Below is quoted from USA Federal Register Goverment Website as of above topic:
By inserting the following new U.S. note 20(nn) to subchapter III of chapter 99 in numerical sequence:“(nn) The U.S. Trade Representative determined to establish a process by which particular products classified in heading 9903.88.03 and provided for in U.S. notes 20(e) and (f) to this subchapter could be excluded from the additional duties imposed by heading 9903.88.03, and by which particular products classified in heading 9903.88.04 and provided for in U.S. note 20(g) to this subchapter could be excluded from the additional duties imposed by heading 9903.88.04. See 83 FR 47974 (September 21, 2018) and 84 FR 29576 (June 24, 2019). Pursuant to the product exclusion process, the U.S. Trade Representative has determined that the additional duties provided for in heading 9903.88.03 shall not apply to the following particular products, which are provided for in the enumerated statistical reporting numbers:
......
(15) Starter motors for internal combustion gasoline engines designed for use in the lawn, automotive, watercraft, motorcycle, industrial and garden industries (described in statistical reporting number 8511.40.0000)
......
If you still want to know the USA tariff duty exclusion as of China alternators, you can send email to Mr. Carl Li at carl.li@longjim.com.
Mr. Carl Li is global supply chain expert with certification of purchasing manger(C.P.M), Six Sigma Black belt and ME & MBA credentials . He has run his Alternator, starter manufacturing and exporting business for 8 years. Before that he led WAI China entities as supply Chain Vice President and General Manager. Mr. Carl Li is the subject matter expert as of Alternator, starter and related parts and offers quality products in good coverage.
Refer to below URL from US government for the detail.
Rough translation in Chinese for key points:
按数字顺序在第99章第三分章中插入以下新的美国注释20（nn）:美国贸易代表决定建立一个程序，使归入第9903.88.03品目并在本分章美国注释20（e）和（f）中规定的特定产品可以从第9903.88.03品目规定的附加关税中排除，并据此将归入第9903.88.04品目和本分章美国注释20（g）规定的特定产品排除在第9903.88.04品目征收的附加关税之外。见83 FR 47974（2018年9月21日）和84 FR 29576（2019年6月24日）。根据产品排除程序，美国贸易代表确定，第9903.88.03品目中规定的附加关税不适用于下列特定产品，这些产品是在列举的统计报告编号中规定的：
......
（15） 用于草坪、汽车、船舶、摩托车、工业和园林工业的内燃机起动马达（统计报告编号8511.40.0000）
......
以上是如何处理美国起动机进口关税的方法。 关于发电机的关税事宜，如有需求可写邮件给李先生： carl.li@longjim.com;
李先生是具有美国注册采购经理、六西格玛黑带认证的资深全球供应链专家， 工学硕士和MBA。李先生曾担任WAI中国和畅博电子/Transpo的总经理以及全球供应链副总裁。李先生自2012年起离开WAI，创建自己的外贸公司，致力于向全球客户提供各种车型、全系列的优质发电机、起动机和相应配件。
