BOSTON (Reuters) - Elon Musk wants to take Tesla Inc TSLA.O private, but some of his big investors may not be able to follow.

FILE PHOTO: A Tesla sales and service center is shown in Costa Mesa, California, U.S., June 28, 2018. REUTERS/Mike Blake/File Photo

Most passive funds would have to drop Tesla if it were removed from stock indexes, experts said on Monday.

Actively-managed funds might need complicated approvals to hold a stake in the company, no matter how much of a mark Musk has made to date as founder and chief executive of the electric carmaker.

“Our inclination is that if we could go private with him, we would, but it’s complicated,” said one large Tesla investor, who spoke on condition of anonymity.

“We would have to call clients and ask for approval, but it’s complicated. And I think it’s complicated for many funds,” the person said.

The uncertainty over how many investors could hold private shares, and would want to, would be a challenge for Musk because the final tally would dictate how much capital he would need to raise to buy out those who would sell the stock.

Musk blogged on Monday he was reaching out to top investors to see “whether they had the ability and desire to remain shareholders in a private Tesla.”

One pool that would likely not be Musk's allies in the effort are big passive vehicles like the Vanguard Total Stock Market Index Fund VSMPX.O.

The fund held about 2 percent of Tesla’s shares at June 30. The fund’s holdings are based on an index tracking shares on the New York Stock Exchange and Nasdaq, according to its prospectus.

Vanguard spokeswoman Carolyn Wegemann declined to comment on Tesla but said by email, “As private companies are not included in indexes, any shares held in an index fund when a company goes private would likely be sold/tendered.”

A factor that could work in Musk’s favor is that more of his top fund investors are active stockpickers. Active funds would generally have more leeway to continue to own Tesla after a go-private transaction.

According to researcher Morningstar, 19 percent of Tesla's fund ownership rests in passive portfolios, compared with 28 percent at General Motors Co GM.N and 56 percent at Ford Motor Co.F.N

Tesla's largest active fund investors include some that have held private companies such as Fidelity Blue Chip Growth Fund. FBGKX.O At June 29 it held securities of private companies like Uber Technologies Inc and Lyft Inc according to a disclosure. A Fidelity spokeswoman declined to comment.

Private shares can be illiquid and hard to price, raising concerns of whether they are appropriate for public portfolios.

David Barclay, chief operating officer of the Center for Research in Security Prices at the University of Chicago’s Booth School of Business, said he knew of no case of a security remaining in an index fund after it went private.