

Journal overlap means the common part overlapped with the main journal and the pin journal (turning radius).
The bigger this overlap is, the higher rigidity and strength are. Therefore, the smaller overlap becomes the lower rigidity and strength.
The design of pin journal follows 45mm as genuine standard.
That is to say, the genuine metal bearings or Maruha’s power metals (F112) can be applied as it is.Compared to 85mm stroke (5mm overlap), 87mm stroke’s overlap is shortened by 1mm, but its stroke is comparatively safe because its overlap keeps 4mm as plus.。

Just for your reference, regarding the crankshaft with 89mm stroke for 2100cc specification, the diameter of pin journal is changed from 44mm (genuine standard) to 48mm to be +4.5mm overlap and to keep good rigidity.

ロードスターNA/NB BP/2.1Lキット ロングストローカーキット
Component parts for 2.1liter kit

Affordable price

Price is the most attention point for a good reputation in the market.
It was not so easy story for us because we had to make our original crankshaft by ourselves.
Maruha took cast process because the more production number is increased, the less unit cost can be decreased.
We have corrected the weak points of genuine crankshaft and achieved saving weight while reducing the cost. No more genuine one is available from Mazda, and our price is less than theirs.
Judging from the above-mentioned contents, even if the genuine crankshaft is reproduced right now, everyone must be very satisfied with Maruha original crankshaft.

Crankshaft for BP 85mm stroke
BP用 85mm 専用クランクシャフト  85mmストローク
code:mar20110085 For NA8C NB8C
Crankshaft for BP 87mm stroke
BP用 87mm 専用クランクシャフト  87mmストローク
code:mar20110087 For NA8C NB8C

We can provide a wide variety of other parts

English Maruha has a lot of engine parts for mazda mx5 !! Please ask us about your idea and request.

こちらも一括手配が可能です。 つまり、MARUHAですべて揃う訳です。

Rapid control is recommended

English Rapid module is quite simple and easy device to optimize your engine. Replace camshafts, pistons, injectors… you probably try to modify your lovely Mx5 !! However ECU control is the main item to get the suitable performance. If you miss this point, everything could come to nothing. Please consider of ECU control more.

制御系の見直しはとても大事です。 例え基本的なOHだとしても、本来のノーマルエンジンからすでに燃調が合っていないエリアも存在します。 そこに、念願のMAHLE製ピストンを入れたり、あるいはFカム程度のライトなカムをいれたり、あるいは87mmロングストローカーを組んだり、あれこれと自分の愛車が生まれ変わることを楽しみにメニュー・予算を組んで行かれることでしょう。しかし、制御を怠っては本末転倒。大事な各アイテムの性能は発揮しきれず・・そんなことに陥らない様に制御系もはっきりとさせておく必要があります。 MARUHAではイタリアDIM社に特注したモジュレーター・RAPID(ラピッド)をご用意できます。


  • 取り付けは比較的簡単、エンジンルーム内で完結、専用コネクターで接続
  • MAP編集が可能(当社及び指定ディーラー様にてのみ)、各エンジンの特徴に合った仕様が作成できます。
  • 通販レベルでも最初からMARUHAオリジナルMAPが入力済(詳しくはお問い合わせください。)
  • 燃料変更、点火時期変更、レブリミッター引き上げ(+1000rpm程度まで)
  • ヘッド面研、ハイカム化、ボアアップ、ロングストローク、ビッグスロットル、インジェクター変更などにも対応可能
  • 取り外し(ノーマル化)も簡単。
RAPiD TPM MX-5 NA6用 ラピッド
code:dim18052900 For NA6
RAPiD TPM MX-5 NA8用 ラピッド
code:dim18052901 For NA8
RAPiD TPM MX-5 NB6用 ラピッド
code:dim19081600 For NB6
RAPiD TPM MX-5 NB8用 ラピッド
code:dim18052902 For NB8

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